lebedev.ri updated this revision to Diff 115482.
lebedev.ri marked 13 inline comments as done.
lebedev.ri added a comment.

Address most of the @JonasToth's review notes.


Repository:
  rL LLVM

https://reviews.llvm.org/D36836

Files:
  clang-tidy/readability/CMakeLists.txt
  clang-tidy/readability/FunctionCognitiveComplexityCheck.cpp
  clang-tidy/readability/FunctionCognitiveComplexityCheck.h
  clang-tidy/readability/ReadabilityTidyModule.cpp
  docs/ReleaseNotes.rst
  docs/clang-tidy/checks/list.rst
  docs/clang-tidy/checks/readability-function-cognitive-complexity.rst
  test/clang-tidy/readability-function-cognitive-complexity.cpp

Index: test/clang-tidy/readability-function-cognitive-complexity.cpp
===================================================================
--- /dev/null
+++ test/clang-tidy/readability-function-cognitive-complexity.cpp
@@ -0,0 +1,839 @@
+// RUN: %check_clang_tidy %s readability-function-cognitive-complexity %t -- -config='{CheckOptions: [{key: readability-function-cognitive-complexity.Threshold, value: 0}]}' -- -std=c++11
+
+// any function should be checked.
+
+extern int ext_func(int x = 0);
+
+int some_func(int x = 0);
+
+static int some_other_func(int x = 0) {}
+
+template<typename T> void some_templ_func(T x = 0) {}
+
+class SomeClass {
+public:
+  int *begin(int x = 0);
+  int *end(int x = 0);
+  static int func(int x = 0);
+  template<typename T> void some_templ_func(T x = 0) {}
+};
+
+// nothing ever decreases cognitive complexity, so we can check all the things
+// in one go. none of the following should increase cognitive complexity:
+void unittest_false() {
+  {};
+  ext_func();
+  some_func();
+  some_other_func();
+  some_templ_func<int>();
+  some_templ_func<bool>();
+  SomeClass::func();
+  SomeClass C;
+  C.some_templ_func<int>();
+  C.some_templ_func<bool>();
+  C.func();
+  C.end();
+  int i = some_func();
+  i = i;
+  i++;
+  --i;
+  i < 0;
+  int j = 0 ?: 1;
+  auto k = new int;
+  delete k;
+  throw i;
+  {
+    throw i;
+  }
+end:
+  return;
+}
+
+#if 1
+#define CC100
+#else
+// this macro has cognitive complexity of 100.
+// it is needed to be able to compare the testcases with the
+// reference Sonar implementation. please place it right after the first
+// CHECK-NOTES in each function
+#define CC100 if(1){if(1){if(1){if(1){if(1){if(1){if(1){if(1){if(1){if(1){if(1){if(1){if(1){}}}}}if(1){}}}}}}}}}
+#endif
+
+//----------------------------------------------------------------------------//
+//------------------------------ B1. Increments ------------------------------//
+//----------------------------------------------------------------------------//
+// Check that every thing listed in B1 of the specification does indeed       //
+// recieve the base increment, and that not-body does not increase nesting    //
+//----------------------------------------------------------------------------//
+
+// break does not increase cognitive complexity.
+// only  break LABEL  does, but it is unavaliable in C or C++
+
+// continue does not increase cognitive complexity.
+// only  continue LABEL  does, but it is unavaliable in C or C++
+
+void unittest_b1_00() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b1_00' has cognitive complexity of 5 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  if (1 ? 1 : 0) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:9: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+  } else if (1 ? 1 : 0) {
+// CHECK-NOTES: :[[@LINE-1]]:10: note: +1, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:16: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+  } else {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: +1, nesting level increased to 1{{$}}
+  }
+}
+
+void unittest_b1_01() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b1_01' has cognitive complexity of 2 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  int i = (1 ? 1 : 0) ? 1 : 0;
+// CHECK-NOTES: :[[@LINE-1]]:23: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:14: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+}
+
+void unittest_b1_02(int x) {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b1_02' has cognitive complexity of 5 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  switch (1 ? 1 : 0) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:13: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+  case -1:
+    return;
+  case 1 ? 1 : 0:
+// CHECK-NOTES: :[[@LINE-1]]:10: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    return;
+  case (1 ? 2 : 0) ... (1 ? 3 : 0):
+// CHECK-NOTES: :[[@LINE-1]]:11: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:27: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    return;
+  default:
+    break;
+  }
+}
+
+void unittest_b1_03(int x) {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b1_03' has cognitive complexity of 4 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  for (x = 1 ? 1 : 0; x < (1 ? 1 : 0); x += 1 ? 1 : 0) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:14: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-3]]:30: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-4]]:47: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    break;
+    continue;
+  }
+}
+
+void unittest_b1_04() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b1_04' has cognitive complexity of 2 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  SomeClass C;
+  for (int i : (1 ? C : C)) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:19: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    break;
+    continue;
+  }
+}
+
+void unittest_b1_05() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b1_05' has cognitive complexity of 2 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  while (1 ? 1 : 0) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:12: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    break;
+    continue;
+  }
+}
+
+void unittest_b1_06() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b1_06' has cognitive complexity of 2 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  do {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    break;
+    continue;
+  } while (1 ? 1 : 0);
+// CHECK-NOTES: :[[@LINE-1]]:14: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+}
+
+void unittest_b1_07() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b1_07' has cognitive complexity of 1 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  try {
+  } catch (...) {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+  }
+}
+
+void unittest_b1_08() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b1_08' has cognitive complexity of 1 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  goto end;
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1{{$}}
+end:
+  return;
+}
+
+void unittest_b1_09_00() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b1_09_00' has cognitive complexity of 34 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  if(1 && 1) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:8: note: +1{{$}}
+  }
+  if(1 && 1 && 1) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:13: note: +1{{$}}
+  }
+  if((1 && 1) && 1) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:15: note: +1{{$}}
+  }
+  if(1 && (1 && 1)) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:8: note: +1{{$}}
+  }
+
+  if(1 && 1 || 1) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:13: note: +1{{$}}
+// CHECK-NOTES: :[[@LINE-3]]:8: note: +1{{$}}
+  }
+  if((1 && 1) || 1) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:15: note: +1{{$}}
+// CHECK-NOTES: :[[@LINE-3]]:9: note: +1{{$}}
+  }
+  if(1 && (1 || 1)) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:8: note: +1{{$}}
+// CHECK-NOTES: :[[@LINE-3]]:14: note: +1{{$}}
+  }
+
+  if(1 || 1) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:8: note: +1{{$}}
+  }
+  if(1 || 1 || 1) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:13: note: +1{{$}}
+  }
+  if((1 || 1) || 1) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:15: note: +1{{$}}
+  }
+  if(1 || (1 || 1)) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:8: note: +1{{$}}
+  }
+
+  if(1 || 1 && 1) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:8: note: +1{{$}}
+// CHECK-NOTES: :[[@LINE-3]]:13: note: +1{{$}}
+  }
+  if((1 || 1) && 1) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:15: note: +1{{$}}
+// CHECK-NOTES: :[[@LINE-3]]:9: note: +1{{$}}
+  }
+  if(1 || (1 && 1)) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:8: note: +1{{$}}
+// CHECK-NOTES: :[[@LINE-3]]:14: note: +1{{$}}
+  }
+}
+
+void unittest_b1_09_01() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b1_09_01' has cognitive complexity of 12 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  if(1 && some_func(1 && 1)) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:8: note: +1{{$}}
+// CHECK-NOTES: :[[@LINE-3]]:23: note: +1{{$}}
+  }
+  if(1 && some_func(1 || 1)) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:8: note: +1{{$}}
+// CHECK-NOTES: :[[@LINE-3]]:23: note: +1{{$}}
+  }
+  if(1 || some_func(1 || 1)) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:8: note: +1{{$}}
+// CHECK-NOTES: :[[@LINE-3]]:23: note: +1{{$}}
+  }
+  if(1 || some_func(1 && 1)) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:8: note: +1{{$}}
+// CHECK-NOTES: :[[@LINE-3]]:23: note: +1{{$}}
+  }
+}
+
+// FIXME: each method in a recursion cycle
+
+//----------------------------------------------------------------------------//
+//---------------------------- B2. Nesting lebel -----------------------------//
+//----------------------------------------------------------------------------//
+// Check that every thing listed in B2 of the specification does indeed       //
+// increase the nesting level                                                 //
+//----------------------------------------------------------------------------//
+
+void unittest_b2_00() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b2_00' has cognitive complexity of 9 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  if (true) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    if(true) {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+    }
+  } else if (true) {
+// CHECK-NOTES: :[[@LINE-1]]:10: note: +1, nesting level increased to 1{{$}}
+    if(true) {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+    }
+  } else {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: +1, nesting level increased to 1{{$}}
+    if(true) {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+    }
+  }
+}
+
+void unittest_b2_01() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b2_01' has cognitive complexity of 5 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  int i = 1 ? (1 ? 1 : 0) : (1 ? 1 : 0);
+// CHECK-NOTES: :[[@LINE-1]]:13: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+// CHECK-NOTES: :[[@LINE-2]]:18: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+// CHECK-NOTES: :[[@LINE-3]]:32: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+// FIXME: would be nice to point at the '?' symbol. does not seem to be possible
+}
+
+void unittest_b2_02(int x) {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b2_02' has cognitive complexity of 3 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  switch (x) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+  case -1:
+    if(true) {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+    }
+    return;
+  default:
+    return;
+  }
+}
+
+void unittest_b2_03() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b2_03' has cognitive complexity of 3 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  for (;;) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    if(true) {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+    }
+  }
+}
+
+void unittest_b2_04() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b2_04' has cognitive complexity of 3 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  SomeClass C;
+  for (int i : C) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    if(true) {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+    }
+  }
+}
+
+void unittest_b2_05() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b2_05' has cognitive complexity of 3 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  while (true) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    if(true) {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+    }
+  }
+}
+
+void unittest_b2_06() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b2_06' has cognitive complexity of 3 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  do {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    if(true) {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+    }
+  } while (true);
+}
+
+void unittest_b2_07() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b2_07' has cognitive complexity of 3 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  try {
+  } catch (...) {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    if(true) {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+    }
+  }
+}
+
+void unittest_b2_08_00() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b2_08_00' has cognitive complexity of 8 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  class X {
+    X() {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: nesting level increased to 1{{$}}
+      CC100;
+
+      if (true) {
+// CHECK-NOTES: :[[@LINE-1]]:7: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+      }
+    }
+
+    ~X() {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: nesting level increased to 1{{$}}
+      CC100;
+
+      if (true) {
+// CHECK-NOTES: :[[@LINE-1]]:7: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+      }
+    }
+
+    void Y() {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: nesting level increased to 1{{$}}
+      CC100;
+
+      if (true) {
+// CHECK-NOTES: :[[@LINE-1]]:7: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+      }
+    }
+
+    static void Z() {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: nesting level increased to 1{{$}}
+      CC100;
+
+      if (true) {
+// CHECK-NOTES: :[[@LINE-1]]:7: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+      }
+    }
+
+// CHECK-NOTES: :[[@LINE-36]]:5: warning: function 'X' has cognitive complexity of 1 (threshold 0) [readability-function-cognitive-complexity]
+// CHECK-NOTES: :[[@LINE-33]]:7: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+
+// CHECK-NOTES: :[[@LINE-30]]:5: warning: function '~X' has cognitive complexity of 1 (threshold 0) [readability-function-cognitive-complexity]
+// CHECK-NOTES: :[[@LINE-27]]:7: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+
+// CHECK-NOTES: :[[@LINE-24]]:10: warning: function 'Y' has cognitive complexity of 1 (threshold 0) [readability-function-cognitive-complexity]
+// CHECK-NOTES: :[[@LINE-21]]:7: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+
+// CHECK-NOTES: :[[@LINE-18]]:17: warning: function 'Z' has cognitive complexity of 1 (threshold 0) [readability-function-cognitive-complexity]
+// CHECK-NOTES: :[[@LINE-15]]:7: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+  };
+}
+
+void unittest_b2_08_01() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b2_08_01' has cognitive complexity of 8 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  struct X {
+    X() {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: nesting level increased to 1{{$}}
+      CC100;
+
+      if (true) {
+// CHECK-NOTES: :[[@LINE-1]]:7: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+      }
+    }
+
+    ~X() {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: nesting level increased to 1{{$}}
+      CC100;
+
+      if (true) {
+// CHECK-NOTES: :[[@LINE-1]]:7: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+      }
+    }
+
+    void Y() {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: nesting level increased to 1{{$}}
+      CC100;
+
+      if (true) {
+// CHECK-NOTES: :[[@LINE-1]]:7: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+      }
+    }
+
+    static void Z() {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: nesting level increased to 1{{$}}
+      CC100;
+
+      if (true) {
+// CHECK-NOTES: :[[@LINE-1]]:7: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+      }
+    }
+
+// CHECK-NOTES: :[[@LINE-36]]:5: warning: function 'X' has cognitive complexity of 1 (threshold 0) [readability-function-cognitive-complexity]
+// CHECK-NOTES: :[[@LINE-33]]:7: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+
+// CHECK-NOTES: :[[@LINE-30]]:5: warning: function '~X' has cognitive complexity of 1 (threshold 0) [readability-function-cognitive-complexity]
+// CHECK-NOTES: :[[@LINE-27]]:7: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+
+// CHECK-NOTES: :[[@LINE-24]]:10: warning: function 'Y' has cognitive complexity of 1 (threshold 0) [readability-function-cognitive-complexity]
+// CHECK-NOTES: :[[@LINE-21]]:7: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+
+// CHECK-NOTES: :[[@LINE-18]]:17: warning: function 'Z' has cognitive complexity of 1 (threshold 0) [readability-function-cognitive-complexity]
+// CHECK-NOTES: :[[@LINE-15]]:7: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+  };
+}
+
+void unittest_b2_08_02() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b2_08_02' has cognitive complexity of 2 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  auto fun = []() {
+// CHECK-NOTES: :[[@LINE-1]]:14: note: nesting level increased to 1{{$}}
+    if (true) {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+    }
+  };
+}
+
+//----------------------------------------------------------------------------//
+//-------------------------- B2. Nesting increments --------------------------//
+//----------------------------------------------------------------------------//
+// Check that every thing listed in B3 of the specification does indeed       //
+// recieve the penalty of the current nesting level                           //
+//----------------------------------------------------------------------------//
+
+void unittest_b3_00() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b3_00' has cognitive complexity of 3 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  if (true) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    if (true) {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+    }
+  }
+}
+
+void unittest_b3_01() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b3_01' has cognitive complexity of 3 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  if (true) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    int i = 1 ? 1 : 0;
+// CHECK-NOTES: :[[@LINE-1]]:15: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+// FIXME: would be nice to point at the '?' symbol. does not seem to be possible
+  }
+}
+
+void unittest_b3_02(int x) {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b3_02' has cognitive complexity of 3 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  if (true) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    switch (x) {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+    case -1:
+      return;
+    default:
+      return;
+    }
+  }
+}
+
+void unittest_b3_03() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b3_03' has cognitive complexity of 3 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  if (true) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    for (;;) {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+    }
+  }
+}
+
+void unittest_b3_04() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b3_04' has cognitive complexity of 3 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  if (true) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    SomeClass C;
+    for (int i : C) {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+    }
+  }
+}
+
+void unittest_b3_05() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b3_05' has cognitive complexity of 3 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  if (true) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    while (true) {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+    }
+  }
+}
+
+void unittest_b3_06() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b3_06' has cognitive complexity of 3 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  if (true) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    do {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+    } while (true);
+  }
+}
+
+void unittest_b3_07() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'unittest_b3_07' has cognitive complexity of 3 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  if (true) {
+// CHECK-NOTES: :[[@LINE-1]]:3: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    try {
+    } catch (...) {
+// CHECK-NOTES: :[[@LINE-1]]:7: note: +2, including nesting penalty of 1, nesting level increased to 2{{$}}
+    }
+  }
+}
+
+//----------------------------------------------------------------------------//
+// Check that functions are being checked                                     //
+//----------------------------------------------------------------------------//
+
+class CheckClass {
+  CheckClass(int x) {
+// CHECK-NOTES: :[[@LINE-1]]:3: warning: function 'CheckClass' has cognitive complexity of 1 (threshold 0) [readability-function-cognitive-complexity]
+    CC100;
+
+    try {
+    } catch (...) {
+// CHECK-NOTES: :[[@LINE-1]]:7: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    }
+  }
+
+  void PrivateMemberFunction() {
+// CHECK-NOTES: :[[@LINE-1]]:8: warning: function 'PrivateMemberFunction' has cognitive complexity of 1 (threshold 0) [readability-function-cognitive-complexity]
+    CC100;
+
+    try {
+    } catch (...) {
+// CHECK-NOTES: :[[@LINE-1]]:7: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    }
+  }
+
+  void PrivateConstMemberFunction() const {
+// CHECK-NOTES: :[[@LINE-1]]:8: warning: function 'PrivateConstMemberFunction' has cognitive complexity of 1 (threshold 0) [readability-function-cognitive-complexity]
+    CC100;
+
+    try {
+    } catch (...) {
+// CHECK-NOTES: :[[@LINE-1]]:7: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    }
+  }
+
+  static void PrivateStaticMemberFunction() {
+// CHECK-NOTES: :[[@LINE-1]]:15: warning: function 'PrivateStaticMemberFunction' has cognitive complexity of 1 (threshold 0) [readability-function-cognitive-complexity]
+    CC100;
+
+    try {
+    } catch (...) {
+// CHECK-NOTES: :[[@LINE-1]]:7: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    }
+  }
+
+public:
+  CheckClass() {
+// CHECK-NOTES: :[[@LINE-1]]:3: warning: function 'CheckClass' has cognitive complexity of 1 (threshold 0) [readability-function-cognitive-complexity]
+    CC100;
+
+    try {
+    } catch (...) {
+// CHECK-NOTES: :[[@LINE-1]]:7: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    }
+  }
+
+  ~CheckClass() {
+// CHECK-NOTES: :[[@LINE-1]]:3: warning: function '~CheckClass' has cognitive complexity of 1 (threshold 0) [readability-function-cognitive-complexity]
+    CC100;
+
+    try {
+    } catch (...) {
+// CHECK-NOTES: :[[@LINE-1]]:7: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    }
+  }
+
+  void PublicMemberFunction() {
+// CHECK-NOTES: :[[@LINE-1]]:8: warning: function 'PublicMemberFunction' has cognitive complexity of 1 (threshold 0) [readability-function-cognitive-complexity]
+    CC100;
+
+    try {
+    } catch (...) {
+// CHECK-NOTES: :[[@LINE-1]]:7: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    }
+  }
+
+  void PublicConstMemberFunction() const {
+// CHECK-NOTES: :[[@LINE-1]]:8: warning: function 'PublicConstMemberFunction' has cognitive complexity of 1 (threshold 0) [readability-function-cognitive-complexity]
+    CC100;
+
+    try {
+    } catch (...) {
+// CHECK-NOTES: :[[@LINE-1]]:7: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    }
+  }
+
+  static void PublicStaticMemberFunction() {
+// CHECK-NOTES: :[[@LINE-1]]:15: warning: function 'PublicStaticMemberFunction' has cognitive complexity of 1 (threshold 0) [readability-function-cognitive-complexity]
+    CC100;
+
+    try {
+    } catch (...) {
+// CHECK-NOTES: :[[@LINE-1]]:7: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    }
+  }
+
+  void PublicFunctionDefinition();
+
+protected:
+  CheckClass(bool b) {
+// CHECK-NOTES: :[[@LINE-1]]:3: warning: function 'CheckClass' has cognitive complexity of 1 (threshold 0) [readability-function-cognitive-complexity]
+    CC100;
+
+    try {
+    } catch (...) {
+// CHECK-NOTES: :[[@LINE-1]]:7: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    }
+  }
+
+  void ProtectedMemberFunction() {
+// CHECK-NOTES: :[[@LINE-1]]:8: warning: function 'ProtectedMemberFunction' has cognitive complexity of 1 (threshold 0) [readability-function-cognitive-complexity]
+    CC100;
+
+    try {
+    } catch (...) {
+// CHECK-NOTES: :[[@LINE-1]]:7: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    }
+  }
+
+  void ProtectedConstMemberFunction() const {
+// CHECK-NOTES: :[[@LINE-1]]:8: warning: function 'ProtectedConstMemberFunction' has cognitive complexity of 1 (threshold 0) [readability-function-cognitive-complexity]
+    CC100;
+
+    try {
+    } catch (...) {
+// CHECK-NOTES: :[[@LINE-1]]:7: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    }
+  }
+
+  static void ProtectedStaticMemberFunction() {
+// CHECK-NOTES: :[[@LINE-1]]:15: warning: function 'ProtectedStaticMemberFunction' has cognitive complexity of 1 (threshold 0) [readability-function-cognitive-complexity]
+    CC100;
+
+    try {
+    } catch (...) {
+// CHECK-NOTES: :[[@LINE-1]]:7: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+    }
+  }
+};
+
+void CheckClass::PublicFunctionDefinition() {
+// CHECK-NOTES: :[[@LINE-1]]:18: warning: function 'PublicFunctionDefinition' has cognitive complexity of 1 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  try {
+  } catch (...) {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+  }
+}
+
+#define uglyfunctionmacro(name)                                                \
+  void name() {                                                                \
+    CC100;                                                                     \
+                                                                               \
+    if (true) {                                                                \
+      try {                                                                    \
+      } catch (...) {                                                          \
+      }                                                                        \
+    }                                                                          \
+  }
+
+uglyfunctionmacro(MacroFunction)
+// CHECK-NOTES: :[[@LINE-1]]:19: warning: function 'MacroFunction' has cognitive complexity of 3 (threshold 0) [readability-function-cognitive-complexity]
+// CHECK-NOTES: :[[@LINE-2]]:1: note: +1, including nesting penalty of 0, nesting level increased to 1
+// CHECK-NOTES: :[[@LINE-10]]:5: note: expanded from macro 'uglyfunctionmacro'
+// CHECK-NOTES: :[[@LINE-4]]:1: note: +2, including nesting penalty of 1, nesting level increased to 2
+// CHECK-NOTES: :[[@LINE-10]]:9: note: expanded from macro 'uglyfunctionmacro'
+
+template<typename T>
+void templatedFunction() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'templatedFunction' has cognitive complexity of 1 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  try {
+  } catch (...) {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+  }
+}
+
+template<>
+void templatedFunction<bool>() {
+// CHECK-NOTES: :[[@LINE-1]]:6: warning: function 'templatedFunction<bool>' has cognitive complexity of 1 (threshold 0) [readability-function-cognitive-complexity]
+  CC100;
+
+  try {
+  } catch (...) {
+// CHECK-NOTES: :[[@LINE-1]]:5: note: +1, including nesting penalty of 0, nesting level increased to 1{{$}}
+  }
+}
+
+template void templatedFunction<int>();
+
+void functionThatCallsTemplatedFunctions() {
+  templatedFunction<int>();
+
+  templatedFunction<bool>();
+
+  templatedFunction<char>();
+
+  templatedFunction<void*>();
+}
Index: docs/clang-tidy/checks/readability-function-cognitive-complexity.rst
===================================================================
--- /dev/null
+++ docs/clang-tidy/checks/readability-function-cognitive-complexity.rst
@@ -0,0 +1,124 @@
+.. title:: clang-tidy - readability-function-cognitive-complexity
+
+readability-function-cognitive-complexity
+=========================================
+
+Checks function Cognitive Complexity metric.
+
+The metric is implemented as per `COGNITIVE COMPLEXITY by SonarSource
+<https://www.sonarsource.com/docs/CognitiveComplexity.pdf>`_ specification
+version 1.2 (19 April 2017).
+
+Options
+-------
+
+.. option:: Threshold
+
+   Flag functions with Cognitive Complexity exceeding this number.
+   The default is `25`.
+
+Building blocks
+---------------
+
+There are three basic building blocks of a Cognitive Complexity metric:
+
+Increment
+^^^^^^^^^
+
+The following structures increase the function's Cognitive Complexity metric
+(by 1):
+   * Conditional operators:
+      * `if()`
+      * `else if()`
+      * `else`
+      * `cond ? true : false`
+   * `switch()`
+   * Loops:
+      * `for()
+      * C++11 range-based `for()`
+      * `while()`
+      * `do while()`
+   * `catch () {}`
+   * `goto LABEL`
+   * sequences of binary logical operators:
+      * `boolean1 || boolean2`
+      * `boolean1 && boolean2`
+
+Nesting level
+^^^^^^^^^^^^^
+
+While by itself the nesting level not change the function's Cognitive Complexity
+metric, it is tracked, and is used by the next, third building block.
+The following structures increase the nesting level (by 1):
+   * Conditional operators:
+      * `if()`
+      * `else if()`
+      * `else`
+      * `cond ? true : false`
+   * `switch()`
+   * Loops:
+      * `for()
+      * C++11 range-based `for()`
+      * `while()`
+      * `do while()`
+   * `catch () {}`
+   * Nested functions:
+      * C++11 Lambda
+      * Nested `class`
+      * Nested `struct`
+
+Nesting increment
+^^^^^^^^^^^^^^^^^
+
+And this is where the previous basic building block, `Nesting level`_, matters.
+The following structures increase the function's Cognitive Complexity metric by
+the current `Nesting level`_:
+   * Conditional operators:
+      * `if()`
+      * `cond ? true : false`
+   * `switch()`
+   * Loops:
+      * `for()
+      * C++11 range-based `for()`
+      * `while()`
+      * `do while()`
+   * `catch () {}`
+
+Examples
+--------
+
+The simplest case. This function has Cognitive complexity of 0.
+```
+void function0() {}
+```
+
+Slightly better example. This function has Cognitive complexity of 1.
+```
+int function1(bool var) {
+  if(var) // +1
+    return 42;
+  return 0;
+}
+```
+
+Full example. This function has Cognitive complexity of 3.
+```
+int function3(bool var1, bool var2) {
+  if(var1) { // +1, nesting level +1
+    if(var2)  // +2 (1 + current nesting level of 1)
+      return 42;
+  }
+
+  return 0;
+}
+```
+
+Limitations
+-----------
+
+The metric is implemented with two notable exceptions:
+   * `preprocessor conditionals` (`#ifdef`, `#if`, `#elif`, `#else`, `#endif`)
+     are not accounted for. Could be done.
+   * `each method in a recursion cycle` is not accounted for. It can't be fully
+     implemented, because cross-translational-unit analysis would be needed,
+     which is currently not possible in clang-tidy.
Index: docs/clang-tidy/checks/list.rst
===================================================================
--- docs/clang-tidy/checks/list.rst
+++ docs/clang-tidy/checks/list.rst
@@ -186,6 +186,7 @@
    readability-delete-null-pointer
    readability-deleted-default
    readability-else-after-return
+   readability-function-cognitive-complexity
    readability-function-size
    readability-identifier-naming
    readability-implicit-bool-conversion
Index: docs/ReleaseNotes.rst
===================================================================
--- docs/ReleaseNotes.rst
+++ docs/ReleaseNotes.rst
@@ -137,6 +137,12 @@
   Checks if the required file flag ``IN_CLOEXEC`` is present in the argument of
   ``inotify_init1()``.
 
+- New `readability-function-cognitive-complexity
+  <http://clang.llvm.org/extra/clang-tidy/checks/readability-function-cognitive-complexity.html>`_ check
+
+  For each function, calculates Cognitive Complexity metric, and flags the
+  functions with that metric exceeding the configured limit.
+
 - New `readability-static-accessed-through-instance
   <http://clang.llvm.org/extra/clang-tidy/checks/readability-static-accessed-through-instance.html>`_ check
 
Index: clang-tidy/readability/ReadabilityTidyModule.cpp
===================================================================
--- clang-tidy/readability/ReadabilityTidyModule.cpp
+++ clang-tidy/readability/ReadabilityTidyModule.cpp
@@ -16,6 +16,7 @@
 #include "DeleteNullPointerCheck.h"
 #include "DeletedDefaultCheck.h"
 #include "ElseAfterReturnCheck.h"
+#include "FunctionCognitiveComplexityCheck.h"
 #include "FunctionSizeCheck.h"
 #include "IdentifierNamingCheck.h"
 #include "ImplicitBoolConversionCheck.h"
@@ -55,6 +56,8 @@
         "readability-deleted-default");
     CheckFactories.registerCheck<ElseAfterReturnCheck>(
         "readability-else-after-return");
+    CheckFactories.registerCheck<FunctionCognitiveComplexityCheck>(
+        "readability-function-cognitive-complexity");
     CheckFactories.registerCheck<FunctionSizeCheck>(
         "readability-function-size");
     CheckFactories.registerCheck<IdentifierNamingCheck>(
Index: clang-tidy/readability/FunctionCognitiveComplexityCheck.h
===================================================================
--- /dev/null
+++ clang-tidy/readability/FunctionCognitiveComplexityCheck.h
@@ -0,0 +1,44 @@
+//===--- FunctionCognitiveComplexityCheck.h - clang-tidy---------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_READABILITY_FUNCTION_COGNITIVE_COMPLEXITY_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_READABILITY_FUNCTION_COGNITIVE_COMPLEXITY_H
+
+#include "../ClangTidy.h"
+
+namespace clang {
+namespace tidy {
+namespace readability {
+
+/// Checks function Cognitive Complexity metric.
+///
+/// There is only one configuration option:
+///
+///   * `Threshold` - flag functions with Cognitive Complexity exceeding
+///     this number. The default is `25`.
+///
+/// For the user-facing documentation see:
+/// http://clang.llvm.org/extra/clang-tidy/checks/readability-function-cognitive-complexity.html
+class FunctionCognitiveComplexityCheck : public ClangTidyCheck {
+public:
+  FunctionCognitiveComplexityCheck(StringRef Name, ClangTidyContext *Context);
+
+  void storeOptions(ClangTidyOptions::OptionMap &Opts) override;
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+
+private:
+  const unsigned Threshold;
+};
+
+} // namespace readability
+} // namespace tidy
+} // namespace clang
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_READABILITY_FUNCTION_COGNITIVE_COMPLEXITY_H
Index: clang-tidy/readability/FunctionCognitiveComplexityCheck.cpp
===================================================================
--- /dev/null
+++ clang-tidy/readability/FunctionCognitiveComplexityCheck.cpp
@@ -0,0 +1,629 @@
+//===--- FunctionCognitiveComplexityCheck.cpp - clang-tidy-----------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "FunctionCognitiveComplexityCheck.h"
+#include "clang/AST/RecursiveASTVisitor.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang {
+namespace tidy {
+namespace readability {
+namespace {
+
+struct CognitiveComplexity final {
+  /// Limit of 25 is the "upstream"'s default.
+  static constexpr unsigned DefaultLimit = 25U;
+
+  /// Any increment is based on some combination of reasons.
+  /// For details you can look at the Specification at
+  /// https://www.sonarsource.com/docs/CognitiveComplexity.pdf
+  /// or user-facing docs at
+  /// http://clang.llvm.org/extra/clang-tidy/checks/readability-function-cognitive-complexity.html
+  /// Here are all the possible reasons:
+  enum Criteria : unsigned char {
+    None = 0U,
+
+    /// B1, increases cognitive complexity (by 1)
+    /// What causes it:
+    /// * if, else if, else, ConditionalOperator (not BinaryConditionalOperator)
+    /// * SwitchStmt
+    /// * ForStmt, CXXForRangeStmt
+    /// * WhileStmt, DoStmt
+    /// * CXXCatchStmt
+    /// * GotoStmt (but not BreakStmt, ContinueStmt)
+    /// * sequences of binary logical operators (BinOpLAnd, BinOpLOr)
+    /// * each method in a recursion cycle (not implemented)
+    Increment = 1U << 0,
+
+    /// B2, increases current nesting level (by 1)
+    /// What causes it:
+    /// * if, else if, else, ConditionalOperator (not BinaryConditionalOperator)
+    /// * SwitchStmt
+    /// * ForStmt, CXXForRangeStmt
+    /// * WhileStmt, DoStmt
+    /// * CXXCatchStmt
+    /// * nested CXXConstructor, CXXDestructor, CXXMethod (incl. C++11 Lambda)
+    IncrementNesting = 1U << 1,
+
+    /// B3, increases cognitive complexity by the current nesting level
+    /// Applied before IncrementNesting
+    /// What causes it:
+    /// * IfStmt, ConditionalOperator (not BinaryConditionalOperator)
+    /// * SwitchStmt
+    /// * ForStmt, CXXForRangeStmt
+    /// * WhileStmt, DoStmt
+    /// * CXXCatchStmt
+    PenalizeNesting = 1U << 2,
+
+    All = Increment | PenalizeNesting | IncrementNesting,
+  };
+
+  /// All the possible messages that can be outputed. The choice of the message
+  /// to use is based of the combination of the Criterias
+  static const std::array<const char *const, 4> Msgs;
+
+  /// The helper struct used to record one increment occurence, with all the
+  /// details nessesary
+  struct Detail final {
+    const SourceLocation Loc;     /// What caused the increment?
+    const unsigned short Nesting; /// How deeply nested is Loc located?
+    const Criteria C : 3;         /// The criteria of the increment
+
+    /// Criteria C is a bitfield. Even though Criteria is an unsigned char; and
+    /// only using 3 bits will still result in padding, the fact that it is a
+    /// bitfield is a reminder that it is important to min(sizeof(Detail))
+
+    Detail(SourceLocation SLoc, unsigned short CurrentNesting, Criteria Crit)
+        : Loc(SLoc), Nesting(CurrentNesting), C(Crit) {}
+
+    /// To minimize the sizeof(Detail), we only store the minimal info there.
+    /// This function is used to convert from the stored info into the usable
+    /// information - what message to output, how much of an increment did this
+    /// occurence actually result in
+    std::pair<const char *, unsigned short> process() const {
+      assert(C != Criteria::None && "invalid criteria");
+
+      const char *Msg;          /// The message to output
+      unsigned short Increment; /// How much of an increment?
+
+      if (C == Criteria::All) {
+        Increment = 1 + Nesting;
+        Msg = Msgs[0];
+      } else if (C == (Criteria::Increment | Criteria::IncrementNesting)) {
+        Increment = 1;
+        Msg = Msgs[1];
+      } else if (C == Criteria::Increment) {
+        Increment = 1;
+        Msg = Msgs[2];
+      } else if (C == Criteria::IncrementNesting) {
+        Increment = 0; /// Unused in this message.
+        Msg = Msgs[3];
+      } else
+        llvm_unreachable("should not get to here.");
+
+      return std::make_pair(Msg, Increment);
+    }
+  };
+  // static_assert(sizeof(Detail) <= 8, "it's best to keep the size minimal");
+
+  /// Based on the publicly-avaliable numbers for some big open-source projects
+  /// https://sonarcloud.io/projects?languages=c%2Ccpp&size=5   we can estimate:
+  /// value ~20 would result in no allocs for 98% of functions, ~12 for 96%, ~10
+  /// for 91%, ~8 for 88%, ~6 for 84%, ~4 for 77%, ~2 for 64%, and ~1 for 37%
+  SmallVector<Detail, DefaultLimit> Details; // 25 elements is 200 bytes.
+  /// Yes, 25 is a magic number. This is the seemingly-sane default for the
+  /// upper limit for function cognitive complexity. Thus it would make sense
+  /// to avoid allocations for any function that does not violate the limit.
+
+  /// The grand total Cognitive Complexity of the function
+  unsigned Total = 0;
+
+  /// The function used to store new increment, calculate the total complexity
+  void account(SourceLocation Loc, unsigned short Nesting, Criteria C);
+};
+
+/// Refer to the enum Criteria for detailed explaination.
+const std::array<const char *const, 4> CognitiveComplexity::Msgs = {{
+    /// B1 + B2 + B3
+    "+%0, including nesting penalty of %1, nesting level increased to %2",
+
+    /// B1 + B2
+    "+%0, nesting level increased to %2",
+
+    /// B1
+    "+%0",
+
+    /// B2
+    "nesting level increased to %2",
+}};
+
+/// Criteria is a bitset, thus a few helpers are needed
+CognitiveComplexity::Criteria operator|(CognitiveComplexity::Criteria LHS,
+                                        CognitiveComplexity::Criteria RHS) {
+  return static_cast<CognitiveComplexity::Criteria>(
+      static_cast<std::underlying_type<CognitiveComplexity::Criteria>::type>(
+          LHS) |
+      static_cast<std::underlying_type<CognitiveComplexity::Criteria>::type>(
+          RHS));
+}
+CognitiveComplexity::Criteria operator&(CognitiveComplexity::Criteria LHS,
+                                        CognitiveComplexity::Criteria RHS) {
+  return static_cast<CognitiveComplexity::Criteria>(
+      static_cast<std::underlying_type<CognitiveComplexity::Criteria>::type>(
+          LHS) &
+      static_cast<std::underlying_type<CognitiveComplexity::Criteria>::type>(
+          RHS));
+}
+CognitiveComplexity::Criteria &operator|=(CognitiveComplexity::Criteria &LHS,
+                                          CognitiveComplexity::Criteria RHS) {
+  LHS = operator|(LHS, RHS);
+  return LHS;
+}
+CognitiveComplexity::Criteria &operator&=(CognitiveComplexity::Criteria &LHS,
+                                          CognitiveComplexity::Criteria RHS) {
+  LHS = operator&(LHS, RHS);
+  return LHS;
+}
+
+void CognitiveComplexity::account(SourceLocation Loc, unsigned short Nesting,
+                                  Criteria C) {
+  C &= Criteria::All;
+  assert(C != Criteria::None && "invalid criteria");
+
+  Details.emplace_back(Loc, Nesting, C);
+  const Detail &D(Details.back());
+
+  const std::pair<const char *, unsigned short> Reasoning(D.process());
+  Total += Reasoning.second;
+}
+
+class FunctionASTVisitor final
+    : public RecursiveASTVisitor<FunctionASTVisitor> {
+  using Base = RecursiveASTVisitor<FunctionASTVisitor>;
+
+  /// The current nesting level (increased by Criteria::IncrementNesting)
+  unsigned short CurrentNestingLevel = 0;
+
+  /// Used to efficiently know the last type of binary sequence operator that
+  /// was encountered. It would make sense for the function call to start the
+  /// new sequence, thus it is a stack.
+  std::stack</*********/ Optional<BinaryOperator::Opcode>,
+             SmallVector<Optional<BinaryOperator::Opcode>, 4>>
+      BinaryOperatorsStack;
+
+public:
+  /// Important piece of the information: the Nesting level is increased only
+  /// for the body of whatever statement, not it's init/cond/inc, thus the
+  /// common repetitive code is factored here.
+#define TraverseWithIncreasedNestingLevel(CLASS, Node)                         \
+  ++CurrentNestingLevel;                                                       \
+  ShouldContinue = Base::Traverse##CLASS(Node);                                \
+  --CurrentNestingLevel;
+
+#define IncreasesNestingLevel(CLASS)                                           \
+  bool Traverse##CLASS(CLASS *Node) {                                          \
+    if (!Node)                                                                 \
+      return Base::Traverse##CLASS(Node);                                      \
+                                                                               \
+    bool ShouldContinue;                                                       \
+    TraverseWithIncreasedNestingLevel(CLASS, Node);                            \
+                                                                               \
+    return ShouldContinue;                                                     \
+  }
+
+  /// B2. Nesting level
+  /// The following structures increment the nesting level:
+  IncreasesNestingLevel(DefaultStmt);
+  IncreasesNestingLevel(CXXCatchStmt);
+  /// Other instances are more complex, and need standalone functions
+
+#undef IncreasesNestingLevel
+
+  bool TraverseIfStmt(IfStmt *Node, bool InElseIf = false) {
+    if (!Node)
+      return Base::TraverseIfStmt(Node);
+
+    {
+      CognitiveComplexity::Criteria Reasons =
+          CognitiveComplexity::Criteria::None;
+
+      /// if   increases cognitive complexity
+      Reasons |= CognitiveComplexity::Criteria::Increment;
+      /// if   increases nesting level
+      Reasons |= CognitiveComplexity::Criteria::IncrementNesting;
+
+      /// if   receives a nesting increment commensurate with it's nested depth
+      /// but only if it is not part of  else if
+      if (!InElseIf)
+        Reasons |= CognitiveComplexity::Criteria::PenalizeNesting;
+
+      CC.account(Node->getIfLoc(), CurrentNestingLevel, Reasons);
+    }
+
+    bool ShouldContinue = TraverseStmt(Node->getInit());
+    if (!ShouldContinue)
+      return ShouldContinue;
+
+    ShouldContinue = TraverseStmt(Node->getCond());
+    if (!ShouldContinue)
+      return ShouldContinue;
+
+    /// if   increases nesting level
+    TraverseWithIncreasedNestingLevel(Stmt, Node->getThen());
+
+    if (!ShouldContinue || !Node->getElse())
+      return ShouldContinue;
+
+    /// else  OR  else if   increases cognitive complexity and nesting level
+    /// but  else if  does NOT result in double increase !
+    if (isa<IfStmt>(Node->getElse()))
+      return TraverseIfStmt(dyn_cast<IfStmt>(Node->getElse()), true);
+
+    {
+      CognitiveComplexity::Criteria Reasons =
+          CognitiveComplexity::Criteria::None;
+
+      /// else   increases cognitive complexity
+      Reasons |= CognitiveComplexity::Criteria::Increment;
+      /// else   increases nesting level
+      Reasons |= CognitiveComplexity::Criteria::IncrementNesting;
+      /// else   DOES NOT receive a nesting increment commensurate with it's
+      /// nested depth
+
+      CC.account(Node->getElseLoc(), CurrentNestingLevel, Reasons);
+    }
+
+    TraverseWithIncreasedNestingLevel(Stmt, Node->getElse());
+
+    return ShouldContinue;
+  }
+
+  bool TraverseConditionalOperator(ConditionalOperator *Node) {
+    if (!Node)
+      return Base::TraverseConditionalOperator(Node);
+
+    bool ShouldContinue = TraverseStmt(Node->getCond());
+    if (!ShouldContinue)
+      return ShouldContinue;
+
+    TraverseWithIncreasedNestingLevel(Stmt, Node->getLHS());
+
+    if (!ShouldContinue)
+      return ShouldContinue;
+
+    TraverseWithIncreasedNestingLevel(Stmt, Node->getRHS());
+
+    return ShouldContinue;
+  }
+
+  bool TraverseCaseStmt(CaseStmt *Node) {
+    if (!Node)
+      return Base::TraverseCaseStmt(Node);
+
+    bool ShouldContinue = TraverseStmt(Node->getLHS());
+    if (!ShouldContinue)
+      return ShouldContinue;
+
+    ShouldContinue = TraverseStmt(Node->getRHS());
+    if (!ShouldContinue)
+      return ShouldContinue;
+
+    TraverseWithIncreasedNestingLevel(Stmt, Node->getSubStmt());
+
+    return ShouldContinue;
+  }
+
+  bool TraverseForStmt(ForStmt *Node) {
+    if (!Node)
+      return Base::TraverseForStmt(Node);
+
+    bool ShouldContinue = TraverseStmt(Node->getInit());
+    if (!ShouldContinue)
+      return ShouldContinue;
+
+    ShouldContinue = TraverseStmt(Node->getCond());
+    if (!ShouldContinue)
+      return ShouldContinue;
+
+    ShouldContinue = TraverseStmt(Node->getInc());
+    if (!ShouldContinue)
+      return ShouldContinue;
+
+    TraverseWithIncreasedNestingLevel(Stmt, Node->getBody());
+
+    return ShouldContinue;
+  }
+
+  bool TraverseCXXForRangeStmt(CXXForRangeStmt *Node) {
+    if (!Node)
+      return Base::TraverseCXXForRangeStmt(Node);
+
+    bool ShouldContinue = TraverseDecl(Node->getLoopVariable());
+    if (!ShouldContinue)
+      return ShouldContinue;
+
+    ShouldContinue = TraverseStmt(Node->getRangeInit());
+    if (!ShouldContinue)
+      return ShouldContinue;
+
+    TraverseWithIncreasedNestingLevel(Stmt, Node->getBody());
+
+    return ShouldContinue;
+  }
+
+  bool TraverseWhileStmt(WhileStmt *Node) {
+    if (!Node)
+      return Base::TraverseWhileStmt(Node);
+
+    bool ShouldContinue = TraverseStmt(Node->getCond());
+    if (!ShouldContinue)
+      return ShouldContinue;
+
+    TraverseWithIncreasedNestingLevel(Stmt, Node->getBody());
+
+    return ShouldContinue;
+  }
+
+  bool TraverseDoStmt(DoStmt *Node) {
+    if (!Node)
+      return Base::TraverseDoStmt(Node);
+
+    bool ShouldContinue = TraverseStmt(Node->getCond());
+    if (!ShouldContinue)
+      return ShouldContinue;
+
+    TraverseWithIncreasedNestingLevel(Stmt, Node->getBody());
+
+    return ShouldContinue;
+  }
+
+/// The currently-being-processed stack entry, which is always the top
+#define CurrentBinaryOperator BinaryOperatorsStack.top()
+
+#define TraverseBinOp(CLASS)                                                   \
+  bool TraverseBin##CLASS(BinaryOperator *Op) {                                \
+    if (!Op)                                                                   \
+      return Base::TraverseBin##CLASS(Op);                                     \
+                                                                               \
+    /** Make sure that there is always at least one frame in the stack */      \
+    if (BinaryOperatorsStack.empty())                                          \
+      BinaryOperatorsStack.emplace();                                          \
+                                                                               \
+    /** If this is the first binary operator that we are processing, or the    \
+     ** previous binary operator was different, there is an increment */       \
+    if (!CurrentBinaryOperator || Op->getOpcode() != CurrentBinaryOperator)    \
+      CC.account(Op->getOperatorLoc(), CurrentNestingLevel,                    \
+                 CognitiveComplexity::Criteria::Increment);                    \
+                                                                               \
+    /** We might encounter function call, which starts a new sequence, thus    \
+     ** we need to save the current previous binary operator */                \
+    const Optional<BinaryOperator::Opcode> BinOpCopy(CurrentBinaryOperator);   \
+                                                                               \
+    /** Record the operator that we are currently processing and traverse it   \
+     */                                                                        \
+    CurrentBinaryOperator = Op->getOpcode();                                   \
+    const bool ShouldContinue = Base::TraverseBin##CLASS(Op);                  \
+    /** And restore the previous binary operator, which might be nonexistant   \
+     */                                                                        \
+    CurrentBinaryOperator = BinOpCopy;                                         \
+                                                                               \
+    return ShouldContinue;                                                     \
+  }
+
+  /// In a sequence of binary logical operators, if new operator is different
+  /// from the previous-one, then the cognitive complexity is increased.
+  TraverseBinOp(LAnd);
+  TraverseBinOp(LOr);
+
+#define TraverseCallExpr_(CLASS)                                               \
+  bool Traverse##CLASS(CLASS *Call) {                                          \
+    /** If we are not currently processing any binary operator sequence, then  \
+     ** no special-handling is needed */                                       \
+    if (!Call || BinaryOperatorsStack.empty() || !CurrentBinaryOperator)       \
+      return Base::Traverse##CLASS(Call);                                      \
+                                                                               \
+    /** Else, do add [uninitialized] frame to the stack, and traverse call */  \
+    BinaryOperatorsStack.emplace();                                            \
+    const bool ShouldContinue = Base::Traverse##CLASS(Call);                   \
+    /** And remove the top frame */                                            \
+    BinaryOperatorsStack.pop();                                                \
+                                                                               \
+    return ShouldContinue;                                                     \
+  }
+
+  /// It would make sense for the function call to start the new binary
+  /// operator sequence, thus let's make sure that it creates new stack frame
+  TraverseCallExpr_(CallExpr);
+  TraverseCallExpr_(CXXMemberCallExpr);
+  TraverseCallExpr_(CXXOperatorCallExpr);
+
+  bool TraverseStmt(Stmt *Node) {
+    if (!Node)
+      return Base::TraverseStmt(Node);
+
+    /// Three following switch()'es have huge duplication, but it is better to
+    /// keep them separate, to simplify comparing them with the Specification
+
+    CognitiveComplexity::Criteria Reasons = CognitiveComplexity::Criteria::None;
+    SourceLocation Location = Node->getLocStart();
+
+    /// B1. Increments
+    /// There is an increment for each of the following:
+    switch (Node->getStmtClass()) {
+    /// if, else if, else  are handled in TraverseIfStmt()
+    /// FIXME: each method in a recursion cycle
+    case Stmt::ConditionalOperatorClass:
+    case Stmt::SwitchStmtClass:
+    case Stmt::ForStmtClass:
+    case Stmt::CXXForRangeStmtClass:
+    case Stmt::WhileStmtClass:
+    case Stmt::DoStmtClass:
+    case Stmt::CXXCatchStmtClass:
+    case Stmt::GotoStmtClass:
+      Reasons |= CognitiveComplexity::Criteria::Increment;
+      break;
+    default:
+      /// break LABEL, continue LABEL increase cognitive complexity,
+      /// but they are not supported in C++ or C.
+      /// regular break/continue do not increase cognitive complexity
+      break;
+    }
+
+    /// B2. Nesting level
+    /// The following structures increment the nesting level:
+    switch (Node->getStmtClass()) {
+    /// if, else if, else  are handled in TraverseIfStmt()
+    /// nested methods and such are handled in TraverseDecl
+    case Stmt::ConditionalOperatorClass:
+    case Stmt::SwitchStmtClass:
+    case Stmt::ForStmtClass:
+    case Stmt::CXXForRangeStmtClass:
+    case Stmt::WhileStmtClass:
+    case Stmt::DoStmtClass:
+    case Stmt::CXXCatchStmtClass:
+      Reasons |= CognitiveComplexity::Criteria::IncrementNesting;
+      break;
+    default:
+      break;
+    }
+
+    /// B3. Nesting increments
+    /// The following structures receive a nesting increment
+    /// commensurate with their nested depth inside B2 structures:
+    switch (Node->getStmtClass()) {
+    /// if, else if, else  are handled in TraverseIfStmt()
+    case Stmt::ConditionalOperatorClass:
+    case Stmt::SwitchStmtClass:
+    case Stmt::ForStmtClass:
+    case Stmt::CXXForRangeStmtClass:
+    case Stmt::WhileStmtClass:
+    case Stmt::DoStmtClass:
+    case Stmt::CXXCatchStmtClass:
+      Reasons |= CognitiveComplexity::Criteria::PenalizeNesting;
+      break;
+    default:
+      break;
+    }
+
+    if (Node->getStmtClass() == Stmt::ConditionalOperatorClass) {
+      /// A little beautification.
+      /// For conditional operator "cond ? true : false" point at the "?"
+      /// symbol.
+      ConditionalOperator *COp = dyn_cast<ConditionalOperator>(Node);
+      Location = COp->getQuestionLoc();
+    }
+
+    /// if we have found any reasons, let's account it.
+    if (Reasons & CognitiveComplexity::Criteria::All)
+      CC.account(Location, CurrentNestingLevel, Reasons);
+
+    return Base::TraverseStmt(Node);
+  }
+
+  /// The second non-standard parameter MainAnalyzedFunction is needed to
+  /// differentiate between the cases where TraverseDecl() is the entry point
+  /// from FunctionCognitiveComplexityCheck::check() and the cases where it was
+  /// called from the FunctionASTVisitor itself.
+  /// Explaination: if we get a function definition (e.g. constructor,
+  /// destructor, method), the Cognitive Complexity specification states that
+  /// the Nesting level shall be increased. But if this function is the entry
+  /// point, then the Nesting level should not be increased. Thus that parameter
+  /// is there and is used to fall-through directly to traversing if this is
+  /// the main function that is being analyzed.
+  bool TraverseDecl(Decl *Node, bool MainAnalyzedFunction = false) {
+    if (!Node || MainAnalyzedFunction)
+      return Base::TraverseDecl(Node);
+
+    /// B2. Nesting level
+    /// The following structures increment the nesting level:
+    switch (Node->getKind()) {
+    /// lambda is handled in TraverseLambdaExpr().
+    /// can't have nested functions, so not accounted for
+    case Decl::CXXConstructor:
+    case Decl::CXXDestructor:
+    case Decl::CXXMethod:
+      break;
+    default:
+      return Base::TraverseDecl(Node);
+      break;
+    }
+
+    CC.account(Node->getLocStart(), CurrentNestingLevel,
+               CognitiveComplexity::Criteria::IncrementNesting);
+
+    bool ShouldContinue;
+    TraverseWithIncreasedNestingLevel(Decl, Node);
+
+    return ShouldContinue;
+  }
+
+  bool TraverseLambdaExpr(LambdaExpr *Node) {
+    if (!Node)
+      return Base::TraverseLambdaExpr(Node);
+
+    CC.account(Node->getLocStart(), CurrentNestingLevel,
+               CognitiveComplexity::Criteria::IncrementNesting);
+
+    bool ShouldContinue;
+    TraverseWithIncreasedNestingLevel(LambdaExpr, Node);
+
+    return ShouldContinue;
+  }
+
+  CognitiveComplexity CC;
+};
+} // namespace
+
+FunctionCognitiveComplexityCheck::FunctionCognitiveComplexityCheck(
+    StringRef Name, ClangTidyContext *Context)
+    : ClangTidyCheck(Name, Context),
+      Threshold(Options.get("Threshold", CognitiveComplexity::DefaultLimit)) {}
+
+void FunctionCognitiveComplexityCheck::storeOptions(
+    ClangTidyOptions::OptionMap &Opts) {
+  Options.store(Opts, "Threshold", Threshold);
+}
+
+void FunctionCognitiveComplexityCheck::registerMatchers(MatchFinder *Finder) {
+  Finder->addMatcher(
+      functionDecl(unless(anyOf(isInstantiated(), isImplicit()))).bind("func"),
+      this);
+}
+
+void FunctionCognitiveComplexityCheck::check(
+    const MatchFinder::MatchResult &Result) {
+  const auto *Func = Result.Nodes.getNodeAs<FunctionDecl>("func");
+
+  FunctionASTVisitor Visitor;
+  Visitor.TraverseDecl(const_cast<FunctionDecl *>(Func), true);
+
+  if (Visitor.CC.Total <= Threshold)
+    return;
+
+  diag(Func->getLocation(),
+       "function %0 has cognitive complexity of %1 (threshold %2)")
+      << Func << Visitor.CC.Total << Threshold;
+
+  for (const auto &Detail : Visitor.CC.Details) {
+    const char *IncreaseMessage = nullptr;
+    unsigned short Increase = 0;
+    std::tie(IncreaseMessage, Increase) = Detail.process();
+    assert(IncreaseMessage && "the code should always provide a msg");
+    /// Increase, on the other hand, can be 0, if only nesting is increased.
+
+    diag(Detail.Loc, IncreaseMessage, DiagnosticIDs::Note)
+        << Increase << Detail.Nesting << 1 + Detail.Nesting;
+  }
+}
+
+} // namespace readability
+} // namespace tidy
+} // namespace clang
Index: clang-tidy/readability/CMakeLists.txt
===================================================================
--- clang-tidy/readability/CMakeLists.txt
+++ clang-tidy/readability/CMakeLists.txt
@@ -7,6 +7,7 @@
   DeleteNullPointerCheck.cpp
   DeletedDefaultCheck.cpp
   ElseAfterReturnCheck.cpp
+  FunctionCognitiveComplexityCheck.cpp
   FunctionSizeCheck.cpp
   IdentifierNamingCheck.cpp
   ImplicitBoolConversionCheck.cpp
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