[PATCH] D139400: [clang] Show error when a local variables is passed as default template parameter

[Jens Massberg via Phabricator via cfe-commits]

massberg updated this revision to Diff 481264.
massberg added a comment.

Use isLocalVarDecl to check if we have a local variable.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D139400/new/

https://reviews.llvm.org/D139400

Files:
  clang/include/clang/Basic/DiagnosticSemaKinds.td
  clang/lib/Sema/SemaTemplate.cpp
  clang/test/SemaTemplate/default-template-arguments.cpp


Index: clang/test/SemaTemplate/default-template-arguments.cpp
===================================================================
--- /dev/null
+++ clang/test/SemaTemplate/default-template-arguments.cpp
@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c++20 -verify %s
+
+namespace GH48755 {
+constexpr auto z = 2;
+
+auto f() {
+  const auto x = 1;
+
+  auto lambda1 = [] <auto y = x> {}; // expected-error {{local variable 'x' 
may not be used as default parameter}}
+  auto lambda2 = [] <auto y = 42> {};
+  auto lambda3 = [] <auto y = z> {};
+  auto lambda4 = [] <auto y> {};
+}
+}
Index: clang/lib/Sema/SemaTemplate.cpp
===================================================================
--- clang/lib/Sema/SemaTemplate.cpp
+++ clang/lib/Sema/SemaTemplate.cpp
@@ -8,6 +8,7 @@
 //  This file implements semantic analysis for C++ templates.
 
//===----------------------------------------------------------------------===//
 
+#include <iostream>
 #include "TreeTransform.h"
 #include "clang/AST/ASTConsumer.h"
 #include "clang/AST/ASTContext.h"
@@ -1587,6 +1588,17 @@
 
   // Check the well-formedness of the default template argument, if provided.
   if (Default) {
+    // Local variables may not be used as default template arguments.
+    if (auto *DRE = dyn_cast<DeclRefExpr>(Default)) {
+      if (VarDecl *VD = dyn_cast<VarDecl>(DRE->getDecl())) {
+        if (VD->isLocalVarDecl()) {
+          Diag(DRE->getLocation(),
+               diag::err_local_variable_as_default_in_template)
+              << VD->getNameAsString();
+        }
+      }
+    }
+
     // Check for unexpanded parameter packs.
     if (DiagnoseUnexpandedParameterPack(Default, UPPC_DefaultArgument))
       return Param;
Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -7900,6 +7900,9 @@
   def warn_cxx17_compat_lambda_def_ctor_assign : Warning<
     "%select{default construction|assignment}0 of lambda is incompatible with "
     "C++ standards before C++20">, InGroup<CXXPre20Compat>, DefaultIgnore;
+
+  def err_local_variable_as_default_in_template : Error<
+  "local variable '%0' may not be used as default parameter">;
 }
 
 def err_return_in_captured_stmt : Error<



Index: clang/test/SemaTemplate/default-template-arguments.cpp
===================================================================
--- /dev/null
+++ clang/test/SemaTemplate/default-template-arguments.cpp
@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c++20 -verify %s
+
+namespace GH48755 {
+constexpr auto z = 2;
+
+auto f() {
+  const auto x = 1;
+
+  auto lambda1 = [] <auto y = x> {}; // expected-error {{local variable 'x' may not be used as default parameter}}
+  auto lambda2 = [] <auto y = 42> {};
+  auto lambda3 = [] <auto y = z> {};
+  auto lambda4 = [] <auto y> {};
+}
+}
Index: clang/lib/Sema/SemaTemplate.cpp
===================================================================
--- clang/lib/Sema/SemaTemplate.cpp
+++ clang/lib/Sema/SemaTemplate.cpp
@@ -8,6 +8,7 @@
 //  This file implements semantic analysis for C++ templates.
 //===----------------------------------------------------------------------===//
 
+#include <iostream>
 #include "TreeTransform.h"
 #include "clang/AST/ASTConsumer.h"
 #include "clang/AST/ASTContext.h"
@@ -1587,6 +1588,17 @@
 
   // Check the well-formedness of the default template argument, if provided.
   if (Default) {
+    // Local variables may not be used as default template arguments.
+    if (auto *DRE = dyn_cast<DeclRefExpr>(Default)) {
+      if (VarDecl *VD = dyn_cast<VarDecl>(DRE->getDecl())) {
+        if (VD->isLocalVarDecl()) {
+          Diag(DRE->getLocation(),
+               diag::err_local_variable_as_default_in_template)
+              << VD->getNameAsString();
+        }
+      }
+    }
+
     // Check for unexpanded parameter packs.
     if (DiagnoseUnexpandedParameterPack(Default, UPPC_DefaultArgument))
       return Param;
Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -7900,6 +7900,9 @@
   def warn_cxx17_compat_lambda_def_ctor_assign : Warning<
     "%select{default construction|assignment}0 of lambda is incompatible with "
     "C++ standards before C++20">, InGroup<CXXPre20Compat>, DefaultIgnore;
+
+  def err_local_variable_as_default_in_template : Error<
+  "local variable '%0' may not be used as default parameter">;
 }
 
 def err_return_in_captured_stmt : Error<

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