lebedev.ri created this revision.
lebedev.ri added reviewers: aaron.ballman, rsmith, rtrieu, rjmccall, dblaikie.
I have seen such a problem when reviewing https://reviews.llvm.org/D43341.
https://godbolt.org/g/aJYcaa
#include <utility>
struct S {};
void test(S a) {
std::move(a);
}
Since `std::move()` is not marked with `nodiscard` attribute in the standard
(should it be? how complicated would it be to write such a proposal?),
nothing diagnoses such code. But i really don't see why one would intentionally
write that.
You have either forgot to assign/pass the result, or you wanted to cast to
`void`.
Stage-2 self-hosting is green, no preparatory changes needed!
Repository:
rC Clang
https://reviews.llvm.org/D45163
Files:
docs/ReleaseNotes.rst
include/clang/Basic/DiagnosticSemaKinds.td
lib/AST/Expr.cpp
lib/Sema/SemaStmt.cpp
test/Analysis/MisusedMovedObject.cpp
test/SemaCXX/warn-unused-result-nodiscard.cpp
test/SemaCXX/warn-unused-xvalue.cpp
Index: test/SemaCXX/warn-unused-xvalue.cpp
===================================================================
--- /dev/null
+++ test/SemaCXX/warn-unused-xvalue.cpp
@@ -0,0 +1,97 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+// definitions for std::move
+namespace std {
+inline namespace foo {
+template <class T>
+struct remove_reference { typedef T type; };
+template <class T>
+struct remove_reference<T &> { typedef T type; };
+template <class T>
+struct remove_reference<T &&> { typedef T type; };
+
+template <class T>
+typename remove_reference<T>::type &&move(T &&t);
+} // namespace foo
+} // namespace std
+
+int f0();
+int &&f1();
+
+int global;
+
+void test_negative() {
+ // Ok. Should not warn
+ f0();
+ f1();
+
+ static_cast<int>(f0()); // expected-warning {{expression result unused}}
+ static_cast<int>(f1()); // expected-warning {{expression result unused}}
+
+ static_cast<int>(global); // expected-warning {{expression result unused}}
+
+ (void)std::move(global);
+ (void)noexcept(std::move(global));
+ (void)sizeof(std::move(global));
+}
+
+// Unevaluated contexts should not trigger unused result warnings.
+template <typename T>
+auto foo(T) -> decltype(std::move(global), bool()) { // Should not warn.
+ return true;
+}
+
+void test_simple() {
+ int x = 1;
+
+ static_cast<int &&>(x); // expected-warning {{ignoring return value of cast to rvalue reference of type 'int'}}
+ std::move(x); // expected-warning {{ignoring return value of std::move() function returning rvalue reference of type 'int'}}
+
+ using std::move;
+ move(x); // expected-warning {{ignoring return value of std::move() function returning rvalue reference of type 'int'}}
+}
+
+void test_global() {
+ static_cast<int &&>(global); // expected-warning {{ignoring return value of cast to rvalue reference of type 'int'}}
+ std::move(global); // expected-warning {{ignoring return value of std::move() function returning rvalue reference of type 'int'}}
+
+ using std::move;
+ move(global); // expected-warning {{ignoring return value of std::move() function returning rvalue reference of type 'int'}}
+}
+
+void test_call() {
+ static_cast<int &&>(f0()); // expected-warning {{ignoring return value of cast to rvalue reference of type 'int'}}
+ static_cast<int &&>(f1()); // expected-warning {{ignoring return value of cast to rvalue reference of type 'int'}}
+ std::move(f0()); // expected-warning {{ignoring return value of std::move() function returning rvalue reference of type 'int'}}
+ std::move(f1()); // expected-warning {{ignoring return value of std::move() function returning rvalue reference of type 'int'}}
+
+ using std::move;
+ move(f0()); // expected-warning {{ignoring return value of std::move() function returning rvalue reference of type 'int'}}
+ move(f1()); // expected-warning {{ignoring return value of std::move() function returning rvalue reference of type 'int'}}
+}
+
+void arg(int a) {
+ static_cast<int &&>(a); // expected-warning {{ignoring return value of cast to rvalue reference of type 'int'}}
+
+ std::move(a); // expected-warning {{ignoring return value of std::move() function returning rvalue reference of type 'int'}}
+}
+
+struct A {};
+struct B {
+ A a;
+};
+struct C {
+ C(){};
+ ~C() {}
+};
+void struct_test() {
+ A a;
+ std::move(a); // expected-warning{{ignoring return value of std::move() function returning rvalue reference of type 'A'}}
+
+ B b;
+ std::move(b); // expected-warning{{ignoring return value of std::move() function returning rvalue reference of type 'B'}}
+ std::move(b.a); // expected-warning{{ignoring return value of std::move() function returning rvalue reference of type 'A'}}
+
+ C c;
+ std::move(c); // expected-warning{{ignoring return value of std::move() function returning rvalue reference of type 'C'}}
+}
Index: test/SemaCXX/warn-unused-result-nodiscard.cpp
===================================================================
--- /dev/null
+++ test/SemaCXX/warn-unused-result-nodiscard.cpp
@@ -0,0 +1,44 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++17 %s
+
+// definitions for somthing similar to the std::move.
+namespace notstd {
+inline namespace foo {
+template <class T>
+struct remove_reference { typedef T type; };
+template <class T>
+struct remove_reference<T &> { typedef T type; };
+template <class T>
+struct remove_reference<T &&> { typedef T type; };
+
+// This is just like the std::move(), but marked with warn_unused_result attribute
+template <class T>
+__attribute__((warn_unused_result)) typename remove_reference<T>::type &¬move1(T &&t);
+template <class T>
+[[clang::warn_unused_result]] typename remove_reference<T>::type &¬move2(T &&t);
+template <class T>
+[[nodiscard]] typename remove_reference<T>::type &¬move3(T &&t);
+} // namespace foo
+} // namespace notstd
+
+// definitions for std::move, but actually marked with nodiscard attribute
+namespace std {
+inline namespace foo {
+template <class T>
+struct remove_reference { typedef T type; };
+template <class T>
+struct remove_reference<T &> { typedef T type; };
+template <class T>
+struct remove_reference<T &&> { typedef T type; };
+
+template <class T>
+[[nodiscard]] typename remove_reference<T>::type &&move(T &&t);
+} // namespace foo
+} // namespace std
+
+void arg(int a) {
+ notstd::notmove1(a); // expected-warning {{ignoring return value of function declared with 'warn_unused_result' attribute}}
+ notstd::notmove2(a); // expected-warning {{ignoring return value of function declared with 'warn_unused_result' attribute}}
+ notstd::notmove3(a); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+
+ std::move(a); // expected-warning {{ignoring return value of std::move() function returning rvalue reference of type 'int'}}
+}
Index: test/Analysis/MisusedMovedObject.cpp
===================================================================
--- test/Analysis/MisusedMovedObject.cpp
+++ test/Analysis/MisusedMovedObject.cpp
@@ -185,7 +185,7 @@
{
A a;
if (i == 1) { // expected-note {{Taking false branch}} expected-note {{Taking false branch}}
- std::move(a);
+ std::move(a); // expected-warning {{ignoring return value of std::move() function returning rvalue reference of type 'A'}}
}
if (i == 2) { // expected-note {{Taking false branch}} expected-note {{Taking false branch}}
a = A();
@@ -409,7 +409,7 @@
// Moves of global variables are not reported.
A global_a;
void globalVariablesTest() {
- std::move(global_a);
+ std::move(global_a); // expected-warning {{ignoring return value of std::move() function returning rvalue reference of type 'A'}}
global_a.foo(); // no-warning
}
Index: lib/Sema/SemaStmt.cpp
===================================================================
--- lib/Sema/SemaStmt.cpp
+++ lib/Sema/SemaStmt.cpp
@@ -264,6 +264,10 @@
// is written in a macro body, only warn if it has the warn_unused_result
// attribute.
if (const Decl *FD = CE->getCalleeDecl()) {
+ if (CE->isCallToStdMove()) {
+ Diag(Loc, diag::warn_unused_stdmove) << E->getType().getCanonicalType();
+ return;
+ }
if (const Attr *A = isa<FunctionDecl>(FD)
? cast<FunctionDecl>(FD)->getUnusedResultAttr()
: FD->getAttr<WarnUnusedResultAttr>()) {
@@ -329,6 +333,11 @@
}
}
+ if (E->isXValue() && isa<CastExpr>(E)) {
+ Diag(Loc, diag::warn_unused_xvalue_cast) << E->getType().getCanonicalType();
+ return;
+ }
+
if (E->isGLValue() && E->getType().isVolatileQualified()) {
Diag(Loc, diag::warn_unused_volatile) << R1 << R2;
return;
Index: lib/AST/Expr.cpp
===================================================================
--- lib/AST/Expr.cpp
+++ lib/AST/Expr.cpp
@@ -2196,7 +2196,7 @@
//
// Note: If new cases are added here, DiagnoseUnusedExprResult should be
// updated to match for QoI.
- if (HasWarnUnusedResultAttr ||
+ if (CE->isCallToStdMove() || HasWarnUnusedResultAttr ||
FD->hasAttr<PureAttr>() || FD->hasAttr<ConstAttr>()) {
WarnE = this;
Loc = CE->getCallee()->getLocStart();
Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -7277,6 +7277,12 @@
def warn_unused_call : Warning<
"ignoring return value of function declared with %0 attribute">,
InGroup<UnusedValue>;
+def warn_unused_stdmove : Warning<
+ "ignoring return value of std::move() function returning rvalue reference of type %0">,
+ InGroup<UnusedValue>;
+def warn_unused_xvalue_cast : Warning<
+ "ignoring return value of cast to rvalue reference of type %0">,
+ InGroup<UnusedValue>;
def warn_side_effects_unevaluated_context : Warning<
"expression with side effects has no effect in an unevaluated context">,
InGroup<UnevaluatedExpression>;
Index: docs/ReleaseNotes.rst
===================================================================
--- docs/ReleaseNotes.rst
+++ docs/ReleaseNotes.rst
@@ -59,6 +59,9 @@
``-Wno-c++98-compat-extra-semi``, so if you want that diagnostic, you need
to explicitly re-enable it (e.g. by appending ``-Wextra-semi``).
+- ``-Wunused-value`` diagnostic was taught to detect unused return value of
+ ``std::move()`` function.
+
Non-comprehensive list of changes in this release
-------------------------------------------------
_______________________________________________
cfe-commits mailing list
cfe-commits@lists.llvm.org
http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
