[Clang][Sema]: Allow copy constructor side effects

Author: vinayakdsci

clang/docs/ReleaseNotes.rst

@@ -173,6 +173,10 @@ Bug Fixes in This Version
   for logical operators in C23.
   Fixes (`#64356 <https://github.com/llvm/llvm-project/issues/64356>`_).
 
+- Clang now doesn't produce false-positive warning `-Wunused-variable`
+  for variables created through copy initialization having side-effects.
+  Fixes (`#79518 <https://github.com/llvm/llvm-project/issues/79518>`_).
+
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

clang/lib/Sema/SemaDecl.cpp

@@ -2044,7 +2044,8 @@ static bool ShouldDiagnoseUnusedDecl(const LangOptions &LangOpts,
           return false;
 
         if (Init) {
-          const auto *Construct = dyn_cast<CXXConstructExpr>(Init);
+          const auto *Construct =
+              dyn_cast<CXXConstructExpr>(Init->IgnoreImpCasts());
           if (Construct && !Construct->isElidable()) {
             const CXXConstructorDecl *CD = Construct->getConstructor();
             if (!CD->isTrivial() && !RD->hasAttr<WarnUnusedAttr>() &&

clang/test/SemaCXX/warn-unused-variables.cpp

@@ -1,5 +1,7 @@
 // RUN: %clang_cc1 -fsyntax-only -Wunused-variable -Wunused-label -Wno-c++1y-extensions -verify %s
 // RUN: %clang_cc1 -fsyntax-only -Wunused-variable -Wunused-label -Wno-c++1y-extensions -verify -std=gnu++11 %s
+// RUN: %clang_cc1 -fsyntax-only -Wunused-variable -Wunused-label -Wno-c++1y-extensions -verify -std=gnu++14 %s
+// RUN: %clang_cc1 -fsyntax-only -Wunused-variable -Wunused-label -Wno-c++1y-extensions -verify -std=gnu++17 %s
 template<typename T> void f() {
   T t;
   t = 17;
@@ -183,7 +185,12 @@ void foo(int size) {
   NonTriviallyDestructible array[2];  // no warning
   NonTriviallyDestructible nestedArray[2][2]; // no warning
 
+  // Copy initialzation gives warning before C++17
+#if __cplusplus <= 201402L
   Foo fooScalar = 1; // expected-warning {{unused variable 'fooScalar'}}
+#else
+  Foo fooScalar = 1; // no warning
+#endif
   Foo fooArray[] = {1,2}; // expected-warning {{unused variable 'fooArray'}}
   Foo fooNested[2][2] = { {1,2}, {3,4} }; // expected-warning {{unused variable 'fooNested'}}
 }
@@ -297,3 +304,29 @@ void RAIIWrapperTest() {
 }
 
 } // namespace gh54489
+
+// Ensure that -Wunused-variable does not emit warning
+// on copy constructors with side effects (C++17 and later)
+#if __cplusplus >= 201703L
+namespace gh79518 {
+
+struct S {
+    S(int);
+};
+
+// With an initializer list
+struct A {
+  int x;
+  A(int x) : x(x) {}
+};
+
+void foo() {
+    S s(0); // no warning
+    S s2 = 0; // no warning
+    S s3{0}; // no warning
+
+    A a = 1; // no warning
+}
+
+} // namespace gh79518
+#endif