[clang] [Clang][Sema]: Allow copy constructor side effects (PR #81127)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Vinayak Dev (vinayakdsci)

<details>
<summary>Changes</summary>

Fixes #<!-- -->79518

Copy constructors can have initialization with side effects, and thus clang should not emit a warning when -Wunused-variable is used in this context. Currently however, a warning is emitted.

Now, compilation happens without warnings. 

---
Full diff: https://github.com/llvm/llvm-project/pull/81127.diff


2 Files Affected:

- (modified) clang/lib/Sema/SemaDecl.cpp (+1-1) 
- (modified) clang/test/SemaCXX/warn-unused-variables.cpp (+26-2) 


``````````diff
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 18a5d93ab8e8c..7f78aef33f1e3 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -2044,7 +2044,7 @@ 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>() &&
diff --git a/clang/test/SemaCXX/warn-unused-variables.cpp b/clang/test/SemaCXX/warn-unused-variables.cpp
index b649c7d808935..92032a9c2b812 100644
--- a/clang/test/SemaCXX/warn-unused-variables.cpp
+++ b/clang/test/SemaCXX/warn-unused-variables.cpp
@@ -1,5 +1,5 @@
 // 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++17 %s
 template<typename T> void f() {
   T t;
   t = 17;
@@ -183,7 +183,7 @@ void foo(int size) {
   NonTriviallyDestructible array[2];  // no warning
   NonTriviallyDestructible nestedArray[2][2]; // no warning
 
-  Foo fooScalar = 1; // expected-warning {{unused variable 'fooScalar'}}
+  Foo fooScalar = 1; // no warning
   Foo fooArray[] = {1,2}; // expected-warning {{unused variable 'fooArray'}}
   Foo fooNested[2][2] = { {1,2}, {3,4} }; // expected-warning {{unused variable 'fooNested'}}
 }
@@ -297,3 +297,27 @@ void RAIIWrapperTest() {
 }
 
 } // namespace gh54489
+
+// Ensure that -Wunused-variable does not emit warning
+// on copy constructors with side effects
+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

``````````

</details>


https://github.com/llvm/llvm-project/pull/81127