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

[Vinayak Dev via cfe-commits]

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

>From d4df9a7bef0f993d0c092af5f5af61202dc3effe Mon Sep 17 00:00:00 2001
From: Vinayak Dev <vinayakdev.sci at gmail.com>
Date: Thu, 8 Feb 2024 17:29:44 +0530
Subject: [PATCH] [Clang][Sema]: Allow copy constructor side effects

---
 clang/docs/ReleaseNotes.rst                  |  4 +++
 clang/lib/Sema/SemaDecl.cpp                  |  3 +-
 clang/test/SemaCXX/warn-unused-variables.cpp | 33 ++++++++++++++++++--
 3 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index f44fef28b9f17f..2aff72733c8b77 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -218,6 +218,10 @@ Bug Fixes in This Version
   for logical operators in C23.
   Fixes (`#64356 <https://github.com/llvm/llvm-project/issues/64356>`_).
 
+- Clang no longer produces a false-positive `-Wunused-variable` warning
+  for variables created through copy initialization having side-effects in C++17 and later.
+  Fixes (`#79518 <https://github.com/llvm/llvm-project/issues/79518>`_).
+
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 9fdd8eb236d1ee..6289cf75e17413 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/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>() &&
diff --git a/clang/test/SemaCXX/warn-unused-variables.cpp b/clang/test/SemaCXX/warn-unused-variables.cpp
index b649c7d8089355..29e8d08d37d8c6 100644
--- a/clang/test/SemaCXX/warn-unused-variables.cpp
+++ b/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=expected,cxx98-14 -std=gnu++11 %s
+// RUN: %clang_cc1 -fsyntax-only -Wunused-variable -Wunused-label -Wno-c++1y-extensions -verify=expected,cxx98-14 -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 +185,8 @@ void foo(int size) {
   NonTriviallyDestructible array[2];  // no warning
   NonTriviallyDestructible nestedArray[2][2]; // no warning
 
-  Foo fooScalar = 1; // expected-warning {{unused variable 'fooScalar'}}
+  // Copy initialzation gives warning before C++17
+  Foo fooScalar = 1; // cxx98-14-warning {{unused variable 'fooScalar'}}
   Foo fooArray[] = {1,2}; // expected-warning {{unused variable 'fooArray'}}
   Foo fooNested[2][2] = { {1,2}, {3,4} }; // expected-warning {{unused variable 'fooNested'}}
 }
@@ -297,3 +300,29 @@ 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