[Sema] Improve side effect checking for unused-lambda-capture warning

Summary:
Don't warn about unused lambda captures that involve copying a
value of a type that cannot be trivially copied and destroyed.

Fixes PR31977

Reviewers: rsmith, aaron.ballman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D30327

llvm-svn: 296602

1 files changed, 24 insertions, 2 deletions

diff --git a/clang/lib/Sema/SemaLambda.cpp b/clang/lib/Sema/SemaLambda.cpp
index c17f9fbd07e..fc823f4aca3 100644
--- a/clang/lib/Sema/SemaLambda.cpp
+++ b/clang/lib/Sema/SemaLambda.cpp
@@ -1438,13 +1438,35 @@ mapImplicitCaptureStyle(CapturingScopeInfo::ImplicitCaptureStyle ICS) {
   llvm_unreachable("Unknown implicit capture style");
 }
 
-void Sema::DiagnoseUnusedLambdaCapture(const LambdaScopeInfo::Capture &From) {
+bool Sema::CaptureHasSideEffects(const LambdaScopeInfo::Capture &From) {
   if (!From.isVLATypeCapture()) {
     Expr *Init = From.getInitExpr();
     if (Init && Init->HasSideEffects(Context))
-      return;
+      return true;
   }
 
+  if (!From.isCopyCapture())
+    return false;
+
+  const QualType T = From.isThisCapture()
+                         ? getCurrentThisType()->getPointeeType()
+                         : From.getCaptureType();
+
+  if (T.isVolatileQualified())
+    return true;
+
+  const Type *BaseT = T->getBaseElementTypeUnsafe();
+  if (const CXXRecordDecl *RD = BaseT->getAsCXXRecordDecl())
+    return !RD->isCompleteDefinition() || !RD->hasTrivialCopyConstructor() ||
+           !RD->hasTrivialDestructor();
+
+  return false;
+}
+
+void Sema::DiagnoseUnusedLambdaCapture(const LambdaScopeInfo::Capture &From) {
+  if (CaptureHasSideEffects(From))
+    return;
+
   auto diag = Diag(From.getLocation(), diag::warn_unused_lambda_capture);
   if (From.isThisCapture())
     diag << "'this'";