[clang] [Clang][Sema] Synthesize a memcpy body for defaulted union assignment (PR #206579)

[Adam Smith via cfe-commits]

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@@ -15383,6 +15401,31 @@ static void diagnoseDeprecatedCopyOperation(Sema &S, CXXMethodDecl *CopyOp) {
   }
 }
 
+// A defaulted copy or move assignment operator for a union copies the object
+// representation as if by a memcpy, the same way the defaulted union copy
+// constructor does.  The memberwise loops in DefineImplicitCopyAssignment and
+// DefineImplicitMoveAssignment skip union members, so the whole-object copy is
+// emitted here instead.  Marks AssignOp invalid and returns false on failure.
+static bool buildUnionAssignmentCopy(Sema &S, SourceLocation Loc,
+                                     CXXRecordDecl *ClassDecl,
+                                     std::optional<RefBuilder> &ExplicitObject,
+                                     std::optional<DerefBuilder> &DerefThis,
+                                     const ExprBuilder &From,
+                                     CXXMethodDecl *AssignOp,
+                                     SmallVectorImpl<Stmt *> &Statements) {
+  ExprBuilder &To = ExplicitObject ? static_cast<ExprBuilder &>(*ExplicitObject)
+                                   : static_cast<ExprBuilder &>(*DerefThis);
+  StmtResult Copy = buildMemcpyForAssignmentOp(
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adams381 wrote:

I traced the same path and land on the same `DiagIfReachable` fix.  The `warn_cxxstruct_memaccess` from the synthesized `__builtin_memcpy` runs through `DiagRuntimeBehavior`, gets parked in the function scope's `PossiblyUnreachableDiags`, and is flushed later by `~SynthesizedFunctionScope`.  Because `setIgnoreAllWarnings` is a global flag read at emit time with no source location attached, a narrow save/restore around the call has already been undone by the time the queue flushes, so the warning still comes out.

`CheckMemaccessArguments` also queues the `(void*)` fixit note right after the warning, as its own deferred diagnostic.  In normal immediate emission the engine already drops a note whose parent warning was ignored (Diagnostic.cpp:676).  If the early-return only covers the warning, that note still lands in the queue and can flush on its own with no parent, so I will have the skip drop the trailing note too when all warnings are ignored, matching the immediate path.

With that in place I will remove the `SuppressMemaccessCheck` flag and add the release note.


https://github.com/llvm/llvm-project/pull/206579