================
@@ -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(
----------------
AaronBallman wrote:

> No, I don't think we should be messing with the 'CheckMemaccessArguments' bit 
> either. I suspect the solution is for `DiagRuntimeBehavior ` to be taught to 
> pay attention to whether the warning it is trying to emit is ignored. WDYT 
> @AaronBallman ?

I think more debugging is needed -- the diagnostic engine is what decides to 
skip rendering an ignored diagnostic: 
https://github.com/llvm/llvm-project/blob/01891953eea17544f6181bb51c973711a382702b/clang/lib/Basic/Diagnostic.cpp#L676
 so why is that logic not working? 

> ... which in a potentially-evaluated context defers it via DiagIfReachable 
> into the function scope's PossiblyUnreachableDiags. Those flush only when 
> ~SynthesizedFunctionScope runs PopFunctionScopeInfo at the end of the 
> synthesized body. The ignore state is read at that flush rather than at the 
> call. S

It makes sense that we process those when popping the function scope, but why 
is the location of the diagnostics incorrect? I can't seem to induce a failure 
through using the pragma directly: https://godbolt.org/z/T31b1T8f1

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