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

[Erich Keane 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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erichkeane wrote:

> We actually do that hack with HTML rewriting; I think we'd want an RAII object to restore the old diagnostics when we're done, though.

Right, my thought was to just disable it for this line.
 
> We only want to suppress warnings though, right? e.g., `DiagnosticsEngine::setIgnoreAllWarnings()` would work as well (can save/restore that instead of installing a whole new consumer).

I'm actually amazed we don't have such a RAII container already!  But I think that is probably more than good here(and probably better than my suggestions).  

BUT Diagnostic.h could very well get a `IgnoreAllWarningDiagRAII` object that would be very useful I think elsewhere too.

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