[clang] Reapply "[Clang][CWG1815] Support lifetime extension of temporary created by aggregate initialization using a default member initializer" (PR #97308)

[Shafik Yaghmour via cfe-commits]

================
@@ -5578,19 +5579,35 @@ ExprResult Sema::BuildCXXDefaultInitExpr(SourceLocation 
Loc, FieldDecl *Field) {
   ImmediateCallVisitor V(getASTContext());
   if (!NestedDefaultChecking)
     V.TraverseDecl(Field);
-  if (V.HasImmediateCalls) {
+
+  // CWG1815
+  // Support lifetime extension of temporary created by aggregate
+  // initialization using a default member initializer. We should always 
rebuild
+  // the initializer in a lifetime extension context (if the initializer
+  // expression is an ExprWithCleanups). Then make sure the normal lifetime
+  // extension code recurses into the default initializer and does lifetime
+  // extension when warranted.
+  bool ContainsAnyTemporaries =
+      isa_and_present<ExprWithCleanups>(Field->getInClassInitializer());
+  if (Field->getInClassInitializer() &&
+      !Field->getInClassInitializer()->containsErrors() &&
+      (V.HasImmediateCalls ||
+       (InLifetimeExtendingContext && ContainsAnyTemporaries))) {
     ExprEvalContexts.back().DelayedDefaultInitializationContext = {Loc, Field,
                                                                    CurContext};
     ExprEvalContexts.back().IsCurrentlyCheckingDefaultArgumentOrInitializer =
         NestedDefaultChecking;
-
+    // Pass down lifetime extending flag, and collect temporaries in
+    // CreateMaterializeTemporaryExpr when we rewrite the call argument.
+    keepInLifetimeExtendingContext();
     EnsureImmediateInvocationInDefaultArgs Immediate(*this);
     ExprResult Res;
+
     runWithSufficientStackSpace(Loc, [&] {
       Res = Immediate.TransformInitializer(Field->getInClassInitializer(),
                                            /*CXXDirectInit=*/false);
     });
-    if (!Res.isInvalid())
+    if (Res.isUsable())
----------------
shafik wrote:

Can you explain what cases this makes a difference in?

https://github.com/llvm/llvm-project/pull/97308
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