[clang] [Clang] Implement P2718R0 "Lifetime extension in range-based for loops" (PR #76361)

[via cfe-commits]

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@@ -6375,12 +6383,16 @@ ExprResult Sema::BuildCXXDefaultInitExpr(SourceLocation Loc, FieldDecl *Field) {
   ImmediateCallVisitor V(getASTContext());
   if (!NestedDefaultChecking)
     V.TraverseDecl(Field);
-  if (V.HasImmediateCalls) {
+  if (V.HasImmediateCalls || InLifetimeExtendingContext) {
     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();
+    keepInMaterializeTemporaryObjectContext();
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yronglin wrote:

I try to extend lifetime of the `MaterializedTemporaryExpr`s in `CXXDefaultInitExpr`, but looking from the generated IR, `MaterializedTemporaryExpr ::setExtendingDecl` does not take effect. I think some modifications may be needed in CodeGen.

https://github.com/llvm/llvm-project/pull/76361