[clang] [Clang][CWG1815] Support lifetime extension of temporary created by aggregate initialization using a default member initializer (PR #87933)

[via cfe-commits]

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@@ -711,6 +711,26 @@ void InitListChecker::FillInEmptyInitForField(unsigned Init, FieldDecl *Field,
       if (VerifyOnly)
         return;
 
+      // Enter a lifetime extension context, then we can support lifetime
+      // extension of temporary created by aggregate initialization using a
+      // default member initializer (DR1815 https://wg21.link/CWG1815).
+      //
+      // In a lifetime extension context, BuildCXXDefaultInitExpr will clone the
+      // initializer expression on each use that would lifetime extend its
+      // temporaries.
+      EnterExpressionEvaluationContext LifetimeExtensionContext(
+          SemaRef, Sema::ExpressionEvaluationContext::PotentiallyEvaluated,
+          /*LambdaContextDecl=*/nullptr,
+          Sema::ExpressionEvaluationContextRecord::EK_Other, true);
+
+      // Lifetime extension in default-member-init.
+      auto &LastRecord = SemaRef.ExprEvalContexts.back();
+
+      // Just copy previous record, make sure we haven't forget anything.
+      LastRecord =
+          SemaRef.ExprEvalContexts[SemaRef.ExprEvalContexts.size() - 2];
+      LastRecord.InLifetimeExtendingContext = true;
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yronglin wrote:

Hmm, thanks point it out, somehow I misunderstand here. Can I extend `InLifetimeExtendingContext` from `bool` to an `enum{NoExtend, ExtendTopLevel, ExtendAll}` to guide lifetime extension behavior? I think it can reduce unnecessery rebuild action.

https://github.com/llvm/llvm-project/pull/87933