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

[via cfe-commits]

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@@ -7849,13 +7854,17 @@ class Sema final : public SemaBase {
   /// keepInLifetimeExtendingContext - Pull down InLifetimeExtendingContext
   /// flag from previous context.
   void keepInLifetimeExtendingContext() {
-    if (ExprEvalContexts.size() > 2 &&
-        parentEvaluationContext().InLifetimeExtendingContext) {
-      auto &LastRecord = ExprEvalContexts.back();
-      auto &PrevRecord = parentEvaluationContext();
-      LastRecord.InLifetimeExtendingContext =
-          PrevRecord.InLifetimeExtendingContext;
-    }
+    if (ExprEvalContexts.size() > 2)
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cor3ntin wrote:

I wonder if this `2` is correct. Pre-existing but it should probably be `1` (ie, if there are more than 1 context, there is a parent. Maybe we should just assert >1 actually
(there is always going a context bot the default init + 1 for the translation unit)

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