[clang] [clang] Fix a crash from nested ArrayInitLoopExpr (PR #67722)

[Richard Smith via cfe-commits]

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@@ -10950,6 +10950,9 @@ bool ArrayExprEvaluator::VisitCXXParenListOrInitListExpr(
 }
 
 bool ArrayExprEvaluator::VisitArrayInitLoopExpr(const ArrayInitLoopExpr *E) {
+
+  FullExpressionRAII Scope(Info);
+
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zygoloid wrote:

This should be done after we evaluate the common expression, not before, and should be *inside* the `for` loop below so that we destroy temporaries after each iteration of the initialization loop. That is, the individual element initialization should be treated as a full-expression, but the evaluation of the source (typically array) expression should not.

This fixes a conformance issue with default arguments (though it's a little hard to observe given that it's hard to give constant-evaluated default arguments side effects). The relevant standard wording is https://eel.is/c++draft/class.temporary#5; adding a comment saying this is implementing C++ [class.temporary]/5 would be helpful.

IR generation gets this right already; the corresponding implementation there is this `RunCleanupsScope`:https://github.com/llvm/llvm-project/blob/d28a782542aff50abf2b072780748ce57d982705/clang/lib/CodeGen/CGExprAgg.cpp#L1882

https://github.com/llvm/llvm-project/pull/67722