[PATCH] D153701: [WIP][Clang] Implement P2718R0 "Lifetime extension in range-based for loops"

Tue Jul 11 22:06:36 PDT 2023

hubert.reinterpretcast added inline comments.

================
Comment at: clang/lib/Sema/SemaExprCXX.cpp:8901-8914
+  // [P2718R0] Lifetime extension in range-based for loops.
+  //
+  // 6.7.7 [class.temporary] p5:
+  // There are four contexts in which temporaries are destroyed at a different
+  // point than the end of the full-expression.
+  //
+  // 6.7.7 [class.temporary] p6:
----------------
yronglin wrote:
> rsmith wrote:
> > This isn't the right way to model the behavior here -- the presence or absence of an `ExprWithCleanups` is just a convenience to tell consumers of the AST whether they should expect to see cleanups later or not, and doesn't carry an implication of affecting the actual temporary lifetimes and storage durations.
> > 
> > The outcome that we should be aiming to reach is that all `MaterializeTemporaryExpr`s created as part of processing the for-range-initializer are marked as being lifetime-extended by the for-range variable. Probably the simplest way to handle that would be to track the current enclosing for-range-initializer variable in the `ExpressionEvaluationContextRecord`, and whenever a `MaterializeTemporaryExpr` is created, if there is a current enclosing for-range-initializer, mark that `MaterializeTemporaryExpr` as being lifetime-extended by it.
> Awesome! Thanks a lot for your advice, this is very helpful! I want to take a longer look at it.
As mentioned in D139586, `CXXDefaultArgExpr`s may need additional handling. Similarly for `CXXDefaultInitExpr`s.

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  https://reviews.llvm.org/D153701/new/

https://reviews.llvm.org/D153701