[clang] [Clang] Implement C++26 P2748R5 "Disallow Binding a Returned Glvalue to a Temporary" (PR #89942)

[via cfe-commits]

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@@ -8340,8 +8340,17 @@ void Sema::checkInitializerLifetime(const InitializedEntity &Entity,
             << Entity.getType()->isReferenceType() << CLE->getInitializer() << 2
             << DiagRange;
       } else {
-        Diag(DiagLoc, diag::warn_ret_local_temp_addr_ref)
-         << Entity.getType()->isReferenceType() << DiagRange;
+        // P2748R5: Disallow Binding a Returned Glvalue to a Temporary.
+        // [stmt.return]/p6: In a function whose return type is a reference,
+        // other than an invented function for std::is_convertible ([meta.rel]),
+        // a return statement that binds the returned reference to a temporary
+        // expression ([class.temporary]) is ill-formed.
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yronglin wrote:

Sorry, I misunderstanding before. I have seen [LWG3400](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2748r5.html#ref-LWG3400), and there has a comments `This issue should be resolved by LWG [4028](https://cplusplus.github.io/LWG/lwg-active.html#4028).`, therefor, what I mean here is that should we makes `std::is_nothrow_convertible_v<int, Weird>` evaluated as `true`?

https://github.com/llvm/llvm-project/pull/89942