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

[Brian Bi 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.
+        if (getLangOpts().CPlusPlus26)
+          Diag(DiagLoc, diag::err_ret_local_temp_addr_ref)
+              << Entity.getType()->isReferenceType() << DiagRange;
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t3nsor wrote:

It's not normatively ill-formed because of the difficulty of specifying exactly which constructs of this nature are supposed to be ill-formed. In the pedantic mode or whatever it's called, you'd have to accept it, even though it's bad.

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