[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 +8340,54 @@ 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.
+        //
+        // C++0x [meta.rel]p4:
+        //   Given the following function prototype:
+        //
+        //     template <class T>
+        //       typename add_rvalue_reference<T>::type create();
+        //
+        //   the predicate condition for a template specialization
+        //   is_convertible<From, To> shall be satisfied if and only if
+        //   the return expression in the following code would be
+        //   well-formed, including any implicit conversions to the return
+        //   type of the function:
+        //
+        //     To test() {
+        //       return create<From>();
+        //     }
+        //
+        //   Access checking is performed as if in a context unrelated to To and
+        //   From. Only the validity of the immediate context of the expression
+        //   of the return-statement (including conversions to the return type)
+        //   is considered.
+        //
+        // We skip to check whether we are evaluating one of {__is_convertible,
+        // __is_nothrow_convertible, __is_convertible_to} type traits
+        // expression, because we model the initialization as a
+        // copy-initialization of a temporary of the appropriate type, which for
+        // this expression is identical to the return statement (since NRVO
+        // doesn't apply), and not really build a `return create<From>()` in
+        // type traits expression evaluation. Therefor, P2748R5 has no impact
+        // for evaluation of {__is_convertible, __is_nothrow_convertible,
+        // __is_convertible_to}.
+        //
+        // Clang can correctly handle cases like the following:
+        //
+        // static_assert(__is_convertible(int, const int &));
+        // static_assert(__is_nothrow_convertible(int, const int &));
+        // static_assert(__is_convertible_to(int, const int &));
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cor3ntin wrote:

I don't think this comment is useful in this context at this point in time.
If we ever need to do further surgery here, we can add the comment back

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