[Bug c++/112908] __reference_{converts,constructs}_from_temporary checks for move constructor when binding prvalue to reference of the same type

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112908

--- Comment #4 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Marek Polacek <[email protected]>:

https://gcc.gnu.org/g:83a903f339af9a1ecde675261191cc69e7b1b721

commit r17-2665-g83a903f339af9a1ecde675261191cc69e7b1b721
Author: Marek Polacek <[email protected]>
Date:   Tue Jul 21 12:59:40 2026 -0400

    c++: implement LWG 3819, reference_xes_from_temporary [PR112908]

    This is an attempt to implement
<https://cplusplus.github.io/LWG/issue3819>.

    My understanding of this issue is that previously, ref_xes_from_temporary
    was defined by using is_constructible, which is implemented by seeing
    if
      T t(declval<Args>()...);
    is well-formed.  But declval always yields an xvalue, never a prvalue.
    In practice this means that for

        struct U {
          U();
          U(U&&) = delete;
        };

        struct T {
          T(U);
        };

    reference_constructs_from_temporary_v<const T&, U> is false due
    to the deleted move ctor.  But if we have a prvalue, then the
    call to the move ctor should be elided and so it doesn't matter
    that it's deleted.  So the result should be 'true'.

    Our ref_xes_from_temporary already doesn't check is_constructible<T, U>
    as the comment says, but we always use build_trait_object which
    gives us an xvalue.  What we need is to implement [meta.unary.prop]/5.2:
    Otherwise [not a reference or function type], VAL<T> is a prvalue that
    initially has type T.  For this I've added build_prvalue_trait_object.

    The finish_trait_expr change is so that get_target_expr doesn't crash
    on an incomplete type.  This change should be correct since
    https://cplusplus.github.io/LWG/issue2939 didn't adjust
    reference_xes_from_temporary the same way as is_convertible/constructible.

            PR c++/112908

    gcc/cp/ChangeLog:

            * method.cc (build_prvalue_trait_object): New.
            (ref_xes_from_temporary): Use build_prvalue_trait_object.
            Use deferring_access_check_sentinel and cp_unevaluated.  Don't
            call force_rvalue or rvalue.
            * semantics.cc (finish_trait_expr)
            <case CPTK_REF_CONSTRUCTS_FROM_TEMPORARY>: Actually check
            completeness.

    gcc/testsuite/ChangeLog:

            * g++.dg/ext/is_constructible8.C: Move
            __reference_*_from_temporary testing to a new test.
            * g++.dg/ext/reference_xes_from_temporary2.C: New test.
            * g++.dg/ext/reference_xes_from_temporary3.C: New test.

    Reviewed-by: Jason Merrill <[email protected]>