[Bug c++/126343] [reflection] type_of not working properly on reflection of info-typed object

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126343

--- Comment #8 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-16 branch has been updated by Jakub Jelinek
<[email protected]>:

https://gcc.gnu.org/g:c68a5d17726fe3b57bcea127d21bf71ff53794a3

commit r16-9472-gc68a5d17726fe3b57bcea127d21bf71ff53794a3
Author: Jakub Jelinek <[email protected]>
Date:   Fri Jul 31 08:37:18 2026 +0200

    c++: Improve diagnostics for nullptr_t/info [PR126343]

    We currently print std::nullptr_t or std::meta::info in diagnostics
    when seeing a NULLPTR_TYPE or META_TYPE, when they aren't type aliases
    (or when they are exactly those type aliases).
    I think that isn't a bad idea, the aliases is what users usually
    use for those.  There are 2 problems with this though.
    We print
    decltype(nullptr)
    and
    decltype(nullptr) const volatile
    exactly the same, both as std::nullptr_t, so the qualifiers are lost.
    And, e.g. in case of a static assertion failure when people want to find
    out why some reflections aren't equal we can print
    note: the comparison reduces to '(^^std::meta::info == ^^std::meta:info)'
    and the user then has no idea what is going on.  Is it because one of
    those is a type alias (which one), or because of cv-qual differences,
    or both?
    The following patch prints the aliases in normal %qT etc. printing,
    if cv qualified prints qualifications after them (so e.g.
    'std::nullptr_t const' or 'std::meta::info volatile').
    And, when printing a reflection expression, it differentiates even
    between the type alias case and non-alias, so for non-aliases prints
    'decltype(nullptr)' or 'decltype(^^int) const volatile' etc.

    2026-07-31  Jakub Jelinek  <[email protected]>

            PR c++/126343
            * error.cc (dump_type) <case NULLPTR_TYPE>: Call
            pp_c_type_qualifier_list.
            (dump_type) <case META_TYPE>: Likewise.
            (dump_expr) <case REFLECT_EXPR>: For REFLECT_EXPR on
            non-typedef META_TYPE or NULLPTR_TYPE print
            decltype(^^int) or decltype(nullptr).

            * g++.dg/reflect/pr126343.C: New test.
            * g++.dg/reflect/diag6.C: Adjust expected diagnostic wording.
            * g++.dg/reflect/init2.C: Likewise.
            * g++.dg/cpp0x/pr124489.C: Likewise.

    Reviewed-by: Jason Merrill <[email protected]>
    (cherry picked from commit 525494c18742ef917ef046584af6dd78f713a449)