[Bug c++/112594] New: Non-type template parameter created with converting constructor sometimes has original type

[Bug c++/112594] New: Non-type template parameter created with converting constructor sometimes has original type

[mital at mitalashok dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112594

            Bug ID: 112594
           Summary: Non-type template parameter created with converting
                    constructor sometimes has original type
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mital at mitalashok dot co.uk
  Target Milestone: ---

The following code complains that `N` (which was declared `n N`) is `3` (of
type `int`) instead of `n(3)`:

<https://godbolt.org/z/rv3777Ycv>

    template<typename T>
    concept C = true;

    struct n {
        constexpr n(int) {}
        static int i;
    };

    template<n N>
    using get_n_i_type = decltype(N.i);

    template<int X>
    int f() {
        using iii = get_n_i_type<X>;
    #if 1  // Change to 0 and this compiles
        static_assert(C<iii>);
    #endif
        return iii{};
    }

    template int f<3>();

Compiler error given:

    <source>: In instantiation of 'int f() [with int X = 3]':
    <source>:21:19:   required from here
       21 | template int f<3>();
          |                   ^
    <source>:10:33: error: request for member 'i' in '3', which is of non-class
type 'int'
       10 | using get_n_i_type = decltype(N.i);
          |                               ~~^

This compiles fine if Clang and MSVC or if you remove the static_assert or
change `template<int X>` to `template<auto X>`.

[Bug c++/112594] Non-type template parameter created with converting constructor sometimes has original type

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112594

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2023-11-20

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

This fails in a similar way so it is not related to i being static here:
```

    template<typename T>
    concept C = true;

    struct n {
        constexpr n(int a) : i(a) {}
        int i;
    };

    template<n N>
    using get_n_i_type = decltype(N.i);

    template<int X>
    int f() {
        using iii = get_n_i_type<X>;
    #if 1  // Change to 0 and this compiles
        static_assert(C<iii>);
    #endif
        return iii{};
    }

    template int f<3>();
```


I will file another related bug seperately which seems like a regression on the
trunk.

[Bug c++/112594] Non-type template parameter created with converting constructor sometimes has original type

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112594

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |rejects-valid

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #1)
> 
> I will file another related bug seperately which seems like a regression on
> the trunk.

Filed PR 112632 for that case, basically using `inline constexpr` rather than
`conccept` used to work in GCC 13 but fails on the trunk ...

[Bug c++/112594] Non-type template parameter created with converting constructor sometimes has original type

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112594

--- Comment #3 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Jason Merrill <jason@gcc.gnu.org>:

https://gcc.gnu.org/g:f1e5bf0d83ee4da81b6317c6d7f1278fe7eaa5a0

commit r14-8291-gf1e5bf0d83ee4da81b6317c6d7f1278fe7eaa5a0
Author: Jason Merrill <jason@redhat.com>
Date:   Wed Jan 17 17:29:33 2024 -0500

    c++: alias template argument conversion [PR112632]

    We've had a problem with lost conversions to template parameter types for a
    while now; looking at this PR, it occurred to me that the problem is really
    with alias (and concept) templates, since we do substitution of dependent
    arguments into them in a way that we don't for other templates.  And fixing
    that specific problem is a lot simpler than adding IMPLICIT_CONV_EXPR
around
    all dependent template arguments the way I gave up on for 111357.

    The other part of the fix was changing tsubst_expr to actually call
    convert_nontype_argument instead of assuming it will eventually happen.

    I waffled about stripping the forced conversion when !force_conv
    vs. skipping them in iterative_hash_template_arg and
    template_args_equal (like we already do for some other conversions) and
    decided to go with the former, but that isn't a strong preference if it
    turns out to be somehow problematic.

            PR c++/112632
            PR c++/112594
            PR c++/111357
            PR c++/104594
            PR c++/67898

    gcc/cp/ChangeLog:

            * cp-tree.h (IMPLICIT_CONV_EXPR_FORCED): New.
            * pt.cc (expand_integer_pack): Remove 111357 workaround.
            (maybe_convert_nontype_argument): Add force parm.
            (convert_template_argument): Handle alias template args
            specially.
            (tsubst_expr): Don't ignore IMPLICIT_CONV_EXPR_NONTYPE_ARG.
            * error.cc (dump_expr) [CASE_CONVERT]: Handle null optype.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/alias-decl-nontype1.C: New test.
            * g++.dg/cpp2a/concepts-narrowing1.C: New test.
            * g++.dg/cpp2a/nontype-class63.C: New test.
            * g++.dg/cpp2a/nontype-class63a.C: New test.

[Bug c++/112594] Non-type template parameter created with converting constructor sometimes has original type

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112594

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED
                 CC|                            |jason at gcc dot gnu.org
   Target Milestone|---                         |14.0

--- Comment #4 from Jason Merrill <jason at gcc dot gnu.org> ---
Fixed for GCC 14.