[Bug c++/112632] New: [14 Regression] Non-type template parameter created with converting constructor sometimes has original type

[Bug c++/112632] New: [14 Regression] 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=112632

            Bug ID: 112632
           Summary: [14 Regression]  Non-type template parameter created
                    with converting constructor sometimes has original
                    type
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: needs-bisection, rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

Take:
```
    template<typename T>
    inline constexpr bool 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>();
```

This is rejected on the trunk with:
```
<source>: In instantiation of 'int f() [with int X = 3]':
<source>:22:23:   required from here
   22 |     template int f<3>();
      |                       ^
<source>:11:37: error: request for member 'i' in '3', which is of non-class
type 'int'
   11 |     using get_n_i_type = decltype(N.i);
      |                                   ~~^
```

But was accepted in GCC 13.  Note the difference between this and PR 112594 is
that C here is declared an `inline constexpr` rather than a concept.

[Bug c++/112632] [14 Regression] 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=112632

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |14.0

[Bug c++/112632] [14 Regression] Non-type template parameter created with converting constructor sometimes has original type

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112632

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|needs-bisection             |
                 CC|                            |ppalka at gcc dot gnu.org
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2023-11-20
     Ever confirmed|0                           |1

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Started with r14-2170-g4cf64d9cc2faf4, although that just makes the more
general problem affect variable templates as well.

[Bug c++/112632] [14 Regression] Non-type template parameter created with converting constructor sometimes has original type

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112632

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P1

[Bug c++/112632] [14 Regression] 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=112632

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |jason at gcc dot gnu.org
                 CC|                            |jason at gcc dot gnu.org
             Status|NEW                         |ASSIGNED

[Bug c++/112632] [14 Regression] 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=112632

--- Comment #2 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++/112632] [14 Regression] 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=112632

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED

--- Comment #3 from Jason Merrill <jason at gcc dot gnu.org> ---
Fixed for GCC 14.