[Bug c++/102412] New: Template argument deduction fails when using concept as defaulted non-type template parameter

[Bug c++/102412] New: Template argument deduction fails when using concept as defaulted non-type template parameter

[joeloser93 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102412

            Bug ID: 102412
           Summary: Template argument deduction fails when using concept
                    as defaulted non-type template parameter
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: joeloser93 at gmail dot com
  Target Milestone: ---

When using a defaulted non-type template parameter whose value is from a
concept, GCC is unable to deduce the template correctly when calling the
function template. Clang accepts this code and it should be valid. 

```
#include <concepts>

template<class T, class U>
concept different_from = not std::is_same_v<T, U>;

template<class T, auto B = different_from<T, int>>
void f() {}

template<class T, auto B = not std::is_same_v<T, int>>
void g() {}

int main() {
    f<double>(); // fails to compile
    g<double>(); // compiles
}

```

See it live at https://godbolt.org/z/n1zrzGPoq

[Bug c++/102412] Template argument deduction fails when using concept as defaulted non-type template parameter

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102412

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ppalka at gcc dot gnu.org
             Status|UNCONFIRMED                 |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot gnu.org
   Target Milestone|---                         |11.3
   Last reconfirmed|                            |2021-09-20
     Ever confirmed|0                           |1

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Confirmed.

[Bug c++/102412] Template argument deduction fails when using concept as defaulted non-type template parameter

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102412

--- Comment #2 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:9329344a6d81a6a5e3bd171167ebc7b158bb44f4

commit r12-3822-g9329344a6d81a6a5e3bd171167ebc7b158bb44f4
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Sep 22 11:16:53 2021 -0400

    c++: concept-ids and value-dependence [PR102412]

    The problem here is that uses_template_parms returns true for all
    concept-ids (even those with non-dependent arguments), so when a concept-id
    is used as a default template argument then during deduction the default
    argument is considered dependent even after substituting into it, which
    leads to deduction failure (from type_unification_real).

    This patch fixes this by implementing the resolution of CWG 2446 which
    says a concept-id is dependent only if its arguments are.

            DR 2446
            PR c++/102412

    gcc/cp/ChangeLog:

            * constexpr.c (cxx_eval_constant_expression)
            <case TEMPLATE_ID_EXPR>: Check value_dependent_expression_p
            instead of processing_template_decl.
            * pt.c (value_dependent_expression_p) <case TEMPLATE_ID_EXPR>:
            Return true only if any_dependent_template_arguments_p.
            (instantiation_dependent_r) <case CALL_EXPR>: Remove this case.
            <case TEMPLATE_ID_EXPR>: Likewise.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/concepts-nondep2.C: New test.
            * g++.dg/cpp2a/concepts-nondep3.C: New test.

[Bug c++/102412] Template argument deduction fails when using concept as defaulted non-type template parameter

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102412

--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Patrick Palka
<ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:5a411459b73b65d8a1235a16d6e2aac4eed17338

commit r11-9082-g5a411459b73b65d8a1235a16d6e2aac4eed17338
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Sep 22 11:16:53 2021 -0400

    c++: concept-ids and value-dependence [PR102412]

    The problem here is that uses_template_parms returns true for all
    concept-ids (even those with non-dependent arguments), so when a concept-id
    is used as a default template argument then during deduction the default
    argument is considered dependent even after substituting into it, which
    leads to deduction failure (from type_unification_real).

    This patch fixes this by implementing the resolution of CWG 2446 which
    says a concept-id is dependent only if its arguments are.

            DR 2446
            PR c++/102412

    gcc/cp/ChangeLog:

            * constexpr.c (cxx_eval_constant_expression)
            <case TEMPLATE_ID_EXPR>: Check value_dependent_expression_p
            instead of processing_template_decl.
            * pt.c (value_dependent_expression_p) <case TEMPLATE_ID_EXPR>:
            Return true only if any_dependent_template_arguments_p.
            (instantiation_dependent_r) <case CALL_EXPR>: Remove this case.
            <case TEMPLATE_ID_EXPR>: Likewise.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp2a/concepts-nondep2.C: New test.
            * g++.dg/cpp2a/concepts-nondep3.C: New test.

    (cherry picked from commit 9329344a6d81a6a5e3bd171167ebc7b158bb44f4)

[Bug c++/102412] Template argument deduction fails when using concept as defaulted non-type template parameter

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102412

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

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

--- Comment #4 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Fixed for GCC 11.3 and 12.