[Bug c++/101463] New: Using a constexpr variable template specialization as default argument for non-type template parameter of reference type leads gcc to reject function call

[Bug c++/101463] New: Using a constexpr variable template specialization as default argument for non-type template parameter of reference type leads gcc to reject function call

[michael.kenzel at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101463

            Bug ID: 101463
           Summary: Using a constexpr variable template specialization as
                    default argument for non-type template parameter of
                    reference type leads gcc to reject function call
           Product: gcc
           Version: 11.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: michael.kenzel at gmail dot com
  Target Milestone: ---

The following code will reproduce the issue:

    extern const int a;

    template <typename T>
    constexpr const int& blub = a;

    template <typename T, const int& b = blub<T>>
    void f() {}

    int main()
    {
        f<void>();              // error: no matching function for call to
'f<void>()'
        f<void, blub<void>>();  // ok
    }

Note: gcc will accept the function call when the exact same template argument
is explicitly specified instead.

godbolt link: https://godbolt.org/z/jd3EGWEfo

[Bug c++/101463] [11/12/13/14 Regression]Using a constexpr variable template specialization as default argument for non-type template parameter of reference type leads gcc to reject function call

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101463

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ppalka at gcc dot gnu.org
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
      Known to work|                            |6.2.0
            Summary|Using a constexpr variable  |[11/12/13/14
                   |template specialization as  |Regression]Using a
                   |default argument for        |constexpr variable template
                   |non-type template parameter |specialization as default
                   |of reference type leads gcc |argument for non-type
                   |to reject function call     |template parameter of
                   |                            |reference type leads gcc to
                   |                            |reject function call
   Last reconfirmed|                            |2023-12-17
   Target Milestone|---                         |11.5

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
GCC 6.1/6.2 accepts (with -std=c++17), we started ICEing on this testcase after
r7-5370-gb4ba0852099ad2, and then rejecting it after r8-7014-ge8ef9fdfb9fd4b.

[Bug c++/101463] [11/12/13/14 Regression] Using a constexpr variable template specialization as default argument for non-type template parameter of reference type leads gcc to reject function call

[law at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101463

Jeffrey A. Law <law at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |law at gcc dot gnu.org
           Priority|P3                          |P2