[Bug c++/122132] New: Template parameter default argument of pre-declared nested classes not recognised if implementation has constructor

[sandro.boehler at gmx dot de via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122132

            Bug ID: 122132
           Summary: Template parameter default argument of pre-declared
                    nested classes not recognised if implementation has
                    constructor
           Product: gcc
           Version: 15.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sandro.boehler at gmx dot de
  Target Milestone: ---

Discovered with
gcc.exe (x86_64-posix-seh-rev0, Built by MinGW-Builds project) 13.2.0
According to compiler explorer (godbolt) still present on trunk (at least for
both x86-x64 and ARM7 standard builds), though (clang included for comparison):
https://godbolt.org/z/n94E9nKes

In the following case the default argument for a pre-declared nested template
is not recognised:

struct wrapper
{
    template <typename T = long>
    struct nested;

};

template <typename T>
struct wrapper::nested
{
    nested(int) { }
};

int main()
{
    wrapper::nested n(7);
    return 0;
}


Compiler error output (details omitted):

error: class template argument deduction failed:
   18 |     outer::inner t(7);
error: no matching function for call to 'inner(int)'

The error does NOT occur if the affected template class
* is not a nested class
* does not have a constructor unrelated to the template parameter

template <typename T>
struct wrapper::nested
{
    // nested(int) { } // omitted!
};

template <typename T = long>
struct non_nested;

template <typename T = long>
struct non_nested
{
   non_nested(int);
};

int main()
{
    wrapper::nested n;
    non_nested nn;
    return 0;
}


Repeating the default template argument allows compilation:

struct wrapper
{
    template <typename T = long>
    struct nested;

};

template <typename T = long> // !!!
struct wrapper::nested
{
    nested(int) { }
};


This does not conform to the standard, though, and actually should be rejected
just as in bug 50370 (opened in 2011, resolved in 2022):

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=50370