[Bug c++/115656] New: Templated ctor use rejected in non-deduced context if class template has template template parameter

[s.murthy at outlook dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115656

            Bug ID: 115656
           Summary: Templated ctor use rejected in non-deduced context if
                    class template has template template parameter
           Product: gcc
           Version: 14.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: s.murthy at outlook dot com
  Target Milestone: ---

Using a class constructor in a non-deduced context (when no CTAD is being
attempted, saying that in case my terminology is incorrect) causes an error if
the class template has a template template parameter (lass template A in
repro). Oddly, the error disappears if CTAD is engaged before the erring code.

This is a regression: no issue in GCC 10.5, but issue since GCC 11.1.

Aside, clang accepts this code (from 11.0 to current version)

PS: I searched the bug database and found no report matching this issue, but I
apologize if this issue is known.

Repro below (online https://sigcpp.godbolt.org/z/M1eoE485x):
------------------------------------------------------------

//stub for use in class template A
template<std::unsigned_integral U = unsigned> class dv;

//class template A with template template parameter
template<std::unsigned_integral U = unsigned,
         template <std::unsigned_integral> class V = dv
        >
struct A 
{
    U u_;

    A(U u) : u_(u) {};

    template<std::signed_integral S>
    A(S s) : u_( s < 0 ? -s : s) {}
};


//same as class A but no template template parameter
template<std::unsigned_integral U = unsigned>
struct B 
{
    U u_;

    B(U u) : u_(u) {};

    template<std::signed_integral S>
    B(S s) : u_( s < 0 ? -s : s) {}
};

int main()
{
    //A a1(45ul);
    A a2(45l); //Error CGCC 11.5+: uncomment line above, the error disappears

    B b2(45l); //OK
}