[Bug c++/111602] New: "Error: symbol is already defined" for variable template dependent on default argument lambda used in sfinae and non-sfinae

["ed at catmur dot uk" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111602

            Bug ID: 111602
           Summary: "Error: symbol is already defined" for variable
                    template dependent on default argument lambda used in
                    sfinae and non-sfinae
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ed at catmur dot uk
  Target Milestone: ---

Since gcc 8.1,

template<class> constexpr bool V = true;
struct F { template<class T, bool = V<T>> F(T) {} };
template<class T> struct C { C(F = []{}) {} };
template<class T> decltype(T()) g() { return T(); }
auto f() { struct T {} t; return t; };
int main() { g<C<decltype(f())>>(); }

compiled at -O0 (and -std=c++14 or above) causes assembler error:

/tmp/cccVZwwU.s: Assembler messages:
/tmp/cccVZwwU.s:106: Error: symbol `_Z1VIZN1CIZ1fvE1TEC4E1FEd_UlvE_E' is
already defined

The symbol demangles as 'V<C<f()::T>::C(F)::{default arg#1}::{lambda()#1}>'.