[Bug c++/94894] New: Premature instantiation of conversion function template during overload resolution

["ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94894

            Bug ID: 94894
           Summary: Premature instantiation of conversion function
                    template during overload resolution
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ppalka at gcc dot gnu.org
  Target Milestone: ---

The following testcase was reduced from a build failure with GCC 10 when
compiling LLVM.

When considering the overload candidate 'void f(int&)' for the overloaded call
'f(S{})', we try to build up a conversion sequence from 'S' to 'int&' that goes
through the conversion operator template with T=int.  In doing so we
instantiate the conversion operator template, which triggers an instantiation
of F<int>::k which triggers a hard error.

$ cat testcase.C
template <typename T, bool B = false>
struct F
{
  static_assert(B);
};

struct S
{
  template <typename T, int = F<T>::k> operator T();
};

void f(int&);
void f(S);

void foo()
{
  f(S{});
}

$ g++ testcase.C
testcase.C: In instantiation of ‘struct F<int, false>’:
testcase.C:9:37:   required by substitution of ‘template<class T, int
<anonymous> > S::operator T() [with T = int; int <anonymous> = <missing>]’
testcase.C:18:6:   required from here
testcase.C:4:17: error: static assertion failed
    4 |   static_assert(B);
      |                 ^