[Bug c++/96560] New: Substitution triggers compile-time error when it shouldn't

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96560

            Bug ID: 96560
           Summary: Substitution triggers compile-time error when it
                    shouldn't
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mpolacek at gcc dot gnu.org
  Target Milestone: ---

This triggers the static_assert while it shouldn't, because DR 1227 says "The
substitution proceeds in lexical order and stops when a condition that causes
deduction to fail is encountered."  Compiles with clang++/EDG.

template<bool, typename = void>
struct enable_if { };

template<typename T>
struct enable_if<true, T> {
  using type = T;
};

template<class T>
struct hard_error {
  static_assert(sizeof(T) == 0);
  static inline constexpr bool value = true;
};

template<class T>
struct always_false {
  static inline constexpr bool value = false;
};

template<class T>
int foo (int, typename enable_if<always_false<T>::value, int>::type = 0,
              typename enable_if<hard_error<T>::value, int>::type = 0 )
{
  return 0;
}

template<class T>
char const *foo (long)
{
  return "";
}

int
main ()
{
  char const *sz = foo<int>(0);
}