c++ - Calling constexpr function in enable_if_t -

consider following example:

#include <type_traits>  template <typename t, typename enable = void> struct a;  template <typename t> constexpr bool f() { return true; }  template <typename t> struct a<t, std::enable_if_t<f<t>()>> {};  int main() {   a<int> f; } 

it compiles fine clang , gcc msvc gives following error:

13 : <source>(13): error c2079: 'f' uses undefined struct 'a<int,void>' 

is bug in msvc or code , how can fix it?

i'm msvc bug.

some testing reveals msvc refuses match specialized template, providing definition , static_assert in primary template can used verify this.

after digging this popped up, , apparently using types in sfinae constructs works, somehow not functions.

the end result this workaround, annoying bearable

#include <type_traits>  template <typename t, typename = void> struct a;  template <typename t> struct f {     constexpr operator bool() {return true;} };  template <typename t> struct a<t, std::enable_if_t<(f<t>())>> {};  // note braces  int main() {     a<int> a; }