Additional note (November, 2020):

Paper P1787R6, adopted at the November, 2020 meeting, partially addresses this issue.

Default function arguments are instantiated only when needed. Is the same true of default template arguments? For example, is the following well-formed?

  #include <type_traits>

  template<class T>
  struct X {
    template<class U = typename T::type>
    static void foo(int){}
    static void foo(...){}
  };

  int main(){
    X<std::enable_if<false>>::foo(0);
  }

Also, is the effect on lookup the same? E.g.,

  struct S {
    template<typename T = U> void f();
    struct U {};
  };