[C++] ADL triggers template type being instantiated.

Reference:

Eric Niebler raised a question:

https://twitter.com/ericniebler/status/1598116211094851584

Answer is elaborated in book "C++ Templates The Complete Guide Second Edition"

13.2.1 Argument-Dependent Lookup

14.3.1 Two-Phase Lookup

i.e.

for first fn call is an ADL call, thus (quoted from @Lewis Baker)

"When passing an argument to an ADL call the compiler needs to build a list of associated entities. This includes looking at associated entities of all template args, which includes inspecting the base classes of those template args, which requires instantiating those types."

 

template <bool B>
struct S {
  static_assert(B);
};

template <class>
struct T {};

template <class U>
void fn(T<U>&&) {}

int main() {
  fn(T<S<false>>{});   // static assert ERROR, why?
  ::fn(T<S<false>>{}); // OK
}