[Bug c++/86384] New: Scoped enumeration instantiated even if not required

[zhonghao at pku dot org.cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86384

            Bug ID: 86384
           Summary: Scoped enumeration instantiated even if not required
           Product: gcc
           Version: 8.0.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zhonghao at pku dot org.cn
  Target Milestone: ---

The code is as follow:

template<typename T>
struct A {
 enum class B {
 X = T::value
 };
};

int main() {
 A<int> a;
}


g++ error:

code0.c.cpp:4:13: error: 'value' is not a member of 'int'
  enum class B {
             ^

This code looks well-formed. Only if we look into the enumeration, as
"A<int>::B::X", the definition of the enumeration is required to exist and thus
implicitly instantiated. This is specified at 14.7.1p1 and p2. I tried clang++.
It accepts the code without any error message.