[Bug c++/85815] New: incorrect "invalid use of incomplete type" in a lambda on valid code

[v at vsamko dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85815

            Bug ID: 85815
           Summary: incorrect "invalid use of incomplete type" in a lambda
                    on valid code
           Product: gcc
           Version: 7.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: v at vsamko dot com
  Target Milestone: ---

This valid code:

template<class T>
class A {
    static A* INSTANCE;
    void foobar();
    void moo() {}
};

template<class T>
A<T>* A<T>::INSTANCE = nullptr;

template<class T>
void A<T>::foobar() {
    auto x = []() {
        INSTANCE->moo();
    };
}

(compiled with -Wall)

Gives error:
<source>: In lambda function:

<source>:14:17: error: invalid use of incomplete type 'class A<T>' [-Werror]

         INSTANCE->moo();

This compiles fine with Clang and GCC 6, but fails with the error above with
GCC 7 and 8.