Given the following code:

struct A
{
    void f(this A self, int arg);
};

int main() {
  A a;
  a.
}

if you invoke completion after a. and accept the proposal for f, the inserted placeholders include the explicit object parameter:

int main() {
  A a;
  a.f(A self, int arg)
}

This is incorrect: only the int arg placeholder should be inserted.

See also #2284 which may have the same or a related cause.