[Bug c++/101288] New: Invalid instantiated redeclaration considered valid by gcc

[eligorkadaf at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101288

            Bug ID: 101288
           Summary: Invalid instantiated redeclaration considered valid by
                    gcc
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eligorkadaf at gmail dot com
  Target Milestone: ---

https://godbolt.org/z/KG5WG43vE

https://eel.is/c++draft/temp.spec#temp.inst-example-4

template<class T, class U>
struct Outer {
  template<class X, class Y> struct Inner;
  template<class Y> struct Inner<T, Y>;         // #1a
  template<class Y> struct Inner<T, Y> { };     // #1b; OK: valid redeclaration
of #1a
  template<class Y> struct Inner<U, Y> { };     // #2
};

Outer<int, int> outer;                          // error at #2

int main() {}