Out of class definition of constrained member function

Having issues finding a syntax for hoisting constrained member function outside of its class that GCC is happy with. At this point I'm starting to think it's a GCC bug.

struct S {
    template<typename Idx>
    static constexpr bool some_check = true;

    template<typename Idx>
    requires some_check<Idx>
    void foo(Idx);
};

template<typename Idx>
requires S::template some_check<Idx>
void S::foo(Idx idx) {}

Clang accepts it but not GCC:

$ g++ -std=c++20 test.cpp
test.cpp:12:6: error: no declaration matches ‘void S::foo(Idx)’
   12 | void S::foo(Idx idx) {}
      |      ^
test.cpp:7:10: note: candidate is: ‘template<class Idx>  requires  some_check<Idx> void S::foo(Idx)’
    7 |     void foo(Idx);
      |          ^~~
test.cpp:1:8: note: ‘struct S’ defined here
    1 | struct S {
      |        ^

Is this program well-formed?