Fix queue/stack friendship.

queue<T, NonSpaceshipContainer> might be instantiated for a
non-three_way_comparable NonSpaceshipContainer. In that case, it
shouldn't attempt to grant friendship to the spaceship operator,
because that would attempt to form
compare_three_way_result_t<NonSpaceshipContainer>.

As far as I can tell, there's no way in the Core Language for a queue
to grant friendship to only one specialization of its non-member
spaceship operator while respecting the constraint. (See
WG21-N4861 [temp.friend]/9.) Instead, we need to template the
friendship declaration, such that queue grants friendship to all of its
spaceship operators. This is safe, just verbose.

Author: StephanTLavavej

stl/inc/queue

@@ -156,9 +156,11 @@ public:
     friend bool operator>  <>(const queue&, const queue&);
     friend bool operator<= <>(const queue&, const queue&);
     friend bool operator>= <>(const queue&, const queue&);
-    #ifdef __cpp_lib_concepts
-    friend compare_three_way_result_t<_Container> operator<=> <>(const queue&, const queue&);
-    #endif // __cpp_lib_concepts
+#ifdef __cpp_lib_concepts
+    template <class _Ty2, three_way_comparable _Container2>
+    friend compare_three_way_result_t<_Container2> operator<=>(
+        const queue<_Ty2, _Container2>&, const queue<_Ty2, _Container2>&);
+#endif // __cpp_lib_concepts
     // clang-format on
 
 protected:

stl/inc/stack

@@ -146,9 +146,11 @@ public:
     friend bool operator>  <>(const stack&, const stack&);
     friend bool operator<= <>(const stack&, const stack&);
     friend bool operator>= <>(const stack&, const stack&);
-    #ifdef __cpp_lib_concepts
-    friend compare_three_way_result_t<_Container> operator<=> <>(const stack&, const stack&);
-    #endif // __cpp_lib_concepts
+#ifdef __cpp_lib_concepts
+    template <class _Ty2, three_way_comparable _Container2>
+    friend compare_three_way_result_t<_Container2> operator<=>(
+        const stack<_Ty2, _Container2>&, const stack<_Ty2, _Container2>&);
+#endif // __cpp_lib_concepts
     // clang-format on
 
 protected: