c++: defaulted friend op== [PR106361]

Now non-member functions can be defaulted, so this assert is wrong.
move_signature_fn_p already checks for ctor or op=.

	PR c++/106361

gcc/cp/ChangeLog:

	* decl.cc (move_fn_p): Remove assert.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/spaceship-eq14.C: New test.

gcc/cp/decl.cc

@@ -15022,8 +15022,6 @@ copy_fn_p (const_tree d)
 bool
 move_fn_p (const_tree d)
 {
-  gcc_assert (DECL_FUNCTION_MEMBER_P (d));
-
   if (cxx_dialect == cxx98)
     /* There are no move constructors if we are in C++98 mode.  */
     return false;

gcc/testsuite/g++.dg/cpp2a/spaceship-eq14.C

@@ -0,0 +1,17 @@
+// PR c++/106361
+// { dg-do compile { target c++20 } }
+
+struct foo {
+  int x;
+};
+
+struct bar {
+  foo f;			// { dg-error "operator==" }
+  friend bool operator==(const bar& a, const bar& b);
+};
+
+bool operator==(const bar& a, const bar& b) = default;
+
+int main() {
+  return bar{} == bar{};	// { dg-error "deleted" }
+}