c++: Fix ICE with implicit operator== [PR94462]

duplicate_decls assumed that any TREE_ARTIFICIAL function at namespace scope
was a built-in function, but now in C++20 it's possible to have an
implicitly declared hidden friend operator==.  We just need to move the
assert into the if condition.

gcc/cp/ChangeLog
2020-04-06  Jason Merrill  <jason@redhat.com>

	PR c++/94462
	* decl.c (duplicate_decls): Fix handling of DECL_HIDDEN_FRIEND_P.

gcc/cp/ChangeLog

+2020-04-06  Jason Merrill  <jason@redhat.com>
+
+	PR c++/94462
+	* decl.c (duplicate_decls): Fix handling of DECL_HIDDEN_FRIEND_P.
+
 2020-04-04  Marek Polacek  <polacek@redhat.com>
 	    Jason Merrill  <jason@redhat.com>
 

gcc/cp/decl.c

@@ -1451,9 +1451,10 @@ duplicate_decls (tree newdecl, tree olddecl, bool newdecl_is_friend)
 
   /* Check for redeclaration and other discrepancies.  */
   if (TREE_CODE (olddecl) == FUNCTION_DECL
-      && DECL_ARTIFICIAL (olddecl))
+      && DECL_ARTIFICIAL (olddecl)
+      /* A C++20 implicit friend operator== uses the normal path (94462).  */
+      && !DECL_HIDDEN_FRIEND_P (olddecl))
     {
-      gcc_assert (!DECL_HIDDEN_FRIEND_P (olddecl));
       if (TREE_CODE (newdecl) != FUNCTION_DECL)
 	{
 	  /* Avoid warnings redeclaring built-ins which have not been

gcc/testsuite/g++.dg/cpp2a/spaceship-eq9.C

+// PR c++/94462
+// { dg-do compile { target c++2a } }
+
+namespace std {
+  struct strong_ordering { };
+}
+
+namespace Synth {
+  struct B {
+    friend std::strong_ordering operator<=>(B, B) = default;
+  };
+
+  struct C {
+    friend bool operator==(C, C);
+  };
+}
+