c++: implicit operator== with previous decl [PR94583]

P2085 clarified that a defaulted comparison operator must be the first
declaration of the function.  Rejecting that avoids the ICE trying to
compare the noexcept-specifications.

gcc/cp/ChangeLog
2020-04-24  Jason Merrill  <jason@redhat.com>

	PR c++/94583
	* decl.c (redeclaration_error_message): Reject defaulted comparison
	operator that has been previously declared.

gcc/cp/ChangeLog

+2020-04-24  Jason Merrill  <jason@redhat.com>
+
+	PR c++/94583
+	* decl.c (redeclaration_error_message): Reject defaulted comparison
+	operator that has been previously declared.
+
 2020-04-25  Patrick Palka  <ppalka@redhat.com>
 
 	* parser.c (cp_parser_diagnose_invalid_type_name): Suggest enabling

gcc/cp/decl.c

@@ -2972,6 +2972,14 @@ redeclaration_error_message (tree newdecl, tree olddecl)
 	    }
 	}
 
+      /* [class.compare.default]: A definition of a comparison operator as
+	 defaulted that appears in a class shall be the first declaration of
+	 that function.  */
+      special_function_kind sfk = special_function_p (olddecl);
+      if (sfk == sfk_comparison && DECL_DEFAULTED_FN (newdecl))
+	return G_("comparison operator %q+D defaulted after "
+		  "its first declaration");
+
       check_abi_tag_redeclaration
 	(olddecl, lookup_attribute ("abi_tag", DECL_ATTRIBUTES (olddecl)),
 	 lookup_attribute ("abi_tag", DECL_ATTRIBUTES (newdecl)));

gcc/testsuite/g++.dg/cpp2a/spaceship-synth6.C

+// PR c++/94583
+// { dg-do compile { target c++2a } }
+
+namespace std { struct strong_ordering { }; }
+
+bool operator==(const struct Q&, const struct Q&);
+struct Q {
+  // { dg-error "defaulted after its first declaration" "" { target *-*-* } .+1 }
+  friend std::strong_ordering operator<=>(const Q&, const Q&) = default;
+};
+bool b = Q() == Q();