c++: Fix ICE-after-error on partial spec [92068]

Here the template arguments for the partial specialization are valid
arguments for the template, but not for a partial specialization, because
'd' can never be deduced to anything other than an empty pack.

gcc/cp/ChangeLog
2020-03-14  Jason Merrill  <jason@redhat.com>

	PR c++/92068
	* pt.c (process_partial_specialization): Error rather than crash on
	extra pack expansion.

gcc/cp/ChangeLog

 2020-03-14  Jason Merrill  <jason@redhat.com>
 
+	PR c++/92068
+	* pt.c (process_partial_specialization): Error rather than crash on
+	extra pack expansion.
+
+2020-03-14  Jason Merrill  <jason@redhat.com>
+
 	PR c++/92909
 	* pt.c (find_parameter_packs_r): [DECL_EXPR]: Walk
 	DECL_ORIGINAL_TYPE of a typedef.

gcc/cp/pt.c

@@ -5038,6 +5038,14 @@ process_partial_specialization (tree decl)
       return decl;
     }
 
+  else if (nargs > DECL_NTPARMS (maintmpl))
+    {
+      error ("too many arguments for partial specialization %qT", type);
+      inform (DECL_SOURCE_LOCATION (maintmpl), "primary template here");
+      /* Avoid crash below.  */
+      return decl;
+    }
+
   /* If we aren't in a dependent class, we can actually try deduction.  */
   else if (tpd.level == 1
 	   /* FIXME we should be able to handle a partial specialization of a
@@ -5064,7 +5072,6 @@ process_partial_specialization (tree decl)
 
      Also, we verify that pack expansions only occur at the
      end of the argument list.  */
-  gcc_assert (nargs == DECL_NTPARMS (maintmpl));
   tpd2.parms = 0;
   for (i = 0; i < nargs; ++i)
     {

gcc/testsuite/g++.dg/cpp0x/variadic178.C

+// PR c++/92068
+// { dg-do compile { target c++11 } }
+
+template <typename, typename> struct a;
+template <typename b, typename c, typename... d>
+struct a<b, c, d...> { };	// { dg-error "arguments" }