PR c++/86932 - missed SFINAE with empty pack.

The issue here was that when processing the explicit template args in
fn_type_unification we added an empty argument pack for the parameter pack,
so we never tried to do any deduction for it, and therefore never looked at
its type.  We need that empty pack behavior for partial ordering, but we
don't want it here, so let's make it conditional on tf_partial.

	* pt.c (coerce_template_parms): Don't add an empty pack if
	tf_partial.
	(fn_type_unification): Pass tf_partial to coerce_template_parms.

From-SVN: r269965

gcc/cp/ChangeLog

 2019-03-26  Jason Merrill  <jason@redhat.com>
 
+	PR c++/86932 - missed SFINAE with empty pack.
+	* pt.c (coerce_template_parms): Don't add an empty pack if
+	tf_partial.
+	(fn_type_unification): Pass tf_partial to coerce_template_parms.
+
 	PR c++/86429 - constexpr variable in lambda.
 	PR c++/82643
 	PR c++/87327

gcc/cp/pt.c

@@ -8436,6 +8436,7 @@ coerce_template_parms (tree parms,
 	arg = NULL_TREE;
 
       if (template_parameter_pack_p (TREE_VALUE (parm))
+	  && (arg || !(complain & tf_partial))
 	  && !(arg && ARGUMENT_PACK_P (arg)))
         {
 	  /* Some arguments will be placed in the
@@ -20077,7 +20078,7 @@ fn_type_unification (tree fn,
 	 substitution context.  */
       explicit_targs
 	= (coerce_template_parms (tparms, explicit_targs, NULL_TREE,
-				  complain,
+				  complain|tf_partial,
 				  /*require_all_args=*/false,
 				  /*use_default_args=*/false));
       if (explicit_targs == error_mark_node)

gcc/testsuite/g++.dg/cpp0x/sfinae65.C

+// PR c++/86932
+// { dg-do compile { target c++11 } }
+
+template<bool, typename T> struct enable_if { using type = T; };
+template<typename T> struct enable_if<false, T> { };
+
+template<typename> struct is_foo { static constexpr bool value = false; };
+
+// { dg-error "enable_if" "" { target *-*-* } .+1 }
+template<class U, typename enable_if<is_foo<U>::value, int>::type...> void f() {}
+
+int main()
+{
+  f<int>();			// { dg-error "no match" }
+}