c++: Fix CTAD with multiple-arg ctor template [93248].

When cp_unevaluated_operand is set, tsubst_decl thinks that if it sees a
PARM_DECL that isn't already in local_specializations, we're in a decltype
in a trailing return type or some such, and so we only want a substitution
for a single PARM_DECL.  In this case, we want the whole chain, so make sure
cp_unevaluated_operand is cleared.

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

	PR c++/93248
	* pt.c (build_deduction_guide): Clear cp_unevaluated_operand for
	substituting DECL_ARGUMENTS.

gcc/cp/ChangeLog

+2020-03-14  Jason Merrill  <jason@redhat.com>
+
+	PR c++/93248
+	* pt.c (build_deduction_guide): Clear cp_unevaluated_operand for
+	substituting DECL_ARGUMENTS.
+
 2020-03-14  Jakub Jelinek  <jakub@redhat.com>
 
 	* logic.cc (formula::formula): Change "a an" to "an" in a comment.

gcc/cp/pt.c

@@ -28071,10 +28071,13 @@ build_deduction_guide (tree type, tree ctor, tree outer_args, tsubst_flags_t com
 				     complain, ctor);
 	  if (fparms == error_mark_node)
 	    ok = false;
-	  fargs = tsubst (fargs, tsubst_args, complain, ctor);
 	  if (ci)
 	    ci = tsubst_constraint_info (ci, tsubst_args, complain, ctor);
 
+	  /* Parms are to have DECL_CHAIN tsubsted, which would be skipped if
+	     cp_unevaluated_operand.  */
+	  cp_evaluated ev;
+	  fargs = tsubst (fargs, tsubst_args, complain, ctor);
 	  current_template_parms = save_parms;
 	}
 

gcc/testsuite/g++.dg/cpp1z/class-deduction71.C

+// PR c++/93248
+// { dg-do compile { target c++17 } }
+
+template <typename T> struct S
+{ template <typename V> S (T, V, long = 0); };
+using U = decltype(S{0, 4u});