c++: Fix array of char typedef in template (PR90966).

Since Martin Sebor's patch for PR 71625 to change braced array initializers
to STRING_CST in some cases, we need to be ready for STRING_CST with types
that are changed by tsubst.  fold_convert doesn't know how to deal with
STRING_CST, which is reasonable; we really shouldn't expect it to here.  So
let's handle STRING_CST separately.

	PR c++/90966
	* pt.c (tsubst_copy) [STRING_CST]: Don't use fold_convert.

gcc/cp/ChangeLog

+2020-01-27  Jason Merrill  <jason@redhat.com>
+
+	PR c++/90966
+	* pt.c (tsubst_copy) [STRING_CST]: Don't use fold_convert.
+
 2020-01-27  Iain Sandoe  <iain@sandoe.co.uk>
 
 	PR c++/93443

gcc/cp/pt.c

@@ -16772,7 +16772,6 @@ tsubst_copy (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 
     case INTEGER_CST:
     case REAL_CST:
-    case STRING_CST:
     case COMPLEX_CST:
       {
 	/* Instantiate any typedefs in the type.  */
@@ -16782,6 +16781,18 @@ tsubst_copy (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 	return r;
       }
 
+    case STRING_CST:
+      {
+	tree type = tsubst (TREE_TYPE (t), args, complain, in_decl);
+	r = t;
+	if (type != TREE_TYPE (t))
+	  {
+	    r = copy_node (t);
+	    TREE_TYPE (r) = type;
+	  }
+	return r;
+      }
+
     case PTRMEM_CST:
       /* These can sometimes show up in a partial instantiation, but never
 	 involve template parms.  */

gcc/testsuite/g++.dg/cpp0x/initlist-array10.C

+// PR c++/90966
+// { dg-do compile { target c++11 } }
+
+template<typename I>
+void f()
+{
+  using S = signed char;
+  constexpr const S v[]{0};
+}
+
+int main()
+{
+  f<int>();
+}