c++: class nttp ICE

The new testcase from P2308 crashed trying to expand 'this' without an
object to refer to, because we stripped the TARGET_EXPR in
create_template_parm_object.  So let's leave it on for giving an error.

gcc/cp/ChangeLog:

	* pt.cc (create_template_parm_object): Pass TARGET_EXPR to
	cxx_constant_value.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/nontype-class64.C: New test.

gcc/cp/pt.cc

@@ -7308,14 +7308,15 @@ invalid_tparm_referent_p (tree type, tree expr, tsubst_flags_t complain)
 static tree
 create_template_parm_object (tree expr, tsubst_flags_t complain)
 {
+  tree orig = expr;
   if (TREE_CODE (expr) == TARGET_EXPR)
     expr = TARGET_EXPR_INITIAL (expr);
 
   if (!TREE_CONSTANT (expr))
     {
       if ((complain & tf_error)
-	  && require_rvalue_constant_expression (expr))
-	cxx_constant_value (expr);
+	  && require_rvalue_constant_expression (orig))
+	cxx_constant_value (orig);
       return error_mark_node;
     }
   if (invalid_tparm_referent_p (TREE_TYPE (expr), expr, complain))

gcc/testsuite/g++.dg/cpp2a/nontype-class64.C

@@ -0,0 +1,23 @@
+// Testcase from P2308R1
+// { dg-do compile { target c++20 } }
+
+template<auto n> struct B { /* ... */ };
+B<5> b1;                        // OK, template parameter type is int
+B<'a'> b2;                      // OK, template parameter type is char
+B<2.5> b3;                      // OK, template parameter type is double
+B<void(0)> b4;		       // { dg-error "void" }
+
+template<int i> struct C { /* ... */ };
+C<{ 42 }> c1;  // OK
+
+struct J1 {
+  J1 *self=this;
+};
+B<J1{}> j1;  // { dg-error "not a constant expression" }
+
+struct J2 {
+  J2 *self=this;
+  constexpr J2() {}
+  constexpr J2(const J2&) {}
+};
+B<J2{}> j2;  // { dg-error "" }