c++: new-expr of array of deduced class tmpl [PR101988]

In r12-1933 I attempted to implement DR2397 aka allowing

  int a[3];
  auto (&r)[3] = a;

by removing the type_uses_auto check in create_array_type_for_decl.
That may have gone too far, because it also allows arrays of
CLASS_PLACEHOLDER_TEMPLATE and it looks like [dcl.type.class.deduct]
prohibits that: "...the declared type of the variable shall be cv T,
where T is the placeholder."  However, in /2 it explicitly states that
"A placeholder for a deduced class type can also be used in the
type-specifier-seq in the new-type-id or type-id of a new-expression."

In this PR, it manifested by making us accept invalid

  template<class T> struct A { A(T); };
  auto p = new A[]{1};

[expr.new]/2 says that such a construct is treated as an invented
declaration of the form

  A x[]{1};

but, I think, that ought to be ill-formed as per above.  So this patch
sort of restores the create_array_type_for_decl check.  I should mention
that the difference between [] and [1] is due to cp_parser_new_type_id:

      if (*nelts == NULL_TREE)
        /* Leave [] in the declarator.  */;

and groktypename returning different types based on that.

	PR c++/101988

gcc/cp/ChangeLog:

	* decl.cc (create_array_type_for_decl): Reject forming an array of
	placeholder for a deduced class type.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/class-deduction-new1.C: New test.
	* g++.dg/cpp23/auto-array2.C: New test.

gcc/cp/decl.cc

@@ -11087,6 +11087,18 @@ create_array_type_for_decl (tree name, tree type, tree size, location_t loc)
   if (type == error_mark_node || size == error_mark_node)
     return error_mark_node;
 
+  /* [dcl.type.class.deduct] prohibits forming an array of placeholder
+     for a deduced class type.  */
+  if (is_auto (type) && CLASS_PLACEHOLDER_TEMPLATE (type))
+    {
+      if (name)
+	error_at (loc, "%qD declared as array of template placeholder "
+		  "type %qT", name, type);
+      else
+	error ("creating array of template placeholder type %qT", type);
+      return error_mark_node;
+    }
+
   /* If there are some types which cannot be array elements,
      issue an error-message and return.  */
   switch (TREE_CODE (type))

gcc/testsuite/g++.dg/cpp1z/class-deduction-new1.C

@@ -0,0 +1,16 @@
+// PR c++/101988
+// { dg-do compile { target c++17 } }
+
+template<typename T>
+struct A {
+  A(T);
+  A();
+};
+auto p1 = new A[]{1}; // { dg-error "creating array of template placeholder type" }
+auto p2 = new A[1]{1}; // { dg-error "invalid use of placeholder" }
+auto p3 = new A<int>[]{1};
+auto p4 = new A<int>[1]{1};
+auto p5 = new A[]{1, 2}; // { dg-error "creating array of template placeholder type" }
+auto p6 = new A<int>[]{1, 2};
+auto p7 = new A<int>[]{A(1), A(1)};
+auto p8 = new A<int>[2]{A(1), A(1)};

gcc/testsuite/g++.dg/cpp23/auto-array2.C

@@ -0,0 +1,11 @@
+// PR c++/101988
+// { dg-do compile { target c++17 } }
+
+template<class T> struct A { A(); };
+A<int> a[3];
+auto (*p)[3] = &a;
+A<int> (*p2)[3] = &a;
+A (*p3)[3] = &a; // { dg-error "template placeholder type" }
+auto (&r)[3] = a;
+A<int> (&r2)[3] = a;
+A (&r3)[3] = a; // { dg-error "template placeholder type" }