c++: missing SFINAE in grok_array_decl [PR111493]

We should guard both the diagnostic and backward compatibilty fallback
code with tf_error, so that in a SFINAE context we don't issue any
diagnostics and correctly treat ill-formed C++23 multidimensional
subscript operator expressions as such.

	PR c++/111493

gcc/cp/ChangeLog:

	* decl2.cc (grok_array_decl): Guard diagnostic and backward
	compatibility fallback code paths with tf_error.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/subscript15.C: New test.

diff --git a/gcc/cp/decl2.cc b/gcc/cp/decl2.cc
index 113b03119ec131f01bc097a078fde035e753c73f..344e19ec98b1f7beccfc78e74c4e7788ef340e3b 100644 (file)
--- a/gcc/cp/decl2.cc
+++ b/gcc/cp/decl2.cc
@@ -459,7 +459,10 @@ grok_array_decl (location_t loc, tree array_expr, tree index_exp,
        {
          expr = build_op_subscript (loc, array_expr, index_exp_list,
                                     &overload, complain & tf_decltype);
-         if (expr == error_mark_node)
+         if (expr == error_mark_node
+             /* Don't do the backward compatibility fallback in a SFINAE
+                context.   */
+             && (complain & tf_error))
            {
              tree idx = build_x_compound_expr_from_vec (*index_exp_list, NULL,
                                                         tf_none);
@@ -510,6 +513,11 @@ grok_array_decl (location_t loc, tree array_expr, tree index_exp,
 
       if (index_exp == NULL_TREE)
        {
+         if (!(complain & tf_error))
+           /* Don't do the backward compatibility fallback in a SFINAE
+              context.  */
+           return error_mark_node;
+
          if ((*index_exp_list)->is_empty ())
            {
              error_at (loc, "built-in subscript operator without expression "
@@ -561,8 +569,9 @@ grok_array_decl (location_t loc, tree array_expr, tree index_exp,
        swapped = true, array_expr = p2, index_exp = i1;
       else
        {
-         error_at (loc, "invalid types %<%T[%T]%> for array subscript",
-                   type, TREE_TYPE (index_exp));
+         if (complain & tf_error)
+           error_at (loc, "invalid types %<%T[%T]%> for array subscript",
+                     type, TREE_TYPE (index_exp));
          return error_mark_node;
        }
 

diff --git a/gcc/testsuite/g++.dg/cpp23/subscript15.C b/gcc/testsuite/g++.dg/cpp23/subscript15.C
new file mode 100644 (file)
index 0000000..fece96b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/subscript15.C
@@ -0,0 +1,25 @@
+// PR c++/111493
+// { dg-do compile { target c++23 } }
+
+template<class T, class... Ts>
+concept CartesianIndexable = requires(T t, Ts... ts) { t[ts...]; };
+
+static_assert(!CartesianIndexable<int>);
+static_assert(!CartesianIndexable<int, int>);
+static_assert(!CartesianIndexable<int, int, int>);
+
+static_assert(!CartesianIndexable<int*>);
+static_assert(CartesianIndexable<int*, int>);
+static_assert(!CartesianIndexable<int*, int, int>);
+static_assert(!CartesianIndexable<int*, int*>);
+
+template<class... Ts>
+struct A {
+  void operator[](Ts...);
+};
+
+static_assert(!CartesianIndexable<A<>, int>);
+static_assert(CartesianIndexable<A<int>, int>);
+static_assert(!CartesianIndexable<A<int>>);
+static_assert(!CartesianIndexable<A<int>, int, int>);
+static_assert(CartesianIndexable<A<int, int>, int, int>);