[gcc r13-7837] c++: missing SFINAE in grok_array_decl [PR111493]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:40d2dec34b58c3c31b1c731049a914204ec252c3

commit r13-7837-g40d2dec34b58c3c31b1c731049a914204ec252c3
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Sep 22 06:27:48 2023 -0400

    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.
    
    (cherry picked from commit 1fea14def849dd38b098b0e2d54e64801f9c1f43)

Diff:
---
 gcc/cp/decl2.cc                          | 15 ++++++++++++---
 gcc/testsuite/g++.dg/cpp23/subscript15.C | 25 +++++++++++++++++++++++++
 2 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/gcc/cp/decl2.cc b/gcc/cp/decl2.cc
index 1dd3c2b13bc..36e1f2ce300 100644
--- 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
index 00000000000..fece96be96b
--- /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>);