[PATCH v2] c++: ICE with auto[] and VLA [PR102414]

[Marek Polacek <polacek@redhat.com>]

On Thu, Jan 27, 2022 at 10:17:00PM -0500, Jason Merrill wrote:
> On 1/27/22 20:02, Marek Polacek wrote:
> > @@ -11159,8 +11159,16 @@ create_array_type_for_decl (tree name, tree type, tree size, location_t loc)
> >     /* Figure out the index type for the array.  */
> >     if (size)
> > -    itype = compute_array_index_type_loc (loc, name, size,
> > -					  tf_warning_or_error);
> > +    {
> > +      itype = compute_array_index_type_loc (loc, name, size,
> > +					    tf_warning_or_error);
> > +      if (type_uses_auto (type)
> > +	  && !TREE_CONSTANT (maybe_constant_value (size)))
> 
> Maybe variably_modified_type_p (itype)?

Ah yes, I missed that v_m_t_p actually looks at TYPE_MAX_VALUE...
Fixed.
 
> > +	{
> > +	  sorry_at (loc, "variable-length array of %<auto%>");
> > +	  return error_mark_node;
> > +	}
> > +    }
> >     return build_cplus_array_type (type, itype);
> >   }
> > diff --git a/gcc/testsuite/g++.dg/cpp23/auto-array3.C b/gcc/testsuite/g++.dg/cpp23/auto-array3.C
> > new file mode 100644
> > index 00000000000..e383a17d0ee
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp23/auto-array3.C
> > @@ -0,0 +1,17 @@
> > +// PR c++/102414
> > +// { dg-do compile { target c++11 } }
> > +// { dg-options "" }
> > +
> > +constexpr int sz () { return 3; }
> > +
> > +void f ()
> > +{
> > +  int a[3];
> > +  auto (*a1)[0/0] = &a; // { dg-message "variable-length array of .auto." }
> > +// { dg-warning "division by zero" "" { target *-*-* } .-1 }
> 
> Let's move the error into the other testcase?

Done.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

-- >8 --
Here we ICE in unify_array_domain when we're trying to deduce the type
of an array, as in

  auto(*p)[i] = (int(*)[i])0;

but unify_array_domain doesn't arbitrarily complex bounds.  Another
test is, e.g.,

  auto (*b)[0/0] = &a;

where the type of the array is

  <<< Unknown tree: template_type_parm >>>[0:(sizetype) ((ssizetype) (0 / 0) - 1)]

It seems to me that we need not handle these.

	PR c++/102414
	PR c++/101874

gcc/cp/ChangeLog:

	* decl.cc (create_array_type_for_decl): Use template_placeholder_p.
	Sorry on a variable-length array of auto.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/auto-array3.C: New test.
	* g++.dg/cpp23/auto-array4.C: New test.
---
 gcc/cp/decl.cc                           | 14 +++++++++++---
 gcc/testsuite/g++.dg/cpp23/auto-array3.C | 16 ++++++++++++++++
 gcc/testsuite/g++.dg/cpp23/auto-array4.C | 14 ++++++++++++++
 3 files changed, 41 insertions(+), 3 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp23/auto-array3.C
 create mode 100644 gcc/testsuite/g++.dg/cpp23/auto-array4.C

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 26ce9bfefec..09eed9ceba6 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -11099,7 +11099,7 @@ create_array_type_for_decl (tree name, tree type, tree size, location_t loc)
 
   /* [dcl.type.class.deduct] prohibits forming an array of placeholder
      for a deduced class type.  */
-  if (is_auto (type) && CLASS_PLACEHOLDER_TEMPLATE (type))
+  if (template_placeholder_p (type))
     {
       if (name)
 	error_at (loc, "%qD declared as array of template placeholder "
@@ -11169,8 +11169,16 @@ create_array_type_for_decl (tree name, tree type, tree size, location_t loc)
 
   /* Figure out the index type for the array.  */
   if (size)
-    itype = compute_array_index_type_loc (loc, name, size,
-					  tf_warning_or_error);
+    {
+      itype = compute_array_index_type_loc (loc, name, size,
+					    tf_warning_or_error);
+      if (type_uses_auto (type)
+	  && variably_modified_type_p (itype, /*fn=*/NULL_TREE))
+	{
+	  sorry_at (loc, "variable-length array of %<auto%>");
+	  return error_mark_node;
+	}
+    }
 
   return build_cplus_array_type (type, itype);
 }
diff --git a/gcc/testsuite/g++.dg/cpp23/auto-array3.C b/gcc/testsuite/g++.dg/cpp23/auto-array3.C
new file mode 100644
index 00000000000..fdf25470fc2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/auto-array3.C
@@ -0,0 +1,16 @@
+// PR c++/102414
+// PR c++/101874
+// { dg-do compile { target c++11 } }
+// { dg-options "" }
+
+constexpr int sz () { return 3; }
+
+void f ()
+{
+  int a[3];
+  const int N = 3;
+  auto (*a2)[N] = &a;
+  constexpr int M = 3;
+  auto (*a3)[M] = &a;
+  auto (*a4)[sz()] = &a;
+}
diff --git a/gcc/testsuite/g++.dg/cpp23/auto-array4.C b/gcc/testsuite/g++.dg/cpp23/auto-array4.C
new file mode 100644
index 00000000000..4385c4a6bf9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/auto-array4.C
@@ -0,0 +1,14 @@
+// PR c++/102414
+// PR c++/101874
+// { dg-do compile { target c++11 } }
+// { dg-options "" }
+
+void
+f (int i)
+{
+  auto x[i] = { 0 }; // { dg-message "variable-length array of .auto." }
+  auto(*p)[i] = (int(*)[i])0; // { dg-message "variable-length array of .auto." }
+  int a[3];
+  auto (*a1)[0/0] = &a; // { dg-message "variable-length array of .auto." }
+// { dg-warning "division by zero" "" { target *-*-* } .-1 }
+}

base-commit: 3a5fdf986dc6ebb6e244087b462132590ad0a184
-- 
2.34.1