Re: [C++ PATCH] Fix constexpr handling of arrays with unknown bound (PR c++/83993)

[Jason Merrill <jason@redhat.com>]

OK.

On Wed, Jan 31, 2018 at 12:45 PM, Jakub Jelinek <jakub@redhat.com> wrote:
> On Wed, Jan 31, 2018 at 11:02:36AM -0500, Jason Merrill wrote:
>> > In this case I was just extending existing warning and wanted
>> > consistency with that.  Both can be changed in a GCC9 follow-up,
>> > or if Jason/Nathan want it now, even for GCC8, sure.
>
> So, do you want %qE as in the patch or %E?  Note, sidx might
> actually not be a INTEGER_CST (as we only verify_constant it which
> allows all kinds of expressions), though usually it will be.
> E.g. the
>       if (!tree_fits_shwi_p (index)
>           || (i = tree_to_shwi (index)) < 0)
>         {
>           diag_array_subscript (ctx, ary, index);
> case will call it if index is some reduced_constant_expression_p
> which is not INTEGER_CST, initializer_constant_valid_p* has lots
> of cases, sure not all apply to integral types.
>
>> > Well, even zero subscript is wrong if lval is false, i.e. a[0] + 1
>> > rather than e.g. &a[0].  So we'd need to have another wordings
>> > for the case when the index is zero.
>>
>> But if lval is false we want the value of the array, and if we know
>> the value we know the bounds, so that shouldn't be needed.
>
> You're right, when dereferencing it we'll error already earlier,
> because the array of unknown bounds can't be constexpr.
>
> I think the " with unknown bounds" part still might be useful to users,
> it can be seen in the type as [], but it might take some time to the users
> to figure out that is the reason why we disallow it.
>
> So like this?
>
> 2018-01-31  Jakub Jelinek  <jakub@redhat.com>
>
>         PR c++/83993
>         * constexpr.c (diag_array_subscript): Emit different diagnostics
>         if TYPE_DOMAIN (arraytype) is NULL.
>         (cxx_eval_array_reference, cxx_eval_store_expression): For arrays
>         with NULL TYPE_DOMAIN use size_zero_node as nelts.
>
>         * g++.dg/init/pr83993-1.C: New test.
>         * g++.dg/cpp0x/pr83993.C: New test.
>
> --- gcc/cp/constexpr.c.jj       2018-01-19 23:34:04.897278768 +0100
> +++ gcc/cp/constexpr.c  2018-01-24 13:38:40.572913190 +0100
> @@ -2270,13 +2270,20 @@ diag_array_subscript (const constexpr_ct
>        tree sidx = fold_convert (ssizetype, index);
>        if (DECL_P (array))
>         {
> -         error ("array subscript value %qE is outside the bounds "
> -                "of array %qD of type %qT", sidx, array, arraytype);
> +         if (TYPE_DOMAIN (arraytype))
> +           error ("array subscript value %qE is outside the bounds "
> +                  "of array %qD of type %qT", sidx, array, arraytype);
> +         else
> +           error ("non-zero array subscript %qE is used with array %qD of "
> +                  "type %qT with unknown bounds", sidx, array, arraytype);
>           inform (DECL_SOURCE_LOCATION (array), "declared here");
>         }
> -      else
> +      else if (TYPE_DOMAIN (arraytype))
>         error ("array subscript value %qE is outside the bounds "
>                "of array type %qT", sidx, arraytype);
> +      else
> +       error ("non-zero array subscript %qE is used with array of type %qT "
> +              "with unknown bounds", sidx, arraytype);
>      }
>  }
>
> @@ -2361,7 +2368,12 @@ cxx_eval_array_reference (const constexp
>
>    tree nelts;
>    if (TREE_CODE (TREE_TYPE (ary)) == ARRAY_TYPE)
> -    nelts = array_type_nelts_top (TREE_TYPE (ary));
> +    {
> +      if (TYPE_DOMAIN (TREE_TYPE (ary)))
> +       nelts = array_type_nelts_top (TREE_TYPE (ary));
> +      else
> +       nelts = size_zero_node;
> +    }
>    else if (VECTOR_TYPE_P (TREE_TYPE (ary)))
>      nelts = size_int (TYPE_VECTOR_SUBPARTS (TREE_TYPE (ary)));
>    else
> @@ -3439,7 +3451,12 @@ cxx_eval_store_expression (const constex
>           tree nelts, ary;
>           ary = TREE_OPERAND (probe, 0);
>           if (TREE_CODE (TREE_TYPE (ary)) == ARRAY_TYPE)
> -           nelts = array_type_nelts_top (TREE_TYPE (ary));
> +           {
> +             if (TYPE_DOMAIN (TREE_TYPE (ary)))
> +               nelts = array_type_nelts_top (TREE_TYPE (ary));
> +             else
> +               nelts = size_zero_node;
> +           }
>           else if (VECTOR_TYPE_P (TREE_TYPE (ary)))
>             nelts = size_int (TYPE_VECTOR_SUBPARTS (TREE_TYPE (ary)));
>           else
> --- gcc/testsuite/g++.dg/init/pr83993-1.C.jj    2018-01-24 13:45:43.430864528 +0100
> +++ gcc/testsuite/g++.dg/init/pr83993-1.C       2018-01-24 13:44:59.352869530 +0100
> @@ -0,0 +1,11 @@
> +// PR c++/83993
> +// { dg-do compile }
> +
> +extern const int a[];
> +const int *const b = &a[0];
> +
> +int
> +foo ()
> +{
> +  return b[0];
> +}
> --- gcc/testsuite/g++.dg/cpp0x/pr83993.C.jj     2018-01-24 14:09:01.846716177 +0100
> +++ gcc/testsuite/g++.dg/cpp0x/pr83993.C        2018-01-24 14:08:41.246718212 +0100
> @@ -0,0 +1,49 @@
> +// PR c++/83993
> +// { dg-do compile { target c++11 } }
> +// { dg-options "" }
> +
> +extern const int a[];
> +const int b[5] = { 1, 2, 3, 4, 5 };
> +extern const int c[4];
> +constexpr const int *d = &a[0];
> +constexpr const int *d2 = a;
> +constexpr const int *e = &a[1];                // { dg-error "non-zero array subscript '1' is used with array 'a' of type 'const int \\\[\\\]' with unknown bounds" }
> +constexpr const int *f = &b[0];
> +constexpr const int *f2 = b;
> +constexpr const int *g = &b[5];
> +constexpr const int *h = &b[6];                // { dg-error "array subscript value '6' is outside the bounds of array 'b' of type 'const int \\\[5\\\]'" }
> +constexpr const int *i = &c[0];
> +constexpr const int *i2 = c;
> +constexpr const int *j = &c[4];
> +constexpr const int *k = &c[5];                // { dg-error "array subscript value '5' is outside the bounds of array 'c' of type 'const int \\\[4\\\]'" }
> +extern const int l[];
> +
> +void
> +foo ()
> +{
> +  extern const int l[3];
> +  constexpr const int *m = &l[0];
> +  constexpr const int *m2 = l;
> +  constexpr const int *n = &l[1];
> +  static_assert (m == m2, "");
> +}
> +
> +constexpr const int *m = &l[0];
> +constexpr const int *m2 = l;
> +constexpr const int *n = &l[1];                // { dg-error "non-zero array subscript '1' is used with array 'l' of type 'const int \\\[\\\]' with unknown bounds" }
> +static_assert (d == d2 && f == f2 && i == i2 && m == m2, "");
> +const int o[] = { 1, 2 };
> +constexpr const int *p = &o[0];
> +constexpr const int *p2 = o;
> +constexpr const int *q = &o[2];
> +constexpr const int *r = &o[3];                // { dg-error "array subscript value '3' is outside the bounds of array 'o' of type 'const int \\\[2\\\]'" }
> +struct S { char a; char b[]; } s;
> +constexpr const char *t = &s.b[0];
> +constexpr const char *t2 = s.b;
> +constexpr const char *u = &s.b[1];     // { dg-error "non-zero array subscript '1' is used with array of type 'char \\\[\\\]' with unknown bounds" }
> +struct V { int a; };
> +extern V v[];
> +constexpr V *w = &v[0];
> +constexpr V *w2 = v;
> +constexpr int *x = &v[0].a;
> +constexpr int y = a[0];                        // { dg-error "the value of 'a' is not usable in a constant expression" }
>
>
>         Jakub