[Bug c++/109756] New: "internal compiler error: tree check" when using the [[assume]] attribute with pack expansion

[Bug c++/109756] New: "internal compiler error: tree check" when using the [[assume]] attribute with pack expansion

[ensadc at mailnesia dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109756

            Bug ID: 109756
           Summary: "internal compiler error: tree check" when using the
                    [[assume]] attribute with pack expansion
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ensadc at mailnesia dot com
  Target Milestone: ---

https://godbolt.org/z/WPn9aq8sr

====
bool f(auto... args) {
    [[assume(args > 0)...]];
    return ((args == 0) && ...);
}

bool g(int a, int b) {
    return f(a, b);
}

====
<source>: In function 'bool f(auto:1 ...)':
<source>:2:28: internal compiler error: tree check: expected tree that contains
'common' structure, have 'expr_pack_expansion' in list_length, at tree.cc:3414
    2 |     [[assume(args > 0)...]];
      |                            ^
0x235945e internal_error(char const*, ...)
        ???:0
0x96da3a tree_contains_struct_check_failed(tree_node const*,
tree_node_structure_enum, char const*, int, char const*)
        ???:0
0xb20632 process_stmt_assume_attribute(tree_node*, tree_node*, unsigned int)
        ???:0
0xc7212f c_parse_file()
        ???:0
0xdb0d99 c_common_parse_file()
        ???:0
Please submit a full bug report, with preprocessed source (by using
-freport-bug).
Please include the complete backtrace with any bug report.
See <https://gcc.gnu.org/bugs/> for instructions.
Compiler returned: 1

====
I believe that [[assume(args > 0)...]] is a valid pack expansion. The standard
says ([temp.variadic]/5):

> Pack expansions can occur in the following contexts:
> (5.8) - In an attribute-list ([dcl.attr.grammar]); the pattern is an attribute.

[Bug c++/109756] "internal compiler error: tree check" when using the [[assume]] attribute with pack expansion

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109756

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
           Keywords|ice-on-valid-code           |
   Last reconfirmed|                            |2023-05-06
     Ever confirmed|0                           |1

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

clang rejects it:
<source>:2:26: error: attribute 'assume' cannot be used as an attribute pack
    [[assume(args > 0)...]];
                         ^

While MSVC accepts it.

I am not sure if it is this is valid or not but GCC should NOT be ICEing either
way.

[Bug c++/109756] "internal compiler error: tree check" when using the [[assume]] attribute with pack expansion

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109756

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
The https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p1774r8.pdf
paper says in the introductory part:
We could therefore hypothetically permit the assume attribute to directly
support pack expansion:
template <int... args>
void f() {
[[assume(args >= 0)...]];
}
However, we do not propose this. It would require substantial additional work
for a very rare use
case. Note that this can instead be expressed with a fold expression, which is
equivalent to the
above and works out of the box without any extra effort:
template <int... args>
void f() {
[[assume(((args >= 0) && ...))]];
}

http://eel.is/c++draft/dcl.attr#grammar-4 says:
"In an attribute-list, an ellipsis may appear only if that attribute's
specification permits it."
And I don't see it being explicitly permitted for any of the standard attribute
(except for alignas which uses a different syntax).
So, I think this is invalid and we should reject it.

[Bug c++/109756] "internal compiler error: tree check" when using the [[assume]] attribute with pack expansion

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109756

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |ice-checking,
                   |                            |ice-on-invalid-code

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
We also correctly reject:
bool f(auto... args) {
    [[assume(((args >= 0) ))]];
}

bool g(int a, int b) {
    return f(a, b);
}

[Bug c++/109756] "internal compiler error: tree check" when using the [[assume]] attribute with pack expansion

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109756

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #3)
> We also correctly reject:
I should say without an ICE.

[Bug c++/109756] "internal compiler error: tree check" when using the [[assume]] attribute with pack expansion

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109756

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org,
                   |                            |ppalka at gcc dot gnu.org

--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Trying other standard attributes, we accept
template <int ...args>
[[noreturn...]] [[deprecated...]] [[nodiscard...]]
int foo (int x)
{
  switch (x)
    {
    case 1:
      [[likely...]];
      [[fallthrough...]];
    case 2:
      [[unlikely...]];

      break;
    default:
      break;
    }
  struct T {};
  struct S { [[no_unique_address...]] T t; };
  for (;;)
    ;
}

int a = foo <1, 2, 3> (4);
and just emit the deprecated warning, while clang++ rejects all those.

[Bug c++/109756] "internal compiler error: tree check" when using the [[assume]] attribute with pack expansion

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109756

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 55019
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55019&action=edit
gcc14-pr109756.patch

Untested patch to diagnose ellipsis after attributes with no arguments.

[Bug c++/109756] "internal compiler error: tree check" when using the [[assume]] attribute with pack expansion

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109756

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |jakub at gcc dot gnu.org

--- Comment #7 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 55020
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55020&action=edit
gcc14-pr109756-2.patch

And untested patch to fix the actually reported problem.

[Bug c++/109756] "internal compiler error: tree check" when using the [[assume]] attribute with pack expansion

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109756

--- Comment #8 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:2499540e9abb55079b5f7b7ccdac97fbc63d9ab4

commit r14-619-g2499540e9abb55079b5f7b7ccdac97fbc63d9ab4
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue May 9 16:05:22 2023 +0200

    c++: Reject pack expansion of assume attribute [PR109756]

    http://eel.is/c++draft/dcl.attr#grammar-4 says
    "In an attribute-list, an ellipsis may appear only if that attribute's
    specification permits it."
    and doesn't explicitly permit it on any standard attribute.
    The https://wg21.link/p1774r8 paper which introduced assume attribute says
    "We could therefore hypothetically permit the assume attribute to directly
    support pack expansion:
    template <int... args>
    void f() {
    [[assume(args >= 0)...]];
    }
    However, we do not propose this. It would require substantial additional
work
    for a very rare use case. Note that this can instead be expressed with a
fold
    expression, which is equivalent to the above and works out of the box
without
    any extra effort:
    template <int... args>
    void f() {
    [[assume(((args >= 0) && ...))]];
    }
    ", but as the testcase shows, GCC 13+ ICEs on assume attribute followed by
    ... if it contains packs.
    The following patch rejects those instead of ICE and for C++17 or later
    suggests using fold expressions instead (it doesn't make sense to suggest
    it for C++14 and earlier when we'd error on the fold expressions).

    2023-05-09  Jakub Jelinek  <jakub@redhat.com>

            PR c++/109756
            * cp-gimplify.cc (process_stmt_assume_attribute): Diagnose pack
            expansion of assume attribute.

            * g++.dg/cpp23/attr-assume11.C: New test.

[Bug c++/109756] "internal compiler error: tree check" when using the [[assume]] attribute with pack expansion

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109756

--- Comment #9 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-13 branch has been updated by Jakub Jelinek
<jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:7bd9a34e9fce0fc307ec26d6db071fe0bc02cd37

commit r13-7312-g7bd9a34e9fce0fc307ec26d6db071fe0bc02cd37
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue May 9 16:05:22 2023 +0200

    c++: Reject pack expansion of assume attribute [PR109756]

    http://eel.is/c++draft/dcl.attr#grammar-4 says
    "In an attribute-list, an ellipsis may appear only if that attribute's
    specification permits it."
    and doesn't explicitly permit it on any standard attribute.
    The https://wg21.link/p1774r8 paper which introduced assume attribute says
    "We could therefore hypothetically permit the assume attribute to directly
    support pack expansion:
    template <int... args>
    void f() {
    [[assume(args >= 0)...]];
    }
    However, we do not propose this. It would require substantial additional
work
    for a very rare use case. Note that this can instead be expressed with a
fold
    expression, which is equivalent to the above and works out of the box
without
    any extra effort:
    template <int... args>
    void f() {
    [[assume(((args >= 0) && ...))]];
    }
    ", but as the testcase shows, GCC 13+ ICEs on assume attribute followed by
    ... if it contains packs.
    The following patch rejects those instead of ICE and for C++17 or later
    suggests using fold expressions instead (it doesn't make sense to suggest
    it for C++14 and earlier when we'd error on the fold expressions).

    2023-05-09  Jakub Jelinek  <jakub@redhat.com>

            PR c++/109756
            * cp-gimplify.cc (process_stmt_assume_attribute): Diagnose pack
            expansion of assume attribute.

            * g++.dg/cpp23/attr-assume11.C: New test.

    (cherry picked from commit 2499540e9abb55079b5f7b7ccdac97fbc63d9ab4)

[Bug c++/109756] "internal compiler error: tree check" when using the [[assume]] attribute with pack expansion

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109756

--- Comment #10 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:39d6d4256d16d676f8b9031c4d1d115ddf4ad76b

commit r14-650-g39d6d4256d16d676f8b9031c4d1d115ddf4ad76b
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Wed May 10 11:37:04 2023 +0200

    c++: Reject attributes without arguments used as pack expansion [PR109756]

    The following testcase shows we silently accept (and ignore) attributes
without
    arguments used as pack expansions.  This is because we call
    make_pack_expansion and that starts with
      if (!arg || arg == error_mark_node)
        return arg;
    Now, an attribute without arguments like [[noreturn...]] is IMHO always
    invalid, in this case for 2 reasons; one is that as it has no arguments,
    no pack can be present and second is that the standard says that
    attributes need to specially permit uses of parameter pack and doesn't
    explicitly permit it for any of the standard attributes (except for
alignas?
    which has different syntax).
    If an attribute has some arguments but doesn't contain packs in those
    arguments, make_pack_expansion will already diagnose it.

    The patch also changes cp_parser_std_attribute, such that for attributes
unknown
    to the compiler (or perhaps registered just for -Wno-attributes=) we
differentiate
    between the attribute having no arguments (in that case we want to diagnose
them
    when followed by ellipsis even if they are unknown, as they can't contain a
pack
    in that case) and the case where they do have arguments but we've just
skipped over
    those arguments because we don't know how to parse them (except that they
are
    a balanced token sequence) - in that case we really don't know if they
contain
    packs or not.

    2023-05-10  Jakub Jelinek  <jakub@redhat.com>

            PR c++/109756
            * parser.cc (cp_parser_std_attribute): For unknown attributes with
            arguments set TREE_VALUE (attribute) to error_mark_node after
skipping
            the balanced tokens.
            (cp_parser_std_attribute_list): If ... is used after attribute
without
            arguments, diagnose it and return error_mark_node.  If
            TREE_VALUE (attribute) is error_mark_node, don't call
            make_pack_expansion nor return early error_mark_node.

            * g++.dg/cpp0x/gen-attrs-78.C: New test.