[Bug c++/94264] New: Array-to-pointer conversion not performed on array prvalues

[Bug c++/94264] New: Array-to-pointer conversion not performed on array prvalues

[ndkrempel at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94264

            Bug ID: 94264
           Summary: Array-to-pointer conversion not performed on array
                    prvalues
           Product: gcc
           Version: 9.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ndkrempel at gmail dot com
  Target Milestone: ---

I think the most clearcut example is:

int main() {
  using T = int[];
  T{1, 2} == nullptr;
}

This compiles fine with clang, and is supported by the standard: the ==
operator explicitly performs array-to-pointer conversion
(https://eel.is/c++draft/expr#eq-1), and array-to-pointer conversion is
explicitly defined for rvalue arrays (https://eel.is/c++draft/expr#conv.array).

Other examples (which again all compile with clang) are:

+T{1, 2};
  Here the standard wording seems to have a minor bug, as unary "+" does not
explicitly perform the array-to-pointer conversion, and the general rubric for
applying it (https://eel.is/c++draft/expr#basic.lval-6) only applies to
glvalues as written.

T{1, 2} + 1;
  Ditto.

*(T{1, 2} + 1);
  Interesting as T{1, 2}[1] should be equivalent to this (modulo the value
category of the result, https://eel.is/c++draft/expr#sub), yet the former is
rejected by gcc and the latter accepted.

[Bug c++/94264] Array-to-pointer conversion not performed on array prvalues

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94264

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2020-04-18
                 CC|                            |mpolacek at gcc dot gnu.org
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
cp_build_binary_op calls cp_default_conversion on its operands and that calls
decay_conversion which has:

 2153       /* Don't let an array compound literal decay to a pointer.  It can
 2154          still be used to initialize an array or bind to a reference.  */
 2155       if (TREE_CODE (exp) == TARGET_EXPR)
 2156         {
 2157           if (complain & tf_error)
 2158             error_at (loc, "taking address of temporary array");
 2159           return error_mark_node;
 2160         }


convert_like_real does

+   /* Take the address explicitly rather than via decay_conversion
+      to avoid the error about taking the address of a temporary.  */
+   array = cp_build_addr_expr (array, complain);
+   array = cp_convert (build_pointer_type (elttype), array);

to avoid that error.

[Bug c++/94264] Array-to-pointer conversion not performed on array prvalues

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94264

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dchneric at gmail dot com

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 89931 has been marked as a duplicate of this bug. ***

[Bug c++/94264] Array-to-pointer conversion not performed on array prvalues

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94264

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |kamkaz at windowslive dot com

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 102568 has been marked as a duplicate of this bug. ***

[Bug c++/94264] Array-to-pointer conversion not performed on array prvalues

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94264

Fedor Chelnokov <fchelnokov at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |fchelnokov at gmail dot com

--- Comment #4 from Fedor Chelnokov <fchelnokov at gmail dot com> ---
Related discussion: https://stackoverflow.com/q/77051011/7325599

And an example from it:

int main() {
    using U = int(&&)[1];
    (void) *U{}; //ok everywhere

    using T = int[1];
    (void) *T{}; //error in GCC
}

Online demo: https://godbolt.org/z/4s8v9PPqT

[Bug c++/94264] Array-to-pointer conversion not performed on array prvalues

[de34 at live dot cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94264

Jiang An <de34 at live dot cn> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |de34 at live dot cn

--- Comment #5 from Jiang An <de34 at live dot cn> ---
Related:
https://cplusplus.github.io/CWG/issues/2548.html

[Bug c++/94264] Array-to-pointer conversion not performed on array prvalues

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94264

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Nick Krempel from comment #0)
> T{1, 2} + 1;
>   Ditto.

That is what clang's issue: https://github.com/llvm/llvm-project/issues/54016
is about even.

[Bug c++/94264] Array-to-pointer conversion not performed on array prvalues

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94264

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org
           Assignee|unassigned at gcc dot gnu.org      |jason at gcc dot gnu.org
         Depends on|                            |53220
             Status|NEW                         |ASSIGNED

--- Comment #7 from Jason Merrill <jason at gcc dot gnu.org> ---
This is due to my PR53220 change to discourage use of compound-literals in ways
that produce a dangling pointer when the C++ compiler treats them as prvalues,
unlike C where they have variable lifetime.

I think the change was always wrong, but wasn't really a problem until we added
array prvalues in C++17.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53220
[Bug 53220] [4.7/4.8 Regression] g++ mis-compiles compound literals

[Bug c++/94264] Array-to-pointer conversion not performed on array prvalues

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94264

--- Comment #8 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Jason Merrill <jason@gcc.gnu.org>:

https://gcc.gnu.org/g:305a2686c99bf9b57490419f25c79f6fb3ae0feb

commit r14-5941-g305a2686c99bf9b57490419f25c79f6fb3ae0feb
Author: Jason Merrill <jason@redhat.com>
Date:   Tue Nov 28 13:54:47 2023 -0500

    c++: prvalue array decay [PR94264]

    My change for PR53220 made array to pointer decay for prvalue arrays
    ill-formed to catch well-defined C code that produces a dangling pointer in
    C++ due to the shorter lifetime of compound literals.  This wasn't really
    correct, but wasn't a problem until C++17 added prvalue arrays, at which
    point it started rejecting valid C++ code.

    I wanted to make sure that we still diagnose the problematic code;
    -Wdangling-pointer covers the array-lit.c case, but I needed to extend
    -Wreturn-local-addr to handle the return case.

            PR c++/94264
            PR c++/53220

    gcc/c/ChangeLog:

            * c-typeck.cc (array_to_pointer_conversion): Adjust -Wc++-compat
            diagnostic.

    gcc/cp/ChangeLog:

            * call.cc (convert_like_internal): Remove obsolete comment.
            * typeck.cc (decay_conversion): Allow array prvalue.
            (maybe_warn_about_returning_address_of_local): Check
            for returning pointer to temporary.

    gcc/testsuite/ChangeLog:

            * c-c++-common/array-lit.c: Adjust.
            * g++.dg/cpp1z/array-prvalue1.C: New test.
            * g++.dg/ext/complit17.C: New test.

[Bug c++/94264] Array-to-pointer conversion not performed on array prvalues

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94264

--- Comment #9 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:5764825aed613f201a8bc47e5b239027a39691f0

commit r14-6342-g5764825aed613f201a8bc47e5b239027a39691f0
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Dec 8 13:33:55 2023 -0500

    c++: undiagnosed error_mark_node from cp_build_c_cast [PR112658]

    When cp_build_c_cast commits to an erroneous const_cast, we neglect to
    replay errors from build_const_cast_1 which can lead to us incorrectly
    accepting (and "miscompiling") the cast, or triggering the assert in
    finish_expr_stmt.

    This patch fixes this oversight.  This was the original fix for the ICE
    in PR112658 before r14-5941-g305a2686c99bf9 made us accept the testcase
    there after all.  I wasn't able to come up with an alternate testcase for
    which this fix has an effect anymore, but below is a reduced version of
    the PR112658 testcase (accepted ever since r14-5941) for good measure.

            PR c++/112658
            PR c++/94264

    gcc/cp/ChangeLog:

            * typeck.cc (cp_build_c_cast): If we're committed to a const_cast
            and the result is erroneous, call build_const_cast_1 a second
            time to issue errors.  Use complain=tf_none instead of =false.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/initlist-array20.C: New test.

[Bug c++/94264] Array-to-pointer conversion not performed on array prvalues

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94264

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
   Target Milestone|---                         |14.0
         Resolution|---                         |FIXED

--- Comment #10 from Jason Merrill <jason at gcc dot gnu.org> ---
Fixed for GCC 14.