[Bug c++/100185] New: transparent_union fails when the union has a destructor

[Bug c++/100185] New: transparent_union fails when the union has a destructor

[gonzalobg88 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100185

            Bug ID: 100185
           Summary: transparent_union fails when the union has a
                    destructor
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gonzalobg88 at gmail dot com
  Target Milestone: ---

Example:

    union  __attribute__((transparent_union)) U {
        int* a;
        ~U() {}
    };

fails with:

    <source>:1:43: error: type transparent 'union U' cannot be made transparent 
    because the type of the first field has a different ABI from the class
overall
        1 | union  __attribute__((transparent_union)) U {
          |                                           ^

I think this should be allowed with the following semantics:

    void f(int*);
    void g(U&);
    void h(U);
    void j(int);

    {
        U u;
        f(u); // passes U as int*
        g(u); // passes U as U&
        h(u); // ERROR: U does not have a copy constructor
        j(u); // ERROR: cannot convert U to int

        //~U() called at the end of the scope
    }

[Bug c++/100185] transparent_union fails when the union has a destructor

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100185

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

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

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
I think the error is right and desirable.  Types with non-trivial ctors or
dtors need to be passed by invisible reference, so can't be passed the same as
the first union member.

[Bug c++/100185] transparent_union fails when the union has a destructor

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100185

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
indeed

[Bug c++/100185] transparent_union fails when the union has a destructor

[gonzalo.gadeschi at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100185

--- Comment #3 from gnzlbg <gonzalo.gadeschi at gmail dot com> ---
@Jakub, is there a GCC equivalent to `[[clang::trivial_abi]]` that one can use
instead ?

[Bug c++/100185] transparent_union fails when the union has a destructor

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100185

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
No and I think it is quite bad idea.

[Bug c++/100185] transparent_union fails when the union has a destructor

[gonzalo.gadeschi at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100185

--- Comment #5 from gnzlbg <gonzalo.gadeschi at gmail dot com> ---
Thank you for the clarification! Will map Rust repr(transparent) to clang's
[[clang::trivial_abi]] then and will gracefully #pragma error to prevent users
from shooting themselves in the foot with GCC.