[Bug c++/110416] New: Error initializing of const union variable with a mutable field

[Bug c++/110416] New: Error initializing of const union variable with a mutable field

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110416

            Bug ID: 110416
           Summary: Error initializing of const union variable with a
                    mutable field
           Product: gcc
           Version: 13.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

This program

union U {
    mutable int x;
    float y = 2;
};

int main() {
    const U u;
    u.x = 1;
}

is accepted in Clang and MSVC, but GCC complains:

<source>: In function 'int main()':
<source>:7:13: error: uninitialized 'const u' [-fpermissive]
    7 |     const U u;
      |             ^
<source>:1:7: note: 'const union U' has no user-provided default constructor
    1 | union U {
      |       ^
<source>:2:17: note: and the implicitly-defined constructor does not initialize
'int U::x'
    2 |     mutable int x;

Online demo: https://gcc.godbolt.org/z/T8bqYTG5f

[Bug c++/110416] Error initializing of const aggregate variable with a mutable field

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110416

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Error initializing of const |Error initializing of const
                   |union variable with a       |aggregate variable with a
                   |mutable field               |mutable field

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

[Bug c++/110416] Error initializing of const union variable with a mutable field

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110416

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
            Summary|Error initializing of const |Error initializing of const
                   |aggregate variable with a   |union variable with a
                   |mutable field               |mutable field
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2023-06-26

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #1)
> struct also fails ...

Well clang does not reject that case but MSVC does.
So I don't think the struct case is a GCC bug; just clang does not have a
check.

[Bug c++/110416] Error initializing of const union variable with a mutable field

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110416

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
[dcl.init.general]/8 (specifically 8.2) is what makes the union case valid.

Though I wonder what mutable field means in an union if not the whole union is
not mutable ...