[Bug c++/105059] New: Inconsistency between paren- and brace-initialization of a union with anonymous struct

[Bug c++/105059] New: Inconsistency between paren- and brace-initialization of a union with anonymous struct

[barry.revzin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105059

            Bug ID: 105059
           Summary: Inconsistency between paren- and brace-initialization
                    of a union with anonymous struct
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

union U {
    struct {
        int x;
        int y;
    };
    long all;
};

constexpr long parens() {
    U u(42);
    return u.all;
}
static_assert(parens() == 42);

constexpr int braces() {
    U u{42};
    return u.all;
}
static_assert(braces() == 42);

gcc currently considers parens() to be a valid constant expression with value
42 (i.e. U(42) initializes U::long) but considers braces() to be invalid
because of reading the wrong member of the union (i.e. U{42} initializes U::x).

clang currently considers parens() ill-formed by rejecting U(42) entirely (and
then rejects braces() for the same reason as gcc does, because it's u.x that
was initialized). 

The standard doesn't even talk about anonymous structs so it's hard to say what
the "right answer" here is, but it seems to me that either U(42) should do the
same thing as U{42} here or at the very least be ill-formed, but definitely not
do some different valid thing.

[Bug c++/105059] Inconsistency between paren- and brace-initialization of a union with anonymous struct

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105059

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|accepts-invalid             |documentation
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2022-12-07

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
With -pedantic:
<source>:2:12: warning: ISO C++ prohibits anonymous structs [-Wpedantic]
    2 |     struct {
      |            ^


So I think this is just a documentation issue of what anonymous struct means
with each of the initializers.

Yes there is an inconsitancy with respect to () vs {} here though.

Confirmed.