[Bug c/108931] New: conditional operator for variably modified types

["muecker at gwdg dot de" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108931

            Bug ID: 108931
           Summary: conditional operator for variably modified types
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: muecker at gwdg dot de
  Target Milestone: ---

Created attachment 54536
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54536&action=edit
patch against trunk

For conditional operators with size expressions in both branches, the behavior
is erratic. While this seems to be undefined behavior (although it shouldn't
be), it is surprising and my cause subtle bugs, e.g. when using such casts or
statement expressions in macros. Also it seems easy to fix by always evaluating
the size of the active branch only and taking this size for the result.

For example, in some cases size expressions from both branches are evaluated:

int foo(void)
{
    return sizeof(*(0 ? (char(*)[f()])0 
                : (char(*)[g()])0));
}

foo:
        sub     rsp, 8
        call    g
        call    f
        add     rsp, 8
        ret

In other cases size from an unevaluated branch is taken as the result:

https://godbolt.org/z/o74e7T16x