[Bug c++/86743] New: Compilation failure when initializing POD structs containing constant member

[ldr709 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86743

            Bug ID: 86743
           Summary: Compilation failure when initializing POD structs
                    containing constant member
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ldr709 at gmail dot com
  Target Milestone: ---

With this code: 

struct A
{
    int i;
    const int b;
};

// The union is to verify that A is a type that can be used in a union.
union U
{
    A a;
    int b;
};

int main()
{
    U a = {1, 1};
    U b = {2, 1};
}

I get an error message:

$ g++ -std=c++03
a.cpp:9:4: error: member ‘A U::a’ with copy assignment operator not allowed in
union
  A a;
    ^
a.cpp:9:4: note: unrestricted unions only available with -std=c++11 or
-std=gnu++11

$ g++ ---version
g++ (GCC) 8.1.1 20180531
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

For reasoning about why this should compile without failure, see
https://stackoverflow.com/questions/27665567/pod-structs-containing-constant-member