[Bug c++/87292] Warnings with Bit Fields

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87292

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2018-09-13
     Ever confirmed|0                           |1

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Nuno Gonçalves from comment #4)
> I found that the issue with enum have been extensively debated at #61414.
> Sorry.

Yes, we already have Bug 61414 for that.

> So actually this bug report is only regarding the warning with initializer
> list:
> 
>     struct{
>         uint8_t c1:6;
>         uint8_t c2:6;
>     } a;
>     auto c = {a.c1, a.c2}; //warning: narrowing conversion of '(unsigned
> char)c1' from 'unsigned char' to 'unsigned char:6' [-Wnarrowing]

Confirmed. The initializers should be promoted before deciding the type of the
std::initializer_list.

This should compile:

#include <initializer_list>

struct {
  int c1 : 6;
  int c2 : 6;
} a;
auto c = { a.c1, a.c2 };
std::initializer_list<int>& r = c;

n.cc:7:14: warning: narrowing conversion of '(int)a.<unnamed struct>::c1' from
'int' to 'signed char:6' [-Wnarrowing]
7 | auto c = { a.c1, a.c2 };
  |            ~~^~
n.cc:7:14: warning: narrowing conversion of 'a.<unnamed struct>::c1' from 'int'
to 'signed char:6' [-Wnarrowing]
n.cc:7:20: warning: narrowing conversion of '(int)a.<unnamed struct>::c2' from
'int' to 'signed char:6' [-Wnarrowing]
7 | auto c = { a.c1, a.c2 };
  |                  ~~^~
n.cc:7:20: warning: narrowing conversion of 'a.<unnamed struct>::c2' from 'int'
to 'signed char:6' [-Wnarrowing]
n.cc:8:33: error: invalid initialization of reference of type
'std::initializer_list<int>&' from expression of type
'std::initializer_list<signed char:6>'
8 | std::initializer_list<int>& r = c;
  |