[Bug c++/106371] New: Bogus narrowing conversion reported due to bitfield

[Bug c++/106371] New: Bogus narrowing conversion reported due to bitfield

[barry.revzin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106371

            Bug ID: 106371
           Summary: Bogus narrowing conversion reported due to bitfield
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

Consider:

#include <cstdint>

struct A {
  uint64_t x : 32;
};

struct B {
  uint32_t x;
};

void f() {
    auto a = A{.x = 1};
    auto b = B{.x = a.x};
}

gcc currently emits a narrowing warning here:

<source>:13:24: warning: narrowing conversion of '(long unsigned int)a.A::x'
from 'long unsigned int' to 'uint32_t' {aka 'unsigned int'} [-Wnarrowing]
   13 |     auto b = B{ .x = a.x };
      |                      ~~^

It is true that a.x is a uint64_t, but also it's only a 32-bit bitfield, there
isn't actually any narrowing possible. This code should be fine. 

clang doesn't warn here (only if A::x were actually a uint64_t, or the width of
the bitfield was larger than 32).

[Bug c++/106371] Bogus narrowing conversion reported due to bitfield

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106371

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
May be a missing unlowered_expr_type call.

[Bug c++/106371] Bogus narrowing conversion reported due to bitfield

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106371

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
MSVC produces an error though:
<source>(14): error C2397: conversion from 'unsigned __int64' to 'unsigned int'
requires a narrowing conversion

[Bug c++/106371] Bogus narrowing conversion reported due to bitfield

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106371

Eric Gallager <egallager at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |egallager at gcc dot gnu.org
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=39170

--- Comment #3 from Eric Gallager <egallager at gcc dot gnu.org> ---
Hm I was thinking that this might also get a warning from -Wconversion due to
bug 39170, but apparently it doesn't...

[Bug c++/106371] Bogus narrowing conversion reported due to bitfield

[leni536 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106371

Lénárd Szolnoki <leni536 at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |leni536 at gmail dot com

--- Comment #4 from Lénárd Szolnoki <leni536 at gmail dot com> ---
Rejects valid code when -pedantic-errors is added:

struct S {
    long l : 8;
};

int main() {
    S s;
    int i = {s.l};
}

https://godbolt.org/z/5bq6cMaMx

Produces wrong code when it's used as a constraint:

struct S {
    long l : 8;
};

template <typename T>
requires requires (T t) {
    int{t.l};
}
void foo(T); // 1

void foo(...); // 2

int main() {
    foo(S{}); // calls 2
}

https://godbolt.org/z/We4M1MP1M