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).
