-Wbitfield-constant-conversion does not warn on 1-bit signed integer bitfield edge case

[Scaless]

In C++, signed integers are represented in two's complement. This also applies to signed bitfields.
A signed bitfield composed of one bit can therefore store a value in the range -1 to 0.
Assigning a value of 1 to such a bitfield should produce a warning since it is out of range of representable values.

MSVC and GCC generate the correct warnings for this situation, clang does not.

clang 13.0.0
-std=c++20 -Wall -Wextra -Wpedantic -Wconversion -Wbitfield-constant-conversion

struct test
{
    signed int v : 1;
};

int main()
{
    test t;
    // MSVC:  warning C4463: overflow; assigning 1 to bit-field that can only hold values from -1 to 0
    // GCC:   overflow in conversion from 'int' to 'signed char:1' changes value from '1' to '-1' [-Woverflow]
    // clang: no warning produced
    t.v = 1;
    return t.v; // Returns -1
}

Godbolt

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[Scaless]

Looks like the root cause is this section here:

llvm-project/clang/lib/Sema/SemaChecking.cpp

Lines 12468 to 12471 in ba6485e

	// Special-case bitfields of width 1: booleans are naturally 0/1, and
	// therefore don't strictly fit into a signed bitfield of width 1.
	if (FieldWidth == 1 && Value == 1)
	return false;

Removing those lines gives me the proper warning:

Boolean types are already excluded from analysis earlier in the function so removing would not affect cases such as bool b : 1;:

llvm-project/clang/lib/Sema/SemaChecking.cpp

Lines 12358 to 12361 in ba6485e

	// White-list bool bitfields.
	QualType BitfieldType = Bitfield->getType();
	if (BitfieldType->isBooleanType())
	return false;

The original code's assumption seems to be that signed int x : 1 = true; should not give a warning, which I (and GCC/MSVC) disagree with. If you want a boolean true/false (1/0) you should be using a bool : 1 or unsigned int : 1 which stores the proper value.

[AaronBallman]

Note, this was further updated by 3b00e48.