const expression in impl really is a constexpr

[dank-openai]

Describe the bug

The compiler incorrectly says that an expression is not constexpr.

pub struct FixedPoint<NUM_BITS: u32, EXPONENT_IS_NEGATIVE:u32, BINARY_UEXPONENT: u32> {
    data: uN[NUM_BITS],  // concatenation of integer and fraction bits
}

impl FixedPoint<NUM_BITS, EXPONENT_IS_NEGATIVE, BINARY_UEXPONENT> {
    const BINARY_EXPONENT : s32 = if EXPONENT_IS_NEGATIVE>u32:0 { -BINARY_UEXPONENT as s32 } else { BINARY_UEXPONENT as s32 };
}

#[test]
fn test_impl() {
    let a = FixedPoint<u32:8, u32:1, u32: 4> { data: u8:0 };
    assert_eq(FixedPoint<u32:8, u32:1, u32: 4>::BINARY_EXPONENT, s32:-4);
}

Now you have to test (not build! there's no problem when I bazel build the dslx library):

Error: INVALID_ARGUMENT: Expression <stuff> was not constexpr: `if EXPONENT_IS_NEGATIVE > u32:0 {
    -BINARY_UEXPONENT as s32
} else {
    BINARY_UEXPONENT as s32
}`

To Reproduce

Copy the code above and test it (again, no problem when building, which is (arguably) a separate bug).

Expected behavior

It to compile and test without error.

Other

The fix might be as simple as lifting the restriction that all parametric types for structs are u32.