"Raw value for enum case is not unique" for Swift enum with Float raw values

Question

According to The Swift Programming Language, I should be able to create a Swift enum with raw values of "strings, characters, or any of the integer or floating-point number types". But when I try:

enum BatteryVoltage: Float {
    case v3v7 = 3.7
    case v5v0 = 5.0
    case v7v4 = 7.4
    case v11v1 = 11.1
    case v12v0 = 12.0
}

... I get a compilation error:

Raw value for enum case is not unique

on the v7v4 line. It compiles fine with that one commented out. But ah, it looks unique to me. If I make the value 7.41, or 7.3 or something else it compiles fine. What's going on? Swift bug?

Answer 1

(From my above comment:)

That looks definitely like a bug. It seems to happen if one enum value is exactly equal to "2 times another enum value", but not equal to an integer.

More generally (as @Sulthan observed) the error occurs if the ratio of the enumeration values is a power of two, such as 3.7/7.4, 1.2/4.8, or 1.1/17.6, but only if both values have a non-zero fractional part. So 1.5/3.0 or 1.25/5.0 do not cause an error.