Conditionally enable generic functions

[Victorious3]

The simple way of making a normal function definition dependant on some sort of condition is to wrap it inside an #if

#if defined WIN32 {
    def foo -> int { return 10 }
}

However, for generic functions it isn't so simple. There is currently no way to "enable" a function if a condition is met.
Take index_of from Vector as an example:

export def index_of(vec: &Vector(type T), elem: T) -> int64 {
    for var i in 0..vec.length {
        if vec[i] == elem {
            return i
        }
    }
    return -1
}

This function only makes sense if equality is defined between element types.
Now there's certainly the option to codify this condition with a type, possibly using this syntax:

type Eq = interface {
    def __eq__(other: Eq) -> bool
}

export def index_of(vec: &Vector(type T > Eq), elem: T) -> int64 {

However more complex conditions can't be expressed in that way.
My idea is to wrap the function body in an #if statement like this:

export def index_of(vec: &Vector(type T), elem: T) -> int64
#if defined __eq__::(T, T) {
    ...
}