Why is alias U8 = Dtype.uint8 not a type?

alias U8 = DType.uint8

@inline_always
fn is_digit(value: U8) -> Bool:
    return value >= 48 && value <= 57

alias U8 = DType.uint8

@inline_always
fn is_digit(value: U8) -> Bool:
    return value >= 48 && value <= 57

alias U8 = DType.uint8

@inline_always
fn is_digit(value: U8) -> Bool:
    return value >= 48 && value <= 57

alias U8 = DType.uint8

@inline_always
fn is_digit(value: U8) -> Bool:
    return value >= 48 && value <= 57

Gives

expected a type, not a value

expected a type, not a value

expected a type, not a value

expected a type, not a value on the

U8

U8

U8

U8 in the function signature. What am I missing?