[estebank]

If you have

    let a: u8 is 0..100 = 1;
    let b: u8 is 0..100 = 2;
    let c = a + b;

The type of c could be u8 in 0..200. If you have holes in the middle, same applies. Which means that if you want to make c u8 between 0..100 you'd have to explicitly clamp/convert/request that, which would have to be a runtime check.

[Jtsummers]

In your example we have enough information to know that the addition is safe. In SPARK, if that were a function with a and b as arguments, for instance, and you don't know what's being passed in you make it a pre-condition. Then it moves the burden of proof to the caller to ensure that the call is safe.

[nine_k]

But obviously the result of a + b is [0..200], so an explicit cast, or an assertion, or a call to clamp() is needed if we want to put it back into a [0..100].

Comptime constant expression evaluation, as in your example, may suffice for the compiler to be able to prove that the result lies in the bounds of the type.