Disjoint constraints for type parameters

I propose adding a new constraint T1, T2, …, Tn disjoint for signifying type parameters T1, T2, …, Tn are provably disjoint at compile-time with exactly the same algorithm as used for switch cases.

It should enable us to write e. g.

function sumMap<Value1, Value2, Return1, Return2>(Value1|Value2 val, Return1(Value1) f1, Return2(Value2) f2)
    given Value1, Value2 disjoint =>
  switch (val)
  case (is Value1) f1(val)
  case (is Value2) f2(val);

or maybe more useful

Return|Error map…<Value, Error, Return>(Value|Error val, Return(Value) f)
    given Value, Error disjoint =>
  switch (val)
  case (is Value) f(val)
  case (is Error) val;

without disabling any warnings.

Maybe examples aren’t comprehensive enough, but it occurred to me this could be a nice feature and maybe even not hard to implement.

[ePaul]

What about given T1 & T2 is Nothing?

Actually, we could also write it the other way around, and reuse the existing syntax: given Nothing satisfies T1 & T2.

This would allow for a future generalization like given String satisfies T1 & T2.

[gavinking]

i.e.

Return|Error map<Value, Error, Return>
   (Value|Error val, Return(Value) fun)
        given Value satisfies ~Error 
        => switch (val)
        case (is Value) fun(val)
        case (is Error) val;