[Feature Request] Allow substitution of child traits for parent traits

[lsh]

Review Mojo's priorities

• I have read the roadmap and priorities and I believe this request falls within the priorities.

What is your request?

If a function takes variadic arguments bound by a trait, then a sub-trait should be allowed to substitute that trait.

What is your motivation for this change?

The following code should be possible with this change:

trait StringableCollectionElement(CollectionElement, Stringable):
   …

struct A[*Ts: StringableCollectionElement]:
    var tup: Tuple[Ts]

Any other details?

No response

[StandinKP]

Where would this be used?

[lsh]

@StandinKP right now tuples holding variadic types aren't particularly useful when trying to work in a generic way. Suppose you have this:

@value
struct MyStruct[*Ts: CollectionElement]:
    var tup: Tuple[ts]

What can you actually do with this? Since the only thing you know about each type is that they conform to CollectionElement, that's the only thing you can use those types for within MyStruct. Basically, all you can do is store, move, and copy the values.

If you tried to add a method:

struct MyStruct[*Ts: CollectionElement]:
    # ...
    fn print_first_value(self):
        print(self.tup[0])

this would fail to compile, since you don't know that each element conforms to Stringable.