Optional positional type parameters

[FMorschel]

I saw this post on X.

The post has this code example:

// Optional error type with default
sealed class Result<T, [E extends Exception]> {...}

// `E` defaults to `Exception`
Result<User> getUser() {...}

// Custom error type
Result<User, ApiError> getUser2() {...}

Similar to our current examples of optional positional parameters, we could have an optional type parameter.

This would make code bases with strict_raw_type: true have maybe less work when the extended type is always the expected type (if we agree that for example it could have a lint similar to avoid_redundant_argument_values).

[FMorschel]

Somewhat related to #4124.

[pierremrtn]

I'm the author of the post, honestly I'm not sure if it's a good idea or worth the effort.
But following the same idea, we can imagine named generics which would allow arbitrary parameter ordering and optional types as well.

class MyClass<{A, B  extends Object, C extends num = int}> {}
 
final v = MyClass<A: SomeType1, B: SomeType2, C: double>();
final v2 = MyClass<B: SomeType2, A: SomeType1>(); // c default to int, params use arbitrary order

[FMorschel]

Also related to #3837