An interface with generic type restrictions that allows for a default type to be specified

I have developed a straightforward generic interface using typescript

interface DateAdapter<T> {
  clone(): T;
}

and I've implemented the interface in a basic class

class StandardDateAdapter implements DateAdapter<StandardDateAdapter> {
  clone: () => StandardDateAdapter;
}

Next, I have a generic Options interface that takes a type extending DateAdapter. I want the default type argument to be StandardDateAdapter, so I declared it as follows:

interface Options<T extends DateAdapter<T> = StandardDateAdapter> {
  until?: T;
}

However, typescript is raising an error with this setup

Type 'StandardDateAdapter' does not satisfy the constraint 'DateAdapter<T>'.
  Types of property 'clone' are incompatible.
    Type '() => StandardDateAdapter' is not assignable to type '() => T'.
      Type 'StandardDateAdapter' is not assignable to type 'T'.

Does anyone have suggestions on how to resolve this issue? Could this be a bug? Thank you!

note: DateAdapter is generic because I want the return type of clone() to match the type of the class implementing DateAdapter. Initially, I tried clone(): this; in the DateAdapter, but typescript correctly recognized that StandardDateAdapter#clone() returning new StandardDateAdapter is not the same as this

Answer №1

It appears that TypeScript does not correctly replace occurrences of T with the default value when checking constraints against the default value of T. It may be worth raising this issue with the TypeScript team and seeking their input on how to address it. As of July 28, 2018, no existing issue has been found related to this issue.

One workaround that seems to work is as follows:

interface Options<T extends DateAdapter<T> = StandardDateAdapter & DateAdapter<T>> {
  until?: T;
}

By using this workaround, the default value effectively becomes

StandardDateAdapter & DateAdapter<{}>
. Although it may not look ideal, it can still be assigned to and from StandardDateAdapter, potentially meeting all your requirements.

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