Suggestion: Allow interface to extend conditional type

An interface cannot currently extend a conditional type. See: (playground link)

type Test<T> = T extends {} ? { test: T } : {}

// Ok: type with selected condition
interface I extends Test<number> { }

// "Error: An interface may only extend a class or another interface."
interface J<T> extends Test<T> { }

// Ok: type without condition
type Test2<T> = { test: T }
interface K<T> extends Test2<T> { }

This example exhibits one of these problems:

• J<T> should be allowed to extend Test<T>, to make it more complete and allow conditional type things in interfaces. –OR–
• The error message should be made more clear, because interfaces can obviously extend types as you can see above.

[m-henderson]

@RyanCavanaugh you mentioned the only thing we can do is update the error message; @mhegazy do you agree with that? if so, what does the diagnostics message need to say so that it is accurate than the current diagnostics message?

[infogulch]

Hmm. If all the possible types the condition could evaluate to are object types, and an interface extends it, the interface is still an object type even if it’s conditional, no? I.e. even though it’s conditional, it’s still guaranteed to resolve to some object type.

If that’s not a strong enough guarantee because of compiler/typesystem/implementation reasons, I won’t argue it any farther. Thank you for your explanations!