ThisType doesn't work when extending it with an interface

[DetachHead]

Bug Report

🔎 Search Terms

thistype interface

🕗 Version & Regression Information

4.6.0-dev.20211116

⏯ Playground Link

Playground link with relevant code

💻 Code

interface Foo {
    foo: () => void;
}

interface NewThisInterface extends ThisType<Foo> {
    hello: () => void
}

type NewThisType = ThisType<Foo> & {
    hello: () => void
}

const functions: NewThisType = {
    hello: function() {
        this.foo(); //works
    }
}

const functions2: NewThisInterface = {
    hello: function() {
        this.foo(); //doesn't work?
    }
}

🙁 Actual behavior

Property 'foo' does not exist on type 'NewThisInterface'.(2339)

🙂 Expected behavior

no error

[MartinJohns]

Possibly because interface and type are not equivalent. Interfaces are subject to declaration merging, and don't extend Record<string, unknown>.

[DetachHead]

yeah i'm aware of the differences between types and interfaces, but i don't see how any of those differences would break ThisType, though it's hard to tell because ThisType seems to be intrinsic, although it isn't marked as such (#46836)

[fatcerberus]

The purpose of ThisType is to change the contextual type of anything assigned to it (e.g. object literals without an explicit this type on their methods). extends isn't assignment so inheriting from it doesn't work.

(Contextual typing being the same mechanism that infers parameter types of callbacks).

[RyanCavanaugh]

extends and & are different operations. extends "copies down" the operand type's members, and ThisType is just a marker type, so effectively nothing happens because there is nothing to copy down. Intersection is intersection, so when contextually typing the function, we can successfully find the nominal ThisType among the intersection's constituents.

This wouldn't be fixable without doing some heavy-duty changing of what extends means in a way that would have observable follow-on effects, and this behavior has been present since 3.3.3 without anyone else noticing, so I'm going to round this up to "intended" behavior.

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.