Extending mapped type doesn't allow extra properties when extended type has string literal values

[asmundg]

TypeScript Version: 3.2.0-dev.20181011

Search Terms: extends mapped literal widening

Code

// Type '{ a: string; c: number; }' is not assignable to type '{ a: "a"; }'.
//   Object literal may only specify known properties, and 'c' does not exist in type '{ a: "a"; }'. [2322]
const f: <T extends { [P in keyof T]: { a: "a" } }>(literal: T) => void = () => {}
f({one: {a: "a", c: 1}})

// works
const a: "a" = "a"
f({one: {a, c: 1}})

// works
const f_: <T extends { [P in keyof T]: { a: string } }>(literal: T) => void = () => {}
f_({one: {a: "a", c: 1}})

// works
const f__: <T extends {one: { a: "a" }}>(literal: T) => void = () => {}
f__({one: {a: "a", c: 1}})

Expected behavior:

Code compiles for all cases.

Actual behavior:

Code fails when extending a mapped type with string literal properties:

test.ts:4:18 - error TS2322: Type '{ a: string; c: number; }' is not assignable to type '{ a: "a"; }'.
  Object literal may only specify known properties, and 'c' does not exist in type '{ a: "a"; }'.

Removing either the mapping, the string literal or the widenable input makes the code pass type checking.

Also notable is that the one.a property of the function argument is only inferred to be defined by the extended type in the first case. E.g. jumping to definition only does something in the first f() call.

This seems related to the a property being widened when mapped over.

Playground Link: http://www.typescriptlang.org/play/#src=%2F%2F%20Type%20'%7B%20a%3A%20string%3B%20c%3A%20number%3B%20%7D'%20is%20not%20assignable%20to%20type%20'%7B%20a%3A%20%22a%22%3B%20%7D'.%0D%0A%2F%2F%20%20%20Object%20literal%20may%20only%20specify%20known%20properties%2C%20and%20'c'%20does%20not%20exist%20in%20type%20'%7B%20a%3A%20%22a%22%3B%20%7D'.%20%5B2322%5D%0D%0Aconst%20f%3A%20%3CT%20extends%20%7B%20%5BP%20in%20keyof%20T%5D%3A%20%7B%20a%3A%20%22a%22%20%7D%20%7D%3E(literal%3A%20T)%20%3D%3E%20void%20%3D%20()%20%3D%3E%20%7B%7D%0D%0Af(%7Bone%3A%20%7Ba%3A%20%22a%22%2C%20c%3A%201%7D%7D)%0D%0A%0D%0A%2F%2F%20works%0D%0Aconst%20a%3A%20%22a%22%20%3D%20%22a%22%0D%0Af(%7Bone%3A%20%7Ba%3A%20a%2C%20c%3A%201%7D%7D)%0D%0A%0D%0A%2F%2F%20works%0D%0Aconst%20f_%3A%20%3CT%20extends%20%7B%20%5BP%20in%20keyof%20T%5D%3A%20%7B%20a%3A%20string%20%7D%20%7D%3E(literal%3A%20T)%20%3D%3E%20void%20%3D%20()%20%3D%3E%20%7B%7D%0D%0Af_(%7Bone%3A%20%7Ba%3A%20%22a%22%2C%20c%3A%201%7D%7D)%0D%0A%0D%0A%2F%2F%20works%0D%0Aconst%20f__%3A%20%3CT%20extends%20%7Bone%3A%20%7B%20a%3A%20%22a%22%20%7D%7D%3E(literal%3A%20T)%20%3D%3E%20void%20%3D%20()%20%3D%3E%20%7B%7D%0D%0Af__(%7Bone%3A%20%7Ba%3A%20%22a%22%2C%20c%3A%201%7D%7D)%0D%0A

Related Issues:

[asmundg]

Possibly related: #27421

[sandersn]

How does T extends { [P in keyof T]: { a: "a" } } differ from T extends { [s: string]: { a: "a" } in the real code that led to this issue? (In your example, you even just use T, so I assume that the real code behind this issue is more complex.)

Note that from the type system's perspective, T extends { [P in keyof T]: { a: "a" } } is circular and so some things don't work the way you would expect.

[asmundg]

The original problem is in react-native's typings (https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/react-native/index.d.ts#L5222) which uses the first form and has some weird behavior related to inference.

I had a hard time describing the problem, so I tried to narrow it down a bit.

The working variants were a result of messing around to try to isolate the issue.

[zhigang1992]

@asmundg I think it might actually be a type definition issue.

Update updating node_modules/@types/react-native/index.d.ts with

-    type NamedStyles<T> = { [P in keyof T]: ViewStyle | TextStyle | ImageStyle };
+    type NamedStyles = { [key: string]: ViewStyle | TextStyle | ImageStyle };

The issue seems to be resolved locally

[asmundg]

As you point out in the DefinitelyTyped PR, extending {[key: string]: ...} now requires the type parameter in export function create<T extends NamedStyles<T>>(styles: T): { [P in keyof T]: RegisteredStyle<T[P]> }; to have a string index, precluding the use of a plain interface without the index.

It seems like the pattern the code aims for is "for each property in an object, the value type must extend V", which seems difficult do do without referring to T or requiring an index signature. Or requiring T to be explicitly provided instead of letting it be inferred.

[sandersn]

I'm closing this issue since the underlying issue is By Design for Typescript itself, and a PR for a fix of @types/react-native is open at DefinitelyTyped/DefinitelyTyped#30564.