Improperly widened computed property keys in the presence of generics + literal types

[carlpaten]

TypeScript Version: 4.2.0-dev.20201202

Search Terms: computed property

Code

function obj<T extends string>(key: T) {
  return { [key]: 0 };
}
const result: { foo: number } = obj<"foo">("foo");

Expected behavior: this should compile without error.

Actual behavior: error TS2741: Property 'foo' is missing in type '{ [x: string]: number; }' but required in type '{ foo: number; }'.

Playground Link: https://www.typescriptlang.org/play?ts=4.2.0-dev.20201202#code/GYVwdgxgLglg9mABHARgKwDwBVEFMAeUuYAJgM6JlQBOMYA5gHwAUA1rgJ4BciWAlIgDeAKESJquKCGpJBiANrsOAXR4AGRAF8A3MM3CICKuNxkQAGyg85wOHB5gQAWxS5qWxAF5k6DACJbOD8WALs-Pl0gA

Related Issues:

• Computed property key names should not be widened #13948 interaction between keyof and computed property keys
• Type checker doesn't understand string literal type in computed property key #15534 same, in the absence of generics; fixed

[RyanCavanaugh]

This is the correct behavior. If you imagine that we inferred obj's return type to be Record<T, number> then this code would be allowed, but it's unsound:

function obj<T extends string>(key: T) {
  return { [key]: 0 };
}

const result: { bar: number } = obj<"foo" | "bar">("foo");
result.bar.toFixed();

[carlpaten]

I thought this was incorrect behavior because the specialized version of the function has the expected type {foo: number}, but now I realize that in order to accomplish that you'd need the return type of obj to be something like T is literal ? Record<T, number> : { [x: string]: number; }, and is literal does not exist in TypeScript. Sorry for the erroneous report, and thank you for the explanation!

[RyanCavanaugh]

np, it's tricky!

[grapereader]

Return type for obj<"foo" | "bar">("foo") should be { foo?: number, bar?: number } but for obj<"foo">("foo") it can be { foo: number }

Even if you do widen the domain of keys to all strings, inferred type should be { [x: string]?: number; } because not every string key will be defined. It is still unsound.