cannot enforce type shape when assigned to a conjunction

[sledorze]

TypeScript Version: 3.1.6, 3.2.0-rc, 3.3.0-dev.20181121

Search Terms:
conjunction assignable wrong type

Code

declare const a: { a: string; b: number }
declare const b: { a: string } & { b: number }
declare function f(x: { [k: string]: string }): void
f(a) // error
f(b) // NO error

Expected behavior:
Errors on both f(a) and f(b)

Actual behavior:
Errors only on f(a)

Playground Link: http://www.typescriptlang.org/play/#src=declare%20const%20a%3A%20%7B%20a%3A%20string%3B%20b%3A%20number%20%7D%0D%0Adeclare%20const%20b%3A%20%7B%20a%3A%20string%20%7D%20%26%20%7B%20b%3A%20number%20%7D%0D%0Adeclare%20function%20f(x%3A%20%7B%20%5Bk%3A%20string%5D%3A%20string%20%7D)%3A%20void%0D%0Af(a)%20%2F%2F%20error%0D%0Af(b)%20%2F%2F%20NO%20error

Related Issues: gcanti/io-ts#234 (comment)
gcanti/fp-ts-routing#37

[sledorze]

I'm guessing this type relaxation is required in certain scenario (with named members), however, with an indexed type on string (covering both 'a' and 'b'), I would rather expect it to fail.

[jack-williams]

From the spec: S is a subtype of T if S is an intersection type and at least one constituent type of S is a subtype of T. (same for assignability).

• T & U is assignable to X if T is assignable to X.
• { a: string } is assignable to { [k: string]: string }

String indexers are unsound like this when properties are 'forgotten'. Another example:

• { a : string; b: number } is assignable to { a: string }
• { a: string } is assignable to { [k: string]: string }

const a: { a: string; b: number } = { a: 'foo', b: 42 };
const b: { a: string } = a
const c: { [k: string]: string } = c;
c.b = "not a number"
const shouldBeNumber: number = a.b;

[sledorze]

Jack Williams (@jack-williams) indeed.
The amount of consequences of the unsoundness is frightening.

[MartinJohns]

Jack Williams (@jack-williams) Quoting the spec is not advised, as the spec was last updated around 3 years ago.

[jakebailey]

This was fixed by #35143, whose description contains effectively this issue's repro