Generics: False negative on type constraint validation involving a conditional type

[UselessPickles]

TypeScript Version: 2.9.2

Search Terms:
conditional type constraint

Code

declare class Model<M extends MR, MR extends {}> {
    // Compiler error: 
    // Type 'M[K]' does not satisfy the constraint 'K extends keyof MR ? MR[K] : M[K]'.
    public getField2<K extends keyof M>(): Field<M[K], K extends keyof MR ? MR[K] : M[K]>
}

declare class Field<T extends TR, TR> {
}

Expected behavior:
No compiler error.

Because M extends MR, and K extends keyof M:

• If K extends keyof MR, then I expect M[K] to satisfy the constraint MR[K].
• Else, I expect M[K] to satisfy the constraint M[K].
• Therefore, it seems that M[K] should satisfy the constraint K extends keyof MR ? MR[K] : M[K]

Actual behavior:
Compiler error: Type 'M[K]' does not satisfy the constraint 'K extends keyof MR ? MR[K] : M[K]'.

Playground Link:
https://www.typescriptlang.org/play/#src=declare%20class%20Model%3CM%20extends%20MR%2C%20MR%20extends%20%7B%7D%3E%20%7B%0D%0A%20%20%20%20%2F%2F%20Compiler%20error%3A%20%0D%0A%20%20%20%20%2F%2F%20Type%20'M%5BK%5D'%20does%20not%20satisfy%20the%20constraint%20'K%20extends%20keyof%20MR%20%3F%20MR%5BK%5D%20%3A%20M%5BK%5D'.%0D%0A%20%20%20%20public%20getField2%3CK%20extends%20keyof%20M%3E()%3A%20Field%3CM%5BK%5D%2C%20K%20extends%20keyof%20MR%20%3F%20MR%5BK%5D%20%3A%20M%5BK%5D%3E%0D%0A%7D%0D%0A%0D%0Adeclare%20class%20Field%3CT%20extends%20TR%2C%20TR%3E%20%7B%0D%0A%7D

Related Issues:
Possibly?

• Conditional types works incorrectly when generics with constraints #24190
• Bug with conditional types and constraints #23843

[UselessPickles]

Interesting followup. As of now, at least, with TypeScript 3.3.3, I can get the correct behavior with a small change: use MR extends Record<string, any> instead of MR extends {} as my type constraint. The original MR extends {} seems to cause problems because type {} has no keys:

Type 'M[K]' does not satisfy the constraint 'K extends keyof MR ? MR[K] : M[K]'.
Type 'MR[K]' is not assignable to type 'K extends keyof MR ? MR[K] : M[K]'.
Type '{}[K]' is not assignable to type 'K extends keyof MR ? MR[K] : M[K]'.

The change to MR extends Record<string, any> seems to clarify that MR is guaranteed to have string keys.

I'm still a bit unclear on when it is appropriate to use type {} as a constraint. Is there still a bug that needs to be investigated to get my original code to compile as I expect it to, or is the change to MR extends Record<string, any>actually the correct solution?

[jack-williams]

I think should be covered by #26933 provided you convert the conditional type to be non-distributive.

[UselessPickles]

@jack-williams I'm not familiar with what a "distributive" vs "non-distributive" type is. Could you elaborate on what you mean by "convert the conditional type to be non-distributive."?

[jack-williams]

@UselessPickles Here is a relevant comment that should have the information (or at least point it).

[UselessPickles]

Thanks. I like the trick for preventing distribution (in this comment, referenced by the comment you pointed me to) :)

[weswigham]

@jack-williams is correct - #26933 + using a non-distributive conditional does fix this. The reason a distributive conditional can't be assumed to work is because the K in either branch of K extends keyof MR ? MR[K] : M[K] is some specific subtype of K (thanks to distribution), while the K in M[K] is just the input K. So if, for example, your input K was "ok" | "not ok", the K in each branch could be one of "ok" or "not ok", and the union of both is not assignable to each individually (it contains the other option, ofc).