Infer of extended generic type

[vlazh]

TypeScript Version: 3.0.1

Code

type JSONModel<T> = T extends object ? string : number;

interface IBase<T> {
  p: JSONModel<T>; // when don't use JSONModel<T> then infered correctly
}

interface I extends IBase<{ a: number }> {}

type A = I extends IBase<infer T> ? T : never;

Expected behavior:
type A is { a: number }

Actual behavior:
type A is {}

[nmain]

This is a similar problem to the unused generic type parameter issue, covered in the Typescript FAQ with various examples: https://github.com/Microsoft/TypeScript/wiki/FAQ#generics

What's happening here is that while T is used in JSONModel, it's not really used. Any T that extends object will give the exact same resulting type:

type Foo = JSONModel<{}> // type Foo = string
type Bar = JSONModel<{ a: number }> // type Bar = string
type Baz = JSONModel<{ french: "bread" }> // type Baz = string

This makes your example go bad when you ask for I extends IBase<infer T>. The actual structural type of I is simply { p: string; }, and it would be the same whether {} or { a: number } was used for T, so Typescript has no way to know what you want it to infer, and the inference it made is correct. Changing p to T or adding another property z: T fixes this because now in order to satisfy I extends IBase<infer T>, { a: number } must be used.

[vlazh]

Changing p to T or adding another property z: T fixes this

Yes, it fixes when I add another property of type T.
Thank you, @nmain!