Generic parameter types are incorrectly inferred as { }

[2A5F]

TypeScript Version: 3.0.1, vscode 1.26.1

Search Terms:

Code

interface A<R> {
 a(): R
}

function some1<R>(a: A<R>): R {
 return null
}

function some2<R, D extends A<R>>(a: D): R {
 return null
}

let out1 = some1({
 a() {
 return 'asd'
 }
})

let out2 = some2({
 a() {
 return 'asd'
 }
})

Expected behavior: out1: string, out2: string

Actual behavior: out1: string, out2: {}

Playground Link

[AlCalzone]

Seems there is not enough context for TS to infer R in some2, since you are not passing any argument with type R. You can fix this by explicitly telling TypeScript to infer R from the interface:

interface A<R> {
 a(): R
}

declare function some2<D extends A<any>>(a: D): D extends A<infer R> ? R : any;

let out2 = some2({
 a() {
 return 'asd'
 }
})
// out2 is string!

[2A5F]

But why would it be {} instead of an error

[AlCalzone]

As far as I understand, {} is the default when inference fails. This keeps the type-checking enabled - as opposed to any, which doesn't.