Redundant intersection type from overloaded function signature

[JasonKleban]

TypeScript Version: 2.8.1

Search Terms:

redundant intersection conditional type function overload

Code

interface ExampleFn {
    (parameters: { a: string, b: string }, option: string[]) : void
    (parameters: { a: string, b: string }, option: string) : void
}

type Implementation<P> = {
    (parameters: P, option: string[]) : void
    (parameters: P, option: string) : void
}

type Factory<X> = 
    X extends Implementation<infer P>
    ? (parameters: P, option: string | string[]) => void
    : never

declare const x : Factory<ExampleFn>;

x()  // const x: (parameters: { a: string; b: string; } & { a: string; b: string; }, option: string | string[]) => void

Expected behavior:

Inferred type should be reduced:

// const x: (parameters: { a: string; b: string; }, option: string | string[]) => void

Actual behavior:

Inferred type is a redundant union:

// const x: (parameters: { a: string; b: string; } & { a: string; b: string; }, option: string | string[]) => void

It works as expected if parameters is string instead of the composite type { a, b }

Playground Link:

https://www.typescriptlang.org/play/#src=%0D%0Ainterface%20ExampleFn%20%7B%0D%0A%20%20%20%20(parameters%3A%20%7B%20a%3A%20string%2C%20b%3A%20string%20%7D%2C%20option%3A%20string%5B%5D)%20%3A%20void%0D%0A%20%20%20%20(parameters%3A%20%7B%20a%3A%20string%2C%20b%3A%20string%20%7D%2C%20option%3A%20string)%20%3A%20void%0D%0A%7D%0D%0A%0D%0Atype%20Implementation%3CP%3E%20%3D%20%7B%0D%0A%20%20%20%20(parameters%3A%20P%2C%20option%3A%20string%5B%5D)%20%3A%20void%0D%0A%20%20%20%20(parameters%3A%20P%2C%20option%3A%20string)%20%3A%20void%0D%0A%7D%0D%0A%0D%0Atype%20Factory%3CX%3E%20%3D%20%0D%0A%20%20%20%20X%20extends%20Implementation%3Cinfer%20P%3E%0D%0A%20%20%20%20%3F%20(parameters%3A%20P%2C%20option%3A%20string%20%7C%20string%5B%5D)%20%3D%3E%20void%0D%0A%20%20%20%20%3A%20never%0D%0A%0D%0Adeclare%20const%20x%20%3A%20Factory%3CExampleFn%3E%3B%0D%0A%0D%0A%0D%0Ax%20%20%2F%2F%20const%20x%3A%20(parameters%3A%20%7B%20a%3A%20string%3B%20b%3A%20string%3B%20%7D%20%26%20%7B%20a%3A%20string%3B%20b%3A%20string%3B%20%7D%2C%20option%3A%20string%20%7C%20string%5B%5D)%20%3D%3E%20void

Related Issues:

none found

[jack-williams]

Related to this? #7705

The display of intersection type does not normalise object literal types.

type X = { x: number } & {x: number };

Hover over X and get { x: number } & {x: number }.

[JasonKleban]

@jack-williams hard to find related items when I confuse union for intersection!

[JasonKleban]

related to #20267