Merge same function signatures for intersection types

[saschanaz]

TypeScript Version: 3.6.0-dev.20190611

Search Terms: function signature merge

Code

interface X { 
    name: string;
    method(): void;
}

interface Y { 
    index: number;
    method(): void;
}

var z: X & Y;

z.method(); // <--- Put your cursor here

Expected behavior: IntelliSense should ideally show only one signature.

Actual behavior: It shows two exactly same signatures. Not harming, but weird.

Playground Link: https://www.typescriptlang.org/play/#src=interface%20X%20%7B%20%0D%0A%20%20%20%20name%3A%20string%3B%0D%0A%20%20%20%20method()%3A%20void%3B%0D%0A%7D%0D%0A%0D%0Ainterface%20Y%20%7B%20%0D%0A%20%20%20%20index%3A%20number%3B%0D%0A%20%20%20%20method()%3A%20void%3B%0D%0A%7D%0D%0A%0D%0Avar%20z%3A%20X%20%26%20Y%3B%0D%0A%0D%0Az.method()

[jcalz]

This is equivalent to asking for repeated overload signatures to be collapsed. Which seems plausible for adjacent overloads and less plausible when distinct signatures appear between them.

[millsp]

@saschanaz I created ts-toolbelt for this purpose

import {O} from 'ts-toolbelt'

interface X { 
    name: string;
    method(): void;
}

interface Y { 
    index: number;
    method(): void;
}

type Z = O.Merge<X, Y>
var z: Z

The last in wins (and, it is compatible with overloads). We should not alter the behavior of &, in my opinion.