Typescript: 3.5 regression: union type as property accessor break function parameter inference

[fregante]

TypeScript Version: 3.5.2 (it works with 3.4.4). Also tested on 3.6.0-dev.20190622

Search Terms:

Code (not real-life, just a repro)

const actions = ['resizeTo', 'resizeBy'] as const;
for (const action of actions) {
    window[action] = (x, y) => {
        window[action](x, y);
    }
}

Expected behavior:

x and y are of type number (like in 3.4)

Actual behavior:

x and y are of type any

Playground Link: (enable noImplicitAny, but it seems to be on 3.4)

https://www.typescriptlang.org/play/#src=%0D%0Aconst%20actions%20%3D%20%5B'resizeTo'%2C%20'resizeBy'%5D%20as%20const%3B%0D%0Afor%20(const%20action%20of%20actions)%20%7B%0D%0A%09window%5Baction%5D%20%3D%20(x%2C%20y)%20%3D%3E%20%7B%0D%0A%09%09window%5Baction%5D(x%2C%20y)%3B%0D%0A%09%7D%0D%0A%7D

Related Issues:

https://github.com/microsoft/TypeScript/issues/32037

[fregante]

This is probably due to https://github.com/microsoft/TypeScript/pull/30769

But the types of window.resizeTo and window.resizeBy are identical so there shouldn't be any difference or ambiguity.

You can find a real-life example here: https://github.com/sindresorhus/refined-github/pull/2164

[ahejlsberg]

Yes, this is caused by #30769. When an indexed access type occurs on the target side of an assignment, we form an intersection instead of a union. When intersecting properties of the same type we just end up with a property of that type, however we don't remove duplicate call/construct signatures. So, in your example the intersection ends up being a type with two (identical) call signatures, at which point we no longer do contextual typing of parameters.

We can fix the issue by removing duplicate call/construct signatures in an intersection.

[fregante]

Great!

[fregante]

I can confirm that this was fixed and works as expected in 3.6.0-dev.20190701

Thank you!