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Assignability of callback parameter defined with conditional tuple type #26013

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@bterlson

Description

@bterlson

TypeScript Version: 3.1.0-dev.20180727

Code

// ConditionalTuple could also return just [] in the false branch, issue still repros
type ConditionalTuple<T> = [T] extends [undefined] ? [] : [T];

// works fine!
declare var f1: (...args: any[]) => void;
declare var f2: <T>(...args: ConditionalTuple<T>) => void;
declare var f3: (...args: ConditionalTuple<number>) => void;
f2 = f1;
f3 = f1;

// but if we move the signatures above into a callback parameter with that signature,
// assignability breaks - the types of cb are incompatible.
declare var cb1: (cb: (...args: any[]) => void) => void;
declare var cb2: <T>(cb: (...args: ConditionalTuple<T>) => void) => void;
declare var cb3: (cb: (...args: ConditionalTuple<number>) => void) => void;

cb2 = cb1; // can't assign to cb2
cb3 = cb1; // this works though

Expected behavior:
I am able to construct callback signatures with generic conditional types and tuples that are assignable with the maximally general callback signature (cb1).

Actual behavior:
cb1 cannot be assigned to cb2, but removing the generic parameter ala cb3 makes assignability work as expected.

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    Working as IntendedThe behavior described is the intended behavior; this is not a bug

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