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Polymorphism doesn't work with mapped type args and strictFunctionTypes #27984
Description
TypeScript Version: 3.2.0-dev.20181019
Search Terms: readonly polymorphism mapped types interface extend
Code
interface Base<P> {
method: (args: Readonly<P>) => void;
}
function fn(arg: Base<any>) { }
// It works
fn({} as Base<{ foo: string }>)
// Why error?
interface Child<P> extends Base<P> { }
fn({} as Child<{ foo: string }>)Expected behavior:
No error
Actual behavior:
Error:
Argument of type 'Child<{ foo: string; }>' is not assignable to parameter of type 'Base<any>'.
Types of property 'method' are incompatible.
Type '(args: Readonly<{ foo: string; }>) => void' is not assignable to type '(args: Readonly<any>) => void'.
Types of parameters 'args' and 'args' are incompatible.
Type 'Readonly<any>' is not assignable to type 'Readonly<{ foo: string; }>'.
Property 'foo' is missing in type 'Readonly<any>'.
Playground Link:
Enable strictFunctionTypes in Options:
https://www.typescriptlang.org/play/index.html#src=interface%20Base%3CP%3E%20%7B%0D%0A%20%20%20%20method%3A%20(args%3A%20Readonly%3CP%3E)%20%3D%3E%20void%3B%0D%0A%7D%0D%0A%0D%0Afunction%20fn(arg%3A%20Base%3Cany%3E)%20%7B%20%7D%0D%0A%0D%0A%2F%2F%20It%20works%0D%0Afn(%7B%7D%20as%20Base%3C%7B%20foo%3A%20string%20%7D%3E)%0D%0A%0D%0A%2F%2F%20Why%20error%3F%0D%0Ainterface%20Child%3CP%3E%20extends%20Base%3CP%3E%20%7B%20%7D%0D%0Afn(%7B%7D%20as%20Child%3C%7B%20foo%3A%20string%20%7D%3E)
Related Issues:
Maybe related to #27484
Activity
diegohaz commentedon Oct 19, 2018
Actually, it errors for any mapped type:
https://www.typescriptlang.org/play/index.html#src=interface%20Base%3CP%3E%20%7B%0D%0A%20%20%20%20method%3A%20(args%3A%20%7B%20%5BK%20in%20keyof%20P%5D%3A%20P%5BK%5D%20%7D)%20%3D%3E%20void%3B%0D%0A%7D%0D%0A%0D%0Afunction%20fn(arg%3A%20Base%3Cany%3E)%20%7B%20%7D%0D%0A%0D%0A%2F%2F%20It%20works%0D%0Afn(%7B%7D%20as%20Base%3C%7B%20foo%3A%20string%20%7D%3E)%0D%0A%0D%0A%2F%2F%20Why%20error%3F%0D%0Ainterface%20Child%3CP%3E%20extends%20Base%3CP%3E%20%7B%20%7D%0D%0Afn(%7B%7D%20as%20Child%3C%7B%20foo%3A%20string%20%7D%3E)
[-]Polymorphism doesn't work with Readonly args and strictFunctionTypes[/-][+]Polymorphism doesn't work with mapped type args and strictFunctionTypes[/+]weswigham commentedon Oct 19, 2018
The root cause of the issue seems to be that for
Base<T>, since it's an interface we type, we see that it matches withBase<{ foo: string }>and measure the variance of each type parameter and perform comparisons accordingly, while forChild<T>, despite it being identical toBase<T>, since it is a different reference, we do not do variance measurements, and instead fall back to structural comparisons, which fail (due to howanyworks and the type having a contravariant position).ahejlsberg commentedon Oct 26, 2018
This is fixed by #27804 because we now measure variance of
Readonly<T>and then just relate the types passed forT.