another strangeness

[zpdDG4gta8XKpMCd]

// tag types: https://github.com/Microsoft/TypeScript/issues/4895
const enum Tag {} // <-- tag (hopefully nominal) type to indicate some assertion taken place over a string value

function foo(
    values: (string & Tag)[],
    callback: (one: string & Tag) => number // <-- only strings that have satisfied the assertion are welcome here
) {
    return undefined;
}

function bar(one: string): number { // <-- function that doesn't require its parameter to be marked
    return undefined
}

foo([], bar); // <-- no problem... why is bar an ok argument? didn't we ask for a function that requires a string & enum?
// are enums not nominal? why is it ok to ignore it here?

[weswigham]

Function Argument Bivariance

When comparing the types of function parameters, assignment succeeds if either the source parameter is assignable to the target parameter, or vice versa. This is unsound because a caller might end up being given a function that takes a more specialized type, but invokes the function with a less specialized type. In practice, this sort of error is rare, and allowing this enables many common JavaScript patterns.

string & Marker is assignable to string, so (one: string) => number is assignable to (one: string & Marker) => number

[zpdDG4gta8XKpMCd]

oh god... so f...ing annoying, hate typescript

[RyanCavanaugh]

🌹

[zpdDG4gta8XKpMCd]

RIP tag types: #4895