TypeScript Version: 2.8.0-dev.20180315

Search Terms:
infer conditional generic function

Code

type Id = <T>(item: T) => T;

type X = ((t: Id) => void) extends (t: (item: any) => any) => void ? "yes" : "no";
// Resolves to "yes", as expected

type Y = ((t: Id) => void) extends (t: (item: any) => infer R) => void ? "yes" : "no";
// Resolves to "no", but "yes" is desired

The code checks whether a function that takes the identity function is assignable to a function that takes some function any => any. This is clearly assignable, and thus results in "yes". However, when replacing one any by infer R, the other branch is taken. I think that replacing any by an infer R type should never change the branch that is taken.

{
	"compilerOptions": {
		"allowJs": true,
		"target": "es6",
		"module": "commonjs",
		"outDir": "dest",
		"strictNullChecks": true,
		"jsx": "preserve",
		"strictFunctionTypes": true
	}
}

Expected behavior:
X and Y resolve to "yes"

Actual behavior:
Y resolves to "no"

Related Issues: