TS 3.7: unlike `x is T`, `asserts x is T` cannot close over generics defined in outer scopes

TypeScript Version: 3.7.0-dev.20191016

Search Terms:
asserts, asserts return, asserts higher order, asserts type, 2775

Code

function literal<T extends keyof any>(lit: T): {
    is(value: any): value is T
    assert(value: any): asserts value is T
} {
    return null as any; // implementation doesn't matter
}

const isHi = literal("hi")
const x: unknown = "test"

if (isHi.is(x)) {
    console.log(x) // x is correctly inferred to be 'hi' :)
}

isHi.assert(x); // error: Assertions require every name in the call target to be declared with an explicit type annotation.(2775)
console.log(x); // x should be inferred to be 'hi' here :(

Expected behavior:

No compile error, x is inferred to be "hi" on the last line.

Actual behavior:

Compile error on isHi.assert. Assertions require every name in the call target to be declared with an explicit type annotation.(2775)

Construction higher order type guards is possible without problem (as shown in the snippet). This mechanism used heavily in libraries like io-ts and mobx-state-tree.

However, when trying to extend the latter library with assertion functionality for more convenient control flow, we run into this issue.

We can build type.is properly, but not type.assert, although they seem to be needing the exact same type / depth of type analysis; if type guards can close over T, so should type assertions?

Playground Link: link

Related Issues:

#34523

Beginne mit der verknüpften TypeScript Playground-Reproduktion und prüfe das zugehörige Issue #34523, wobei du diagnostic 2775 für das Beispiel mit der höhergeordneten Assertion bestätigst. Die Arbeit ist abgeschlossen, wenn das Beispiel ohne diesen Fehler kompiliert wird und die letzte Verwendung von x als "hi" inferiert wird.