Error "The 'this' context of type 'void' is not assignable to method's 'this' ..." when using an intermediate function instead of "!"

[stephanedr]

Same situation than #23040, but using an intermediate function instead of using "!".

TypeScript Version: 2.9.0-dev.20180410

Search Terms: "'this' context of type 'void' is not assignable"

Code

interface Foo {
    required: ((this: Foo) => void);
    optional: ((this: Foo) => void) | null;
}

function notNull<T>(value: T): NonNullable<T> {
    if (value == null) throw Error("notNull() failed");
    return value as NonNullable<T>;
}

declare const foo: Foo;
foo.required();
foo.optional!();   // Fixed by #23040.
notNull(foo.optional)();  // Error.

Expected behavior:
No Error.

Actual behavior:
The 'this' context of type 'void' is not assignable to method's 'this' of type 'Foo'.

Playground Link: https://www.typescriptlang.org/play/index.html#src=interface%20Foo%20%7B%0D%0A%20%20%20%20required%3A%20((this%3A%20Foo)%20%3D%3E%20void)%3B%0D%0A%20%20%20%20optional%3A%20((this%3A%20Foo)%20%3D%3E%20void)%20%7C%20null%3B%0D%0A%7D%0D%0A%0D%0Afunction%20notNull%3CT%3E(value%3A%20T)%3A%20NonNullable%3CT%3E%20%7B%0D%0A%20%20%20%20if%20(value%20%3D%3D%20null)%20throw%20Error(%22notNull()%20failed%22)%3B%0D%0A%20%20%20%20return%20value%20as%20NonNullable%3CT%3E%3B%0D%0A%7D%0D%0A%0D%0Adeclare%20const%20foo%3A%20Foo%3B%0D%0Afoo.required()%3B%0D%0Afoo.optional!()%3B%20%20%20%2F%2F%20Fixed%20by%20%2323040.%0D%0AnotNull(foo.optional)()%3B%20%20%2F%2F%20Error.%0D%0A

Related Issues: #23040

That code will actually fail at runtime:

interface Foo {
    required: ((this: Foo) => void);
    optional: ((this: Foo) => void) | null;
}

function notNull<T>(value: T): NonNullable<T> {
    if (value == null) throw Error("notNull() failed");
    return value as NonNullable<T>;
}

const foo: Foo = {
    required() {},
    optional() {
        this.required();
    },
};
(notNull(foo.optional) as any)();  // Error.

Fails with TypeError: Cannot read property 'required' of undefined, because the call notNull(foo.optional)() doesn't have foo as the this receiver.

[stephanedr]

Indeed, at JS level, call() has to be called, so ideally generated by TS.

The reason is that ! is a "blind" non-null assertion, whereas I definitely prefer checking that, whenever I assume something, it is really true (notNull() is a debug feature that is removed in release mode).
And if I use call() I lose the benefit of argument checking...
Additionally, if one day conditional compilation is implemented (#3538), we'll face the same kind of issue.

foo.optional!() gets a runtime check too -- it throws a TypeError if foo.optional was undefined.

[stephanedr]

Andy (Andrewkraft) (@Andy-MS), fully agree, but in fact my notNull() implementation throws a specific AssertionError (as well as other debug functions) that are reported/routed differently than other errors.

[stephanedr]

To be clearer, AssertionErrors are necessarily programming errors, so directly reported to developers, whereas other errors may be due to user errors.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.