fix(FastCheck): missing namespace on function objects

[marvinhagemeister]

This PR addresses an issue with FastCheck where the following snippet would lead to a type error:

export interface it {
  skip(): void
}
export function it() {}
it.skip = () => {}

it.skip()

Internally, TypeScript generates a namespace for it which tells the compiler that the function has a skip property. The inteface itself is interestingly ignored. This lead to us erroring.

The solution is to generate the namespace declaration which we need to anyway for the dts transform.

export interface it {
    skip(): void;
}
export function it(): void {}
export namespace it {
    var skip: () => void;
}

This matches the exact output that tsc generates.

Fixes denoland/deno#23276

[marvinhagemeister]

This fixes the issue, but it likely needs further iteration. Especially in regards to maybe_infer_type_from_expr which I feel like should return a Return type. What do you think @dsherret ?

[marvinhagemeister]

Okay, went ahead and made maybe_infer_type_from_expr return a Result. To fully support the original pattern in the issue I also expanded our capabilities to infer the type from as const expressions.

[dsherret]

Looks great, but we need to make this work for namespaces.

[dsherret]

We should add this crazy test case and make it work:

export function test() {
}

if (true) {
    test.skip = () => {};
}

test.skip();

export namespace Test {
    export function test() {
    }

    test.other = () => {};

    test.other();
}

export namespace Test {
    test.other2 = () => {};

    test.other2();
}

This is TypeScript's dts output:

export declare function test(): void;
export declare namespace test {
    var skip: () => void;
}
export declare namespace Test {
    function test(): void;
    namespace test {
        var other: () => void;
        var other2: () => void;
    }
}
export declare namespace Test {
}

[dsherret]

We'll save this one for later.

[dsherret]

See here how the myFunc symbol now has these expando properties as exports.