Compiler cannot infer the type of the visitor method parameter if you use enum values as the property names.

[UselessPickles]

Cannot infer the type of the visitor method parameter if you use the enum value as the property name.

I reported this issue on the github TypeScript repo and was promptly informed that the issue is already fixed in development for the "next" version of TypeScript (currently named v2.7.0-dev).

There are two work-arounds to this:

1. Explicity declare the type of the parameter.
2. Use a string literal for the property name.

Example:

enum RGB {
    R = "r",
    G = "g",
    B = "b"
}

function rgbIdentity(rgb: RGB): RGB {
    return visitString(rgb).with<RGB>({
        // ERROR: type of `value` not inferred properly!
        // Type is implicitly 'any'
        [RGB.R]: (value) => {
            return value;
        },
        // Work-around option #1
        [RGB.G]: (value: RGB.G) => {
            return value;
        },
        // Work-around option #2
        "b": (value) => {
            return value;
        }
    });
}

After the new version fo TypeScript is released, add a unit test for this situation.

[UselessPickles]

There's a commented-out sample file that needs to be uncommented to enable unit testing of this situation (after upgrading TypeScript): tests/compile_samples/pass/enum.enum-props.inferred-param-type.ts

[UselessPickles]

This is no longer an issue as of TypeScript 2.7.1