function myValueArg(arg: "value") { }

function myStringArg(arg: string) { }

namespace ns { // Not needed, but shows it's not scope related
    let str = "value";

    if (str === "value") {
        myValueArg(str);
        [].forEach(() => {
            myValueArg(str); // Argument of type 'string' is not assignable to parameter of type '"value"'.
        });
    } else {
        myStringArg(str);
        [].forEach(() => {
            myStringArg(str);
        });
    }
}