问Typescript:类型'Ivalue[T]‘不能用于索引类型'{ '1'：数字；'2'：数字；'3'：数字；}’

EN

const C = {
    METHOD: {
        '1': 2,
        '2': 2,
        '3': 3
    },
    CCSIGNFLAG: {
        '4': 4,
        '5': 5,
        '6': 6
    }
};
interface IKey {
    method: '123123123';
    ccSignFlag: '2222';
    [key: string]: string;
}
interface Ivalue {
    method: '1' | '2' | '3';
    ccSignFlag: '4' | '5' | '6';
    [key: string]: string;
}

function test<T extends keyof IKey>(field: T, value: Ivalue[T]) {
    switch (field) {
        case 'method':
            //                       ↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓
            return C['METHOD'][value as keyof typeof C['METHOD']];
         case 'ccSignFlag':
            //                           ↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓
            return C['CCSIGNFLAG'][value as keyof typeof C['CCSIGNFLAG']]
        default:
            return value;
    }
}
test('ccSignFlag', '4');