enum TYPES {
    ONE = 'one',
    TWO = 'two'
}

interface IOne {
    a: string,
     type: TYPES.ONE
     meta: {
         type: TYPES.ONE
     }
}

interface ITwo {
    b: string,
    type: TYPES.TWO
    meta : {
        type: TYPES.TWO
    }
}

[].map((block:IOne | ITwo) => {
    // This works.
    switch (block.type) {
        case TYPES.ONE:
            block.a 
            break;
        case TYPES.TWO:
            block.b 
            break;
    }

    // This does NOT work.
    switch (block.meta.type) {
        case TYPES.ONE:
            block.a 
            break;
        case TYPES.TWO:
            block.b 
            break;
    }
});