How do I exclude an index signature from a type?

Answer

There is no general way to do this for something that has an index signature.

Alternative

Get the keys before adding the index signature:

interface Foo {}

interface BarCore {
    a: string;
    b: boolean;
    c: Foo;
}


type Bar = BarCore & {
    [index: string]: any;
}

type X = keyof BarCore; // a|b|c

More

PS: try not to mix index signatures with valid prop at root level. Instead use the nested object pattern