Ts Helpers

ts.tsx

/**============== Avoid doing things weird with never ===========================*/

type Exact<A, B> = A extends B ? (B extends A ? A : never) : never;

// This basically says that if and A extends B and B extends A then the types are identical, 
// neither are a subset or superset of the other. 
// So it should allow that type through. 
// If they are not identical, then the type is never which prevents that type from being allowed.
// Now you just need to make your function generic, and enforce that argument to exactly the right type:

function func<T>(param: Exact<T, LayoutType>) {
    console.log(param);
}

/**************************************************/

type IFoo = {
  bar: string; can?: never
} | {
    bar?: never; can: number
  };


let val0: IFoo = { bar: "hello" } // OK only bar
let val1: IFoo = { can: 22 } // OK only can
let val2: IFoo = { bar: "hello",  can: 22 } // Error foo and can
let val3: IFoo = {  } // Error neither foo or can