type a = { a: string };

type b = { b: number };

type c = { a: number };

type d = { a: boolean };

type ComputeIntersection<O> = { [k in keyof O]: O[k] };

type test1 = ComputeIntersection<a & b>; 
// test1 =>  { a: string ; b:number}  That's what I expected

type test2 = ComputeIntersection<a & c>; 
// test2 => { a: never } ，That's what I expected. just like: test2 = { a: string & number }

type test3 = ComputeIntersection<a & c & d>;

// test3 = > never . This puzzles me，According to test2,  test3 is supposed to be "{ a: string & number & boolean}" The result is still "{ a:never }"

// Replace a & c with test2 .
type test4 = ComputeIntersection< test2 & d>;
// test4 => { a : never }. How do we understand this!