type A_or_C_Child = A | C_Child;

interface A {
  a: string;
}

interface B {
  b: string;
}

interface C {
  c: string;
}

interface C_Child extends A, C {
  extra: string;
}

declare function isA(it: any): it is A;
declare function isB(it: any): it is B;
declare function isC(it: any): it is C;
declare function isA_or_C_Child(it: any): it is A_or_C_Child;

let item: any;
if (isA_or_C_Child(item) && !isC(item) && isB(item)) {
  item; // inferred as A & B
}

if (isA_or_C_Child(item) && isB(item) && !isC(item)) {
  item; // inferred as A_or_C_Child & B
}