So I have the following type A, descriminated by the property ‘type’:

type A = { type: 'x', foo: boolean } | { type: 'y', bar: string }

with this type I can make objects such as

{
  type: 'x',
  foo: false
}

or 

{ 
  type: 'y',
  bar: "hmm"
}

and I have a type B, which takes the { type: ‘x’ } discriminated union from A, and allows for an extra baz

type B = Extract<A, { type: 'x' }> & { baz: number }

This type B will allow for objects such as

{
  type: 'x'
  foo: false,
  baz: 3
}

Now, I want to merge this B back into another type, that will basically overwrite the union discriminated by ‘x’ with the definition from B, but leave the original ‘type: y’ from A intact. So like

{
  type: 'x'
  foo: false,
  baz: 3
}

or 

{ 
  type: 'y',
  bar: "hmm"
}