Question about `Struct.evolve` and Type Widening Prevention

const Letter = S.Literal("A", "B", "C")

type Foo = {
  letter: typeof Letter.Type
}

const foo: Foo = {
  letter: "A",
}

const nextFoo: Foo = Struct.evolve(foo, {
  letter: () => "B",
})

// Diagnostics:
// 1. Type '{ letter: string; }' is not assignable to type 'Foo'.
//      Types of property 'letter' are incompatible.
//        Type 'string' is not assignable to type '"A" | "B" | "C"'. [2322]