Composite Type Subtyping

Two composite types are compatible if and only if they refer to the same declaration by name, i.e., nominal typing applies instead of structural typing.

Even if two composite types declare the same fields and functions, the types are only compatible if their names match.

// Declare a structure named `A` which has a function `test`
// which has type `((): Void)`.
//
struct A {
    fun test() {}
}

// Declare a structure named `B` which has a function `test`
// which has type `((): Void)`.
//
struct B {
    fun test() {}
}

// Declare a variable named which accepts values of type `A`.
//
var something: A = A()

// Invalid: Assign a value of type `B` to the variable.
// Even though types `A` and `B` have the same declarations,
// a function with the same name and type, the types' names differ,
// so they are not compatible.
//
something = B()

// Valid: Reassign a new value of type `A`.
//
something = A()