Assignability question in golang specification

Question

While reading go specification "Assignability" section, I tried to execute a couple of examples to get a better understanding of the topic, and now I can't get what am I doing wrong in my code.

According to the specification, One of the cases when a value x is assignable to a variable of type T is as follows:

x's type V and T have identical underlying types and at least one of V or T is not a defined type.

Defined type specification states that

Type definition creates a new, distinct type with the same underlying type and operations as the given type, and binds an identifier to it.

But when I try to run following code, the build fails:

func main() {
    type Defined int32
    var d Defined
    var i int32
    d = i
}

The output is:

cannot use i (type int32) as type Defined in assignment

Meanwhile, the similar example with composite literal works fine:

func main() {
    type MyMap map[string]int
    var x MyMap 
    var y map[string]int
    x = y
}

playground

Folksy

Answer 1

Also from the spec:

https://golang.org/ref/spec#Numeric_types

To avoid portability issues all numeric types are defined types and thus distinct

Group

So, since both are defined types, they cannot be assigned to each other directly. You can however say d = Defined(i) – Concent

Answer 2

Since type Defined int32 defines a new type, d and i don't have identical types; hence, the first clause x's type is identical to T from the assignability spec isn't applicable. The second clause x's type V and T have identical underlying types and at least one of V or T is not a defined type is not applicable as the types of both i and d are defined types. As the remaining clauses from the assignability spec do not apply in this situation, the assignment fails. Changing type Defined int32 to type Defined = int32 (which aliases a type) fixes the error.

x = y due to the T is an interface type and x implements T clause from the assignability spec is applicable.

Synonymize