Now we get to tie a bunch of the past concepts together in a more concrete way, with assignability. Which types of values can be assigned to which types of variables? That’s what this is all about. It’s mostly intuitive, and in fact, I virtually never find myself wondering about these details while coding. But the details do matter. So let’s look at them, with examples.

Assignability

A value x of type V is assignable to a variable of type T (“x is assignable to T”) if one of the following conditions applies:

• V and T are identical.

var x int = 3
var y int = x // Identical types

• V and T have identical underlying types but are not type parameters and at least one of V or T is not a named type.

var x int = 3
type MyInt int
var y MyInt = x // Identical underlying types

• V and T are channel types with identical element types, V is a bidirectional channel, and at least one of V or T is not a named type.

var x chan int // x's type `V` is `chan int`
var y chan<- int = x // Identical element types, V is bidirectional

• T is an interface type, but not a type parameter, and x implements T.

var x *os.File
var y io.Reader = x

• x is the predeclared identifier nil and T is a pointer, function, slice, map, channel, or interface type, but not a type parameter.

var a *int = nil
var b func() error = nil
var c map[string]any = nil
var d chan int = nil
var e io.Reader = nil

• x is an untyped constant representable by a value of type T.

const x 1234
var y int = x
var y2 float64 = x

Quotes from The Go Programming Language Specification Version of December 15, 2022