为什么此分配的类型不匹配？

Question

Why isn't this a type mismatch?

From: https://golang.org/ref/spec#Assignability

A value x is assignable to a variable of type T ("x is assignable to T") in any of these cases: ...snip... x's type V and T have identical underlying types and at least one of V or T is not a named type. ...snip...

Is that because the underlying type of N[] is N[] which is not a named type?

What's the rationale behind it?

package main

import "fmt"

type N []N

func main() {
    n := make([]N, 1)
    fmt.Printf("%T
", n)
    fmt.Printf("%T
", n[0])
    n[0] = n
    //fmt.Println(n)
}

*Output:*
[]main.N
main.N

Answer 1

You're asking if n[0] = n is valid. You've correctly identified the rule from the language spec:

A value x is assignable to a variable of type T ("x is assignable to T") in any of these cases:

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

And here's how it applies here:

• n[0] has type N and underlying type []N (from the language specification: "the type to which N refers in its type declaration").
• n has type []N (with the same underlying type).

So n[0] and n have identical underlying types ([]N), and the type of n is not a named type.