[Go Spec]Confused about Assignability in Golang Specification

[Jimu Yang]

I am reading the doc about Assignability in Golang Specification . It says:

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

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

type str string

func main() {

string1 := "string"

var str1 str = "str1"

str1 = string1 // cannot use string1 (type string) as type str in assignment

}

string1's type string and str have identical underlying type (string) and string is not a defined type. 

So why cannot i assign string1 to str1?

[Jan Mercl]

Both string1 and str1 have defined types while the quoted specs rule says "at least one of V or T is not a defined type.".

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[Ian Lance Taylor]

On Sun, Feb 23, 2020 at 5:19 AM Jimu Yang <jimu...@gmail.com> wrote:
>
> string1's type string and str have identical underlying type (string) and string is not a defined type.

"string" is a defined type. See https://golang.org/ref/spec#String_types.

Ian

[Michael Poole]

According to the language specification (https://golang.org/ref/spec#String_types), string is a defined type, and so it is distinct from your type str.

[Jimu Yang]

Thank you all. 

i read all about defined types on the doc https://golang.org/ref/spec#String_types 

now i know many defined types (string bool int ...      but which are the "undefined" types? i didn't find a direct answer. 

my thinking is that the "literals" which including all type literals are not defined types. The following code confirms that.

could you give me a reference answer? 

type str string
var s str = "sdwf"

// boolean literal

type boolean bool

var b boolean = true

// integer literal

type integer int

var i integer = 1

// slice literal

type strarr []string

var strings []string = []string{"sfw"}

var strs strarr = strings

// pointer literal

type stringptr *string

var string1 string = "11"

var ptr stringptr = &string1

在 2020年2月23日星期日 UTC+8下午9:19:34，Jimu Yang写道：

[Jan Mercl]

Type literals are a prime example of "undefined" types. For example as in `var a []int`.

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