为什么从当前程序包将结构传递给具有文字struct参数的函数与从另一个程序包传递的函数不同？

Question

This works perferctly:

package main

import "fmt"

type Struct struct {
    field string
}
func Fn(arg struct{field string}) {
    fmt.Println(arg)
}

func main() {
    Fn(Struct{"john"})
}

But this gives ./main.go:12: cannot use Struct literal (type Struct) as type struct { field string } in argument to sub.Fn:

main.go

package main

import "go_tests/sub"

type Struct struct {
    field string
}

func main() {
    sub.Fn(Struct{"john"})
}

sub/sub.go

package sub

import "fmt"

func Fn(arg struct{field string}) {
    fmt.Println(arg)
}

Only change in function call is that Fn(Struct{"john"}) was replaced with sub.Fn(Struct{"john"}).

Why moving function to another package affects types logic? Links to the docs will be appreciated.

Answer 1

You need to export your struct field:

type Struct struct {
    Field string
}

and then also change the call to use the exported field:

func Fn(arg struct{Field string}) {
    fmt.Println(arg)
}

From the language spec (specifically the last sentence):

For struct literals the following rules apply:

• A key must be a field name declared in the LiteralType.
• An element list that does not contain any keys must list an element for each struct field in the order in which the fields are declared.
• If any element has a key, every element must have a key.
• An element list that contains keys does not need to have an element for each struct field. Omitted fields get the zero value for that field.
• A literal may omit the element list; such a literal evaluates to the zero value for its type.
• It is an error to specify an element for a non-exported field of a struct belonging to a different package.