英文:
go refspec: assignability of defined/undefined types
问题
https://golang.org/ref/spec#Assignability中提到：
> 如果满足以下条件之一，值x可以分配给类型为T的变量（“x可以分配给T”）：
> * x的类型V和T具有相同的底层类型，并且V或T中至少有一个不是定义类型。
> * ...
我在测试这个时遇到了问题。 “assignability”是否允许类型转换（我猜是的）？
如果是这样，为什么
https://play.golang.org/p/Y-U5ruLH_bd
能够编译，即使type_x和type_y都是定义类型？
如果assignability的概念不允许类型转换，为什么
https://play.golang.org/p/dHDq9nnWSMC
不能编译，即使int和type_x具有相同的底层类型？
英文:
https://golang.org/ref/spec#Assignability 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.
> * ...
I have trouble testing this. Does "assignability" allow for casting (I guess yes)?
If so, why does
https://play.golang.org/p/Y-U5ruLH_bd
compile, even though both type_x and type_y are defined types?
If concept of assignability does not allow casting, why does
https://play.golang.org/p/dHDq9nnWSMC
NOT compile, even though int and type_x have same underlying types?
答案1
得分: 2
我相信你正在寻找的是 https://golang.org/ref/spec#Conversions。
简而言之，你不是将类型为 type_y 的值直接赋给类型为 type_x 的变量，而是在赋值之前将该值转换为 type_x 类型。转换的限制要宽松得多。
直接将 y 的值赋给 x 是不被编译器允许的，原因就是你提供的链接中所述。
英文:
I believe https://golang.org/ref/spec#Conversions was what you were looking for.
In short, you are not assigning a value of type type_y to a variable of type type_x, but rather converting that value to one of type type_x before assigning it. The restrictions on conversions are a lot less tight.
Directly assigning the value of y to x will not be allowed by the compiler, for the reasons you linked to.
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