接口如何具有可比性，而同时又不能发挥作用？

Question

I have the following code-snippet:

type F func()

type I interface {}

func A() {}

func B() {}

func test() {
    var a interface{} = A
    var b interface{} = B
    if A == B { // 1. Compile error 
        // Code
    }

    if a == b { // 2. No compile error
        // Code
    }
}

If functions are not comparable and interfaces are comparable, why can I assign a function to an interface type?

--

To clarify my question another code-snippet:

type I interface {
    DoSomething()
}

type F func()

func (f F) DoSomething() {
    f()
}

func A() {
    fmt.Println("A")
}

func B() {
    fmt.Println("B")
}

func test() {
    var _a F = A
    var _b F = B
    var a I = _a
    var b I = _b

    if a == b { // 2. No compile error but panic
        // Code
    }
}

It seems to me, that I can break the type system with simple assignments.

I do not propose that functions should be comparable. My questions are:

• Why interfaces are comparable by default?
• Why is it not possible to mark an interface as comparable resp. not comparable?
• Why is it not possible do define user defined-equality?
• What is the motivation in this language design?

Answer 1

This is simply how the language is defined. From the spec:

Interface values are comparable. Two interface values are equal if they have identical dynamic types and equal dynamic values or if both have value nil.

A comparison of two interface values with identical dynamic types causes a run-time panic if values of that type are not comparable.

function values are not comparable. However, as a special case, a [...] function value may be compared to the predeclared identifier nil.

That explains why the first if statement in your example fails at compile time, and why the second one fails at runtime.