Equivalent Interfaces - why does this fail to compile?

[Amnon]

See https://play.golang.org/p/5x9JrD55WKc

The interfaces aer and ber look equivalent.
Both contain a function which returns nothing called m1
and a function which returns another an instance of the interface m2.

If we comment out m2, then the code will compile.
But otherwise we get an error:

./prog.go:14:5: cannot use a (type aer) as type ber in argument to bar: aer does not implement ber (wrong type for m2 method) have m2() aer want m2() ber

even though aer and ber should be equivalent!

package main

type aer interface {

m1()

m2() aer

}

type ber interface {

m1()

m2() ber

}

func Foo(a aer) {

bar(a)

}

func bar(b ber) {

}

func main() {

}

[Axel Wagner]

This is answered in the FAQ: https://golang.org/doc/faq#covariant_types

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[Alexander Verbitsky]

Interfaces are not equal: "m2() aer" and "m2() ber" has different return type. Even compiler told you about it.

[Amnon]

yes, but surely the return types are equivalent?

[Axel Wagner]

No, they are not. They are both defined types and as such, are not identical.

To repeat, this is answered in the FAQ.

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