A strange invalid recursive type error

[jsc]

Hi All,

I found a strange “invalid recursive type” compile error. The code will occur “invalid recursive type” error by run go build.

package main

func main() {
}

type A struct {
	i I
}

type B[T I] struct {
	i I
}

type I interface {
	funcA(*B[I])
}

When I change the declaration ordering, put B to the top, go build will work！
Is a go bug?

package main

func main() {
}

type B[T I] struct {
	i I
}

type A struct {
	i I
}

type I interface {
	funcA(*B[I])
}

[christophberger]

Hi @jsc, welcome to the forum.

The generic definition of B is not necessary.
This compiles fine:

package main

func main() {
}

type A struct {
	i I
}

type B struct {
	i I
}

type I interface {
	funcA(*B)
}

(Also, struct B should use the type parameter T inside:

type B[T I] struct {
	i T
}

Although this does not change the error message.)

What do you want to achieve by parametrizing struct B?

[bluefire]

The “invalid recursive type” error occurs in your code because you have defined a recursive type in the interface declaration. In your example, the I interface refers to the B generic type parameter T, which in turn refers back to I, creating a recursive dependency.

To resolve this issue, you can modify your code by removing the recursive reference. Here’s an updated version of your code without the recursive type:

package main

func main() {
}

type A struct {
	i I
}

type B[T any] struct {
	i T
}

type I interface {
	funcA(*B[I])
}

In the updated code:

• The generic type parameter T of the B struct is replaced with T any to allow any type to be used.
• The I interface is defined as before, referring to *B[I] in the funcA method.

By removing the recursive dependency in the type definitions, you should no longer encounter the “invalid recursive type” error when building your code.