why does go2go permits this code?

go2go was only created to experiment with the type parameters proposal when no implementation existed, the fact that it does or does not permit some piece of code doesn't mean much in itself.

cc @griesemer

The error message is pretty clear:

x.go:18:11: a.GetFooer undefined (type *FG has no field or method GetFooer)

The type *FG (pointer to type parameter) doesn't have a GetFooer method, but the type FG does. This code compiles:

package main

import "fmt"

type Fooer interface{ Foo() }

type FooGetter[F Fooer] interface{ GetFooer() F }

type A struct{}

func (A) GetFooer() B { return B{} }

type B struct{}

func (B) Foo() {}

func fun[F Fooer, FG FooGetter[F]](a FG) F { // <<< FG not *FG
	return a.GetFooer()
}

func main() {
	fmt.Printf("%T", fun[B, A](A{})) // <<< A{} not &A{}
}

There's no automatic indirection for pointers to type parameters - maybe there should be (we're still fine-tuning those rules).

I don't know why your original version works on the go2go playground - possibly because the translated code does the automatic indirection (because it's been translated into regular Go types). But it doesn't really matter - we're not going to dig into that - the playground was for experimentation only is not kept up tp date. I recommend you use the latest compiler as the ground truth at this point. Thanks.

Closing as there is nothing to do here.

@griesemer sorry to bother you a little more, but what is the correct way to require pointers on callee side?

Pass a pointer type argument in main: fmt.Printf("%T", fun[B, *A](&A{}))
Also, see the section on constraint type inference. Thanks.