What version of Go are you using (go version)?

$ go version
go2goplay.golang.org
 devel +5f01333bf1 Wed Jun 17 04:20:54 2020 +0000. 

What did you do?

Try to run this code
https://go2goplay.golang.org/p/bBE2ZO_LP36

package main

import (
	"fmt"
)

type List(type T) interface{
	Head() T
	Tail() List(T)
		
	Add(T) List(T)
	Empty() bool
}

func NewList(type T)() List(T){
	return &nill(T){}
}

type list(type T) struct{
	head T
	tail List(T)
}

func (q *list(T)) Head() T{
	return q.head
}

func (q *list(T)) Tail() List(T){
	return q.tail
}

func (q *list(T)) Add(v T) List(T){
	return &list(T){
		head: v,
		tail: q,
	}
}

func (*list(T)) Empty() bool{
	return false
}

type nill(type T) struct{}

func (*nill(T)) Head() T{
	panic("no head in nill list")
	var t T
	return t
}

func (*nill(T)) Tail() List(T){
	return &nill(T){}
}

func (n *nill(T)) Add(v T) List(T){
	return &list(T){
		head: v,
		tail: n,
	}
}

func (q *nill(T)) Empty() bool{
	return true
}

func main() {
	lst := NewList(int)()
	lst=lst.Add(10)
	lst=lst.Add(15)
	lst=lst.Add(20)
	
	for v := lst.Head(); !lst.Empty(); v = lst.Head(){
		fmt.Println(v)
	}
}

What did you expect to see?

I'm not shure, but not the panic.

What did you see instead?

panic: runtime error: index out of range [0] with length 0

goroutine 1 [running]:
go/go2go.(*translator).instantiateTypeDecl(0xc0004099b0, 0x0, 0xc00000c580, 0xc00007b3b0, 0xc000422040, 0x1, 0x1, 0xc000422150, 0x1, 0x1, ...)
	/usr/local/go-faketime/src/go/go2go/instantiate.go:201 +0xc45
go/go2go.(*translator).translateTypeInstantiation(0xc0004099b0, 0xc0003e9780)
	/usr/local/go-faketime/src/go/go2go/rewrite.go:808 +0x4cd
go/go2go.(*translator).translateExpr(0xc0004099b0, 0xc0003e9780)
	/usr/local/go-faketime/src/go/go2go/rewrite.go:616 +0x3bb
go/go2go.(*translator).translateExpr(0xc0004099b0, 0xc0003fa690)
	/usr/local/go-faketime/src/go/go2go/rewrite.go:571 +0xf9
go/go2go.(*translator).translateExpr(0xc0004099b0, 0xc000422030)
	/usr/local/go-faketime/src/go/go2go/rewrite.go:623 +0x1e5
go/go2go.(*translator).translateExprList(0xc0004099b0, 0xc000422030, 0x1, 0x1)
	/usr/local/go-faketime/src/go/go2go/rewrite.go:693 +0x46
go/go2go.(*translator).translateStmt(0xc0004099b0, 0xc000422020)
	/usr/local/go-faketime/src/go/go2go/rewrite.go:504 +0x225
go/go2go.(*translator).translateBlockStmt(0xc0004099b0, 0xc0003f96b0)
	/usr/local/go-faketime/src/go/go2go/rewrite.go:455 +0x52
go/go2go.(*translator).translateFuncDecl(0xc0004099b0, 0xc000422060)
	/usr/local/go-faketime/src/go/go2go/rewrite.go:448 +0xc5
go/go2go.(*translator).translate(0xc0004099b0, 0xc000104300)
	/usr/local/go-faketime/src/go/go2go/rewrite.go:376 +0x391
go/go2go.rewriteAST(0xc000072240, 0xc0000553b0, 0x0, 0x0, 0xc000055b30, 0xc000104300, 0x1, 0xc000055360, 0xc000055b30)
	/usr/local/go-faketime/src/go/go2go/rewrite.go:188 +0x109
go/go2go.RewriteBuffer(0xc0000553b0, 0x7ffe6b956dd8, 0x1e, 0xc000138000, 0x38f, 0x58f, 0x0, 0xc000010a50, 0xc000010a20, 0xc0000109f0, ...)
	/usr/local/go-faketime/src/go/go2go/go2go.go:132 +0x2c5
main.translateFile(0xc0000553b0, 0x7ffe6b956dd8, 0x1e)
	/usr/local/go-faketime/src/cmd/go2go/translate.go:26 +0xa9
main.main()
	/usr/local/go-faketime/src/cmd/go2go/main.go:64 +0x2ea