1 files changed, 100 insertions, 0 deletions
diff --git a/test/typeparam/cons.go b/test/typeparam/cons.go
new file mode 100644
index 0000000..733e579
--- /dev/null
+++ b/test/typeparam/cons.go
@@ -0,0 +1,100 @@
+// run
+
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import "fmt"
+
+// Overriding the predeclare "any", so it can be used as a type constraint or a type
+// argument
+type any interface{}
+
+type Function[a, b any] interface {
+	Apply(x a) b
+}
+
+type incr struct{ n int }
+
+func (this incr) Apply(x int) int {
+	return x + this.n
+}
+
+type pos struct{}
+
+func (this pos) Apply(x int) bool {
+	return x > 0
+}
+
+type compose[a, b, c any] struct {
+	f Function[a, b]
+	g Function[b, c]
+}
+
+func (this compose[a, b, c]) Apply(x a) c {
+	return this.g.Apply(this.f.Apply(x))
+}
+
+type _Eq[a any] interface {
+	Equal(a) bool
+}
+
+type Int int
+
+func (this Int) Equal(that int) bool {
+	return int(this) == that
+}
+
+type List[a any] interface {
+	Match(casenil Function[Nil[a], any], casecons Function[Cons[a], any]) any
+}
+
+type Nil[a any] struct {
+}
+
+func (xs Nil[a]) Match(casenil Function[Nil[a], any], casecons Function[Cons[a], any]) any {
+	return casenil.Apply(xs)
+}
+
+type Cons[a any] struct {
+	Head a
+	Tail List[a]
+}
+
+func (xs Cons[a]) Match(casenil Function[Nil[a], any], casecons Function[Cons[a], any]) any {
+	return casecons.Apply(xs)
+}
+
+type mapNil[a, b any] struct {
+}
+
+func (m mapNil[a, b]) Apply(_ Nil[a]) any {
+	return Nil[b]{}
+}
+
+type mapCons[a, b any] struct {
+	f Function[a, b]
+}
+
+func (m mapCons[a, b]) Apply(xs Cons[a]) any {
+	return Cons[b]{m.f.Apply(xs.Head), Map[a, b](m.f, xs.Tail)}
+}
+
+func Map[a, b any](f Function[a, b], xs List[a]) List[b] {
+	return xs.Match(mapNil[a, b]{}, mapCons[a, b]{f}).(List[b])
+}
+
+func main() {
+	var xs List[int] = Cons[int]{3, Cons[int]{6, Nil[int]{}}}
+	var ys List[int] = Map[int, int](incr{-5}, xs)
+	var xz List[bool] = Map[int, bool](pos{}, ys)
+	cs1 := xz.(Cons[bool])
+	cs2 := cs1.Tail.(Cons[bool])
+	_, ok := cs2.Tail.(Nil[bool])
+	if cs1.Head != false || cs2.Head != true || !ok {
+		panic(fmt.Sprintf("got %v, %v, %v, expected false, true, true",
+			cs1.Head, cs2.Head, ok))
+	}
+}