cmd/compile: fix case where func-valued field of a generic type is called

Added test example orderedmap.go (binary search tree) that requires this
fix (calling function compare in _Map).

Also added new tests slices.go and metrics.go that just work.

Change-Id: Ifa5f42ab6eee9aa54c40f0eca19e00a87f8f608a
Reviewed-on: https://go-review.googlesource.com/c/go/+/301829
Trust: Dan Scales <danscales@google.com>
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

4 files changed, 807 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/noder/stencil.go b/src/cmd/compile/internal/noder/stencil.go
index 78c7ddeabe..64b3a942e2 100644
--- a/src/cmd/compile/internal/noder/stencil.go
+++ b/src/cmd/compile/internal/noder/stencil.go
@@ -389,6 +389,13 @@ func (subst *subster) node(n ir.Node) ir.Node {
 				typecheck.Callee(call.X)
 				call.SetTypecheck(0)
 				typecheck.Call(call)
+			} else if call.X.Op() == ir.ODOT || call.X.Op() == ir.ODOTPTR {
+				// An OXDOT for a generic receiver was resolved to
+				// an access to a field which has a function
+				// value. Typecheck the call to that function, now
+				// that the OXDOT was resolved.
+				call.SetTypecheck(0)
+				typecheck.Call(call)
 			} else if call.X.Op() != ir.OFUNCINST {
 				// A call with an OFUNCINST will get typechecked
 				// in stencil() once we have created & attached the
diff --git a/test/typeparam/metrics.go b/test/typeparam/metrics.go
new file mode 100644
index 0000000000..8a39d9945d
--- /dev/null
+++ b/test/typeparam/metrics.go
@@ -0,0 +1,196 @@
+// run -gcflags=-G=3
+
+// 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 metrics provides tracking arbitrary metrics composed of
+// values of comparable types.
+package main
+
+import (
+	"fmt"
+	"sort"
+	"sync"
+)
+
+// _Metric1 tracks metrics of values of some type.
+type _Metric1[T comparable] struct {
+	mu sync.Mutex
+	m  map[T]int
+}
+
+// Add adds another instance of some value.
+func (m *_Metric1[T]) Add(v T) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	if m.m == nil {
+		m.m = make(map[T]int)
+	}
+	m.m[v]++
+}
+
+// Count returns the number of instances we've seen of v.
+func (m *_Metric1[T]) Count(v T) int {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	return m.m[v]
+}
+
+// Metrics returns all the values we've seen, in an indeterminate order.
+func (m *_Metric1[T]) Metrics() []T {
+	return _Keys(m.m)
+}
+
+type key2[T1, T2 comparable] struct {
+	f1 T1
+	f2 T2
+}
+
+// _Metric2 tracks metrics of pairs of values.
+type _Metric2[T1, T2 comparable] struct {
+	mu sync.Mutex
+	m  map[key2[T1, T2]]int
+}
+
+// Add adds another instance of some pair of values.
+func (m *_Metric2[T1, T2]) Add(v1 T1, v2 T2) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	if m.m == nil {
+		m.m = make(map[key2[T1, T2]]int)
+	}
+	m.m[key2[T1, T2]{v1, v2}]++
+}
+
+// Count returns the number of instances we've seen of v1/v2.
+func (m *_Metric2[T1, T2]) Count(v1 T1, v2 T2) int {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	return m.m[key2[T1, T2]{v1, v2}]
+}
+
+// Metrics returns all the values we've seen, in an indeterminate order.
+func (m *_Metric2[T1, T2]) Metrics() (r1 []T1, r2 []T2) {
+	for _, k := range _Keys(m.m) {
+		r1 = append(r1, k.f1)
+		r2 = append(r2, k.f2)
+	}
+	return r1, r2
+}
+
+type key3[T1, T2, T3 comparable] struct {
+	f1 T1
+	f2 T2
+	f3 T3
+}
+
+// _Metric3 tracks metrics of triplets of values.
+type _Metric3[T1, T2, T3 comparable] struct {
+	mu sync.Mutex
+	m  map[key3[T1, T2, T3]]int
+}
+
+// Add adds another instance of some triplet of values.
+func (m *_Metric3[T1, T2, T3]) Add(v1 T1, v2 T2, v3 T3) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	if m.m == nil {
+		m.m = make(map[key3[T1, T2, T3]]int)
+	}
+	m.m[key3[T1, T2, T3]{v1, v2, v3}]++
+}
+
+// Count returns the number of instances we've seen of v1/v2/v3.
+func (m *_Metric3[T1, T2, T3]) Count(v1 T1, v2 T2, v3 T3) int {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	return m.m[key3[T1, T2, T3]{v1, v2, v3}]
+}
+
+// Metrics returns all the values we've seen, in an indeterminate order.
+func (m *_Metric3[T1, T2, T3]) Metrics() (r1 []T1, r2 []T2, r3 []T3) {
+	for k := range m.m {
+		r1 = append(r1, k.f1)
+		r2 = append(r2, k.f2)
+		r3 = append(r3, k.f3)
+	}
+	return r1, r2, r3
+}
+
+type S struct{ a, b, c string }
+
+func TestMetrics() {
+	m1 := _Metric1[string]{}
+	if got := m1.Count("a"); got != 0 {
+		panic(fmt.Sprintf("Count(%q) = %d, want 0", "a", got))
+	}
+	m1.Add("a")
+	m1.Add("a")
+	if got := m1.Count("a"); got != 2 {
+		panic(fmt.Sprintf("Count(%q) = %d, want 2", "a", got))
+	}
+	if got, want := m1.Metrics(), []string{"a"}; !_SlicesEqual(got, want) {
+		panic(fmt.Sprintf("Metrics = %v, want %v", got, want))
+	}
+
+	m2 := _Metric2[int, float64]{}
+	m2.Add(1, 1)
+	m2.Add(2, 2)
+	m2.Add(3, 3)
+	m2.Add(3, 3)
+	k1, k2 := m2.Metrics()
+
+	sort.Ints(k1)
+	w1 := []int{1, 2, 3}
+	if !_SlicesEqual(k1, w1) {
+		panic(fmt.Sprintf("_Metric2.Metrics first slice = %v, want %v", k1, w1))
+	}
+
+	sort.Float64s(k2)
+	w2 := []float64{1, 2, 3}
+	if !_SlicesEqual(k2, w2) {
+		panic(fmt.Sprintf("_Metric2.Metrics first slice = %v, want %v", k2, w2))
+	}
+
+	m3 := _Metric3[string, S, S]{}
+	m3.Add("a", S{"d", "e", "f"}, S{"g", "h", "i"})
+	m3.Add("a", S{"d", "e", "f"}, S{"g", "h", "i"})
+	m3.Add("a", S{"d", "e", "f"}, S{"g", "h", "i"})
+	m3.Add("b", S{"d", "e", "f"}, S{"g", "h", "i"})
+	if got := m3.Count("a", S{"d", "e", "f"}, S{"g", "h", "i"}); got != 3 {
+		panic(fmt.Sprintf("Count(%v, %v, %v) = %d, want 3", "a", S{"d", "e", "f"}, S{"g", "h", "i"}, got))
+	}
+}
+
+func main() {
+	TestMetrics()
+}
+
+// _Equal reports whether two slices are equal: the same length and all
+// elements equal. All floating point NaNs are considered equal.
+func _SlicesEqual[Elem comparable](s1, s2 []Elem) bool {
+	if len(s1) != len(s2) {
+		return false
+	}
+	for i, v1 := range s1 {
+		v2 := s2[i]
+		if v1 != v2 {
+			isNaN := func(f Elem) bool { return f != f }
+			if !isNaN(v1) || !isNaN(v2) {
+				return false
+			}
+		}
+	}
+	return true
+}
+
+// _Keys returns the keys of the map m.
+// The keys will be an indeterminate order.
+func _Keys[K comparable, V any](m map[K]V) []K {
+	r := make([]K, 0, len(m))
+	for k := range m {
+		r = append(r, k)
+	}
+	return r
+}
diff --git a/test/typeparam/orderedmap.go b/test/typeparam/orderedmap.go
new file mode 100644
index 0000000000..db1b374267
--- /dev/null
+++ b/test/typeparam/orderedmap.go
@@ -0,0 +1,286 @@
+// run -gcflags=-G=3
+
+// 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 orderedmap provides an ordered map, implemented as a binary tree.
+package main
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"runtime"
+)
+
+type Ordered interface {
+        type int, int8, int16, int32, int64,
+                uint, uint8, uint16, uint32, uint64, uintptr,
+                float32, float64,
+                string
+}
+
+// _Map is an ordered map.
+type _Map[K, V any] struct {
+	root    *node[K, V]
+	compare func(K, K) int
+}
+
+// node is the type of a node in the binary tree.
+type node[K, V any] struct {
+	key         K
+	val         V
+	left, right *node[K, V]
+}
+
+// _New returns a new map. It takes a comparison function that compares two
+// keys and returns < 0 if the first is less, == 0 if they are equal,
+// > 0 if the first is greater.
+func _New[K, V any](compare func(K, K) int) *_Map[K, V] {
+	return &_Map[K, V]{compare: compare}
+}
+
+// _NewOrdered returns a new map whose key is an ordered type.
+// This is like _New, but does not require providing a compare function.
+// The map compare function uses the obvious key ordering.
+func _NewOrdered[K Ordered, V any]() *_Map[K, V] {
+	return _New[K, V](func(k1, k2 K) int {
+		switch {
+		case k1 < k2:
+			return -1
+		case k1 == k2:
+			return 0
+		default:
+			return 1
+		}
+	})
+}
+
+// find looks up key in the map, returning either a pointer to the slot of the
+// node holding key, or a pointer to the slot where should a node would go.
+func (m *_Map[K, V]) find(key K) **node[K, V] {
+	pn := &m.root
+	for *pn != nil {
+		switch cmp := m.compare(key, (*pn).key); {
+		case cmp < 0:
+			pn = &(*pn).left
+		case cmp > 0:
+			pn = &(*pn).right
+		default:
+			return pn
+		}
+	}
+	return pn
+}
+
+// Insert inserts a new key/value into the map.
+// If the key is already present, the value is replaced.
+// Reports whether this is a new key.
+func (m *_Map[K, V]) Insert(key K, val V) bool {
+	pn := m.find(key)
+	if *pn != nil {
+		(*pn).val = val
+		return false
+	}
+	*pn = &node[K, V]{key: key, val: val}
+	return true
+}
+
+// Find returns the value associated with a key, or the zero value
+// if not present. The found result reports whether the key was found.
+func (m *_Map[K, V]) Find(key K) (V, bool) {
+	pn := m.find(key)
+	if *pn == nil {
+		var zero V
+		return zero, false
+	}
+	return (*pn).val, true
+}
+
+// keyValue is a pair of key and value used while iterating.
+type keyValue[K, V any] struct {
+	key K
+	val V
+}
+
+// iterate returns an iterator that traverses the map.
+func (m *_Map[K, V]) Iterate() *_Iterator[K, V] {
+	sender, receiver := _Ranger[keyValue[K, V]]()
+	var f func(*node[K, V]) bool
+	f = func(n *node[K, V]) bool {
+		if n == nil {
+			return true
+		}
+		// Stop the traversal if Send fails, which means that
+		// nothing is listening to the receiver.
+		return f(n.left) &&
+			sender.Send(context.Background(), keyValue[K, V]{n.key, n.val}) &&
+			f(n.right)
+	}
+	go func() {
+		f(m.root)
+		sender.Close()
+	}()
+	return &_Iterator[K, V]{receiver}
+}
+
+// _Iterator is used to iterate over the map.
+type _Iterator[K, V any] struct {
+	r *_Receiver[keyValue[K, V]]
+}
+
+// Next returns the next key and value pair, and a boolean that reports
+// whether they are valid. If not valid, we have reached the end of the map.
+func (it *_Iterator[K, V]) Next() (K, V, bool) {
+	keyval, ok := it.r.Next(context.Background())
+	if !ok {
+		var zerok K
+		var zerov V
+		return zerok, zerov, false
+	}
+	return keyval.key, keyval.val, true
+}
+
+func TestMap() {
+	m := _New[[]byte, int](bytes.Compare)
+
+	if _, found := m.Find([]byte("a")); found {
+		panic(fmt.Sprintf("unexpectedly found %q in empty map", []byte("a")))
+	}
+	if !m.Insert([]byte("a"), 'a') {
+		panic(fmt.Sprintf("key %q unexpectedly already present", []byte("a")))
+	}
+	if !m.Insert([]byte("c"), 'c') {
+		panic(fmt.Sprintf("key %q unexpectedly already present", []byte("c")))
+	}
+	if !m.Insert([]byte("b"), 'b') {
+		panic(fmt.Sprintf("key %q unexpectedly already present", []byte("b")))
+	}
+	if m.Insert([]byte("c"), 'x') {
+		panic(fmt.Sprintf("key %q unexpectedly not present", []byte("c")))
+	}
+
+	if v, found := m.Find([]byte("a")); !found {
+		panic(fmt.Sprintf("did not find %q", []byte("a")))
+	} else if v != 'a' {
+		panic(fmt.Sprintf("key %q returned wrong value %c, expected %c", []byte("a"), v, 'a'))
+	}
+	if v, found := m.Find([]byte("c")); !found {
+		panic(fmt.Sprintf("did not find %q", []byte("c")))
+	} else if v != 'x' {
+		panic(fmt.Sprintf("key %q returned wrong value %c, expected %c", []byte("c"), v, 'x'))
+	}
+
+	if _, found := m.Find([]byte("d")); found {
+		panic(fmt.Sprintf("unexpectedly found %q", []byte("d")))
+	}
+
+	gather := func(it *_Iterator[[]byte, int]) []int {
+		var r []int
+		for {
+			_, v, ok := it.Next()
+			if !ok {
+				return r
+			}
+			r = append(r, v)
+		}
+	}
+	got := gather(m.Iterate())
+	want := []int{'a', 'b', 'x'}
+	if !_SliceEqual(got, want) {
+		panic(fmt.Sprintf("Iterate returned %v, want %v", got, want))
+	}
+}
+
+func main() {
+	TestMap()
+}
+
+// _Equal reports whether two slices are equal: the same length and all
+// elements equal. All floating point NaNs are considered equal.
+func _SliceEqual[Elem comparable](s1, s2 []Elem) bool {
+	if len(s1) != len(s2) {
+		return false
+	}
+	for i, v1 := range s1 {
+		v2 := s2[i]
+		if v1 != v2 {
+			isNaN := func(f Elem) bool { return f != f }
+			if !isNaN(v1) || !isNaN(v2) {
+				return false
+			}
+		}
+	}
+	return true
+}
+
+// Ranger returns a Sender and a Receiver. The Receiver provides a
+// Next method to retrieve values. The Sender provides a Send method
+// to send values and a Close method to stop sending values. The Next
+// method indicates when the Sender has been closed, and the Send
+// method indicates when the Receiver has been freed.
+//
+// This is a convenient way to exit a goroutine sending values when
+// the receiver stops reading them.
+func _Ranger[Elem any]() (*_Sender[Elem], *_Receiver[Elem]) {
+	c := make(chan Elem)
+	d := make(chan struct{})
+	s := &_Sender[Elem]{
+		values: c,
+		done:   d,
+	}
+	r := &_Receiver[Elem] {
+		values: c,
+		done:   d,
+	}
+	runtime.SetFinalizer(r, (*_Receiver[Elem]).finalize)
+	return s, r
+}
+
+// A _Sender is used to send values to a Receiver.
+type _Sender[Elem any] struct {
+	values chan<- Elem
+	done   <-chan struct{}
+}
+
+// Send sends a value to the receiver. It reports whether the value was sent.
+// The value will not be sent if the context is closed or the receiver
+// is freed.
+func (s *_Sender[Elem]) Send(ctx context.Context, v Elem) bool {
+	select {
+	case <-ctx.Done():
+		return false
+	case s.values <- v:
+		return true
+	case <-s.done:
+		return false
+	}
+}
+
+// Close tells the receiver that no more values will arrive.
+// After Close is called, the _Sender may no longer be used.
+func (s *_Sender[Elem]) Close() {
+	close(s.values)
+}
+
+// A _Receiver receives values from a _Sender.
+type _Receiver[Elem any] struct {
+	values <-chan Elem
+	done   chan<- struct{}
+}
+
+// Next returns the next value from the channel. The bool result indicates
+// whether the value is valid.
+func (r *_Receiver[Elem]) Next(ctx context.Context) (v Elem, ok bool) {
+	select {
+	case <-ctx.Done():
+	case v, ok = <-r.values:
+	}
+	return v, ok
+}
+
+// finalize is a finalizer for the receiver.
+func (r *_Receiver[Elem]) finalize() {
+	close(r.done)
+}
diff --git a/test/typeparam/slices.go b/test/typeparam/slices.go
new file mode 100644
index 0000000000..149199eb64
--- /dev/null
+++ b/test/typeparam/slices.go
@@ -0,0 +1,318 @@
+// run -gcflags=-G=3
+
+// 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 slices provides functions for basic operations on
+// slices of any element type.
+package main
+
+import (
+	"fmt"
+	"math"
+	"strings"
+)
+
+type Ordered interface {
+        type int, int8, int16, int32, int64,
+                uint, uint8, uint16, uint32, uint64, uintptr,
+                float32, float64,
+                string
+}
+
+type Integer interface {
+        type int, int8, int16, int32, int64,
+                uint, uint8, uint16, uint32, uint64, uintptr
+}
+
+// Max returns the maximum of two values of some ordered type.
+func _Max[T Ordered](a, b T) T {
+        if a > b {
+                return a
+        }
+        return b
+}
+
+// Min returns the minimum of two values of some ordered type.
+func _Min[T Ordered](a, b T) T {
+        if a < b {
+                return a
+        }
+        return b
+}
+
+// _Equal reports whether two slices are equal: the same length and all
+// elements equal. All floating point NaNs are considered equal.
+func _Equal[Elem comparable](s1, s2 []Elem) bool {
+	if len(s1) != len(s2) {
+		return false
+	}
+	for i, v1 := range s1 {
+		v2 := s2[i]
+		if v1 != v2 {
+			isNaN := func(f Elem) bool { return f != f }
+			if !isNaN(v1) || !isNaN(v2) {
+				return false
+			}
+		}
+	}
+	return true
+}
+
+// _EqualFn reports whether two slices are equal using a comparision
+// function on each element.
+func _EqualFn[Elem any](s1, s2 []Elem, eq func(Elem, Elem) bool) bool {
+	if len(s1) != len(s2) {
+		return false
+	}
+	for i, v1 := range s1 {
+		v2 := s2[i]
+		if !eq(v1, v2) {
+			return false
+		}
+	}
+	return true
+}
+
+// _Map turns a []Elem1 to a []Elem2 using a mapping function.
+func _Map[Elem1, Elem2 any](s []Elem1, f func(Elem1) Elem2) []Elem2 {
+	r := make([]Elem2, len(s))
+	for i, v := range s {
+		r[i] = f(v)
+	}
+	return r
+}
+
+// _Reduce reduces a []Elem1 to a single value of type Elem2 using
+// a reduction function.
+func _Reduce[Elem1, Elem2 any](s []Elem1, initializer Elem2, f func(Elem2, Elem1) Elem2) Elem2 {
+	r := initializer
+	for _, v := range s {
+		r = f(r, v)
+	}
+	return r
+}
+
+// _Filter filters values from a slice using a filter function.
+func _Filter[Elem any](s []Elem, f func(Elem) bool) []Elem {
+	var r []Elem
+	for _, v := range s {
+		if f(v) {
+			r = append(r, v)
+		}
+	}
+	return r
+}
+
+// _Max returns the maximum element in a slice of some ordered type.
+// If the slice is empty it returns the zero value of the element type.
+func _SliceMax[Elem Ordered](s []Elem) Elem {
+	if len(s) == 0 {
+		var zero Elem
+		return zero
+	}
+	return _Reduce(s[1:], s[0], _Max[Elem])
+}
+
+// _Min returns the minimum element in a slice of some ordered type.
+// If the slice is empty it returns the zero value of the element type.
+func _SliceMin[Elem Ordered](s []Elem) Elem {
+	if len(s) == 0 {
+		var zero Elem
+		return zero
+	}
+	return _Reduce(s[1:], s[0], _Min[Elem])
+}
+
+// _Append adds values to the end of a slice, returning a new slice.
+// This is like the predeclared append function; it's an example
+// of how to write it using generics. We used to write code like
+// this before append was added to the language, but we had to write
+// a separate copy for each type.
+func _Append[T any](s []T, t ...T) []T {
+	lens := len(s)
+	tot := lens + len(t)
+	if tot <= cap(s) {
+		s = s[:tot]
+	} else {
+		news := make([]T, tot, tot + tot/2)
+		_Copy(news, s)
+		s = news
+	}
+	_Copy(s[lens:tot], t)
+	return s
+}
+
+// _Copy copies values from t to s, stopping when either slice is full,
+// returning the number of values copied. This is like the predeclared
+// copy function; it's an example of how to write it using generics.
+func _Copy[T any](s, t []T) int {
+	i := 0
+	for ; i < len(s) && i < len(t); i++ {
+		s[i] = t[i]
+	}
+	return i
+}
+
+func TestEqual() {
+        s1 := []int{1, 2, 3}
+        if !_Equal(s1, s1) {
+                panic(fmt.Sprintf("_Equal(%v, %v) = false, want true", s1, s1))
+        }
+        s2 := []int{1, 2, 3}
+        if !_Equal(s1, s2) {
+                panic(fmt.Sprintf("_Equal(%v, %v) = false, want true", s1, s2))
+        }
+        s2 = append(s2, 4)
+        if _Equal(s1, s2) {
+                panic(fmt.Sprintf("_Equal(%v, %v) = true, want false", s1, s2))
+        }
+
+        s3 := []float64{1, 2, math.NaN()}
+        if !_Equal(s3, s3) {
+                panic(fmt.Sprintf("_Equal(%v, %v) = false, want true", s3, s3))
+        }
+
+        if _Equal(s1, nil) {
+                panic(fmt.Sprintf("_Equal(%v, nil) = true, want false", s1))
+        }
+        if _Equal(nil, s1) {
+                panic(fmt.Sprintf("_Equal(nil, %v) = true, want false", s1))
+        }
+        if !_Equal(s1[:0], nil) {
+                panic(fmt.Sprintf("_Equal(%v, nil = false, want true", s1[:0]))
+        }
+}
+
+func offByOne[Elem Integer](a, b Elem) bool {
+	return a == b + 1 || a == b - 1
+}
+
+func TestEqualFn() {
+	s1 := []int{1, 2, 3}
+	s2 := []int{2, 3, 4}
+	if _EqualFn(s1, s1, offByOne[int]) {
+		panic(fmt.Sprintf("_EqualFn(%v, %v, offByOne) = true, want false", s1, s1))
+	}
+	if !_EqualFn(s1, s2, offByOne[int]) {
+		panic(fmt.Sprintf("_EqualFn(%v, %v, offByOne) = false, want true", s1, s2))
+	}
+
+	if !_EqualFn(s1[:0], nil, offByOne[int]) {
+		panic(fmt.Sprintf("_EqualFn(%v, nil, offByOne) = false, want true", s1[:0]))
+	}
+
+	s3 := []string{"a", "b", "c"}
+	s4 := []string{"A", "B", "C"}
+	if !_EqualFn(s3, s4, strings.EqualFold) {
+		panic(fmt.Sprintf("_EqualFn(%v, %v, strings.EqualFold) = false, want true", s3, s4))
+	}
+}
+
+func TestMap() {
+	s1 := []int{1, 2, 3}
+	s2 := _Map(s1, func(i int) float64 { return float64(i) * 2.5 })
+	if want := []float64{2.5, 5, 7.5}; !_Equal(s2, want) {
+		panic(fmt.Sprintf("_Map(%v, ...) = %v, want %v", s1, s2, want))
+	}
+
+	s3 := []string{"Hello", "World"}
+	s4 := _Map(s3, strings.ToLower)
+	if want := []string{"hello", "world"}; !_Equal(s4, want) {
+		panic(fmt.Sprintf("_Map(%v, strings.ToLower) = %v, want %v", s3, s4, want))
+	}
+
+	s5 := _Map(nil, func(i int) int { return i })
+	if len(s5) != 0 {
+		panic(fmt.Sprintf("_Map(nil, identity) = %v, want empty slice", s5))
+	}
+}
+
+func TestReduce() {
+	s1 := []int{1, 2, 3}
+	r := _Reduce(s1, 0, func(f float64, i int) float64 { return float64(i) * 2.5 + f })
+	if want := 15.0; r != want {
+		panic(fmt.Sprintf("_Reduce(%v, 0, ...) = %v, want %v", s1, r, want))
+	}
+
+	if got := _Reduce(nil, 0, func(i, j int) int { return i + j}); got != 0 {
+		panic(fmt.Sprintf("_Reduce(nil, 0, add) = %v, want 0", got))
+	}
+}
+
+func TestFilter() {
+	s1 := []int{1, 2, 3}
+	s2 := _Filter(s1, func(i int) bool { return i%2 == 0 })
+	if want := []int{2}; !_Equal(s2, want) {
+		panic(fmt.Sprintf("_Filter(%v, even) = %v, want %v", s1, s2, want))
+	}
+
+	if s3 := _Filter(s1[:0], func(i int) bool { return true }); len(s3) > 0 {
+		panic(fmt.Sprintf("_Filter(%v, identity) = %v, want empty slice", s1[:0], s3))
+	}
+}
+
+func TestMax() {
+	s1 := []int{1, 2, 3, -5}
+	if got, want := _SliceMax(s1), 3; got != want {
+		panic(fmt.Sprintf("_Max(%v) = %d, want %d", s1, got, want))
+	}
+
+	s2 := []string{"aaa", "a", "aa", "aaaa"}
+	if got, want := _SliceMax(s2), "aaaa"; got != want {
+		panic(fmt.Sprintf("_Max(%v) = %q, want %q", s2, got, want))
+	}
+
+	if got, want := _SliceMax(s2[:0]), ""; got != want {
+		panic(fmt.Sprintf("_Max(%v) = %q, want %q", s2[:0], got, want))
+	}
+}
+
+func TestMin() {
+	s1 := []int{1, 2, 3, -5}
+	if got, want := _SliceMin(s1), -5; got != want {
+		panic(fmt.Sprintf("_Min(%v) = %d, want %d", s1, got, want))
+	}
+
+	s2 := []string{"aaa", "a", "aa", "aaaa"}
+	if got, want := _SliceMin(s2), "a"; got != want {
+		panic(fmt.Sprintf("_Min(%v) = %q, want %q", s2, got, want))
+	}
+
+	if got, want := _SliceMin(s2[:0]), ""; got != want {
+		panic(fmt.Sprintf("_Min(%v) = %q, want %q", s2[:0], got, want))
+	}
+}
+
+func TestAppend() {
+	s := []int{1, 2, 3}
+	s = _Append(s, 4, 5, 6)
+	want := []int{1, 2, 3, 4, 5, 6}
+	if !_Equal(s, want) {
+		panic(fmt.Sprintf("after _Append got %v, want %v", s, want))
+	}
+}
+
+func TestCopy() {
+	s1 := []int{1, 2, 3}
+	s2 := []int{4, 5}
+	if got := _Copy(s1, s2); got != 2 {
+		panic(fmt.Sprintf("_Copy returned %d, want 2", got))
+	}
+	want := []int{4, 5, 3}
+	if !_Equal(s1, want) {
+		panic(fmt.Sprintf("after _Copy got %v, want %v", s1, want))
+	}
+}
+func main() {
+	TestEqual()
+	TestEqualFn()
+	TestMap()
+	TestReduce()
+	TestFilter()
+	TestMax()
+	TestMin()
+	TestAppend()
+	TestCopy()
+}