[dev.typeparams] cmd/compile/internal/types2: add *.go2 (generic) tests

Change-Id: I33453736ac05cd7c2e666b280974719c734701fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/263632
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

src/cmd/compile/internal/types2/testdata/builtins.go2

@@ -0,0 +1,53 @@
+// Copyright 2020 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.
+
+// This file tests built-in calls on generic types.
+
+package builtins
+
+type Bmc interface {
+	type map[rune]string, chan int
+}
+
+type Bms interface {
+	type map[string]int, []int
+}
+
+type Bcs interface {
+	type chan bool, []float64
+}
+
+type Bss interface {
+	type []int, []string
+}
+
+func _[T any] () {
+	_ = make(T /* ERROR invalid argument */ )
+	_ = make(T /* ERROR invalid argument */ , 10)
+	_ = make(T /* ERROR invalid argument */ , 10, 20)
+}
+
+func _[T Bmc] () {
+	_ = make(T)
+	_ = make(T, 10)
+	_ = make /* ERROR expects 1 or 2 arguments */ (T, 10, 20)
+}
+
+func _[T Bms] () {
+	_ = make /* ERROR expects 2 arguments */ (T)
+	_ = make(T, 10)
+	_ = make /* ERROR expects 2 arguments */ (T, 10, 20)
+}
+
+func _[T Bcs] () {
+	_ = make /* ERROR expects 2 arguments */ (T)
+	_ = make(T, 10)
+	_ = make /* ERROR expects 2 arguments */ (T, 10, 20)
+}
+
+func _[T Bss] () {
+	_ = make /* ERROR expects 2 or 3 arguments */ (T)
+	_ = make(T, 10)
+	_ = make(T, 10, 20)
+}

src/cmd/compile/internal/types2/testdata/chans.go2

@@ -0,0 +1,62 @@
+package chans
+
+import "runtime"
+
+// 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[T any]() (*Sender[T], *Receiver[T]) {
+	c := make(chan T)
+	d := make(chan bool)
+	s := &Sender[T]{values: c, done: d}
+	r := &Receiver[T]{values: c, done: d}
+	runtime.SetFinalizer(r, r.finalize)
+	return s, r
+}
+
+// A sender is used to send values to a Receiver.
+type Sender[T any] struct {
+	values chan<- T
+	done <-chan bool
+}
+
+// Send sends a value to the receiver. It returns whether any more
+// values may be sent; if it returns false the value was not sent.
+func (s *Sender[T]) Send(v T) bool {
+	select {
+	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[T]) Close() {
+	close(s.values)
+}
+
+// A Receiver receives values from a Sender.
+type Receiver[T any] struct {
+	values <-chan T
+	done chan<- bool
+}
+
+// Next returns the next value from the channel. The bool result
+// indicates whether the value is valid, or whether the Sender has
+// been closed and no more values will be received.
+func (r *Receiver[T]) Next() (T, bool) {
+	v, ok := <-r.values
+	return v, ok
+}
+
+// finalize is a finalizer for the receiver.
+func (r *Receiver[T]) finalize() {
+	close(r.done)
+}

src/cmd/compile/internal/types2/testdata/issues.go2

@@ -0,0 +1,249 @@
+// Copyright 2020 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.
+
+// This file contains regression tests for bugs found.
+
+package p
+
+import "io"
+import "context"
+
+// Interfaces are always comparable (though the comparison may panic at runtime).
+func eql[T comparable](x, y T) bool {
+	return x == y
+}
+
+func _() {
+	var x interface{}
+	var y interface{ m() }
+	eql(x, y /* ERROR does not match */ ) // interfaces of different types
+	eql(x, x)
+	eql(y, y)
+	eql(y, nil)
+	eql[io.Reader](nil, nil)
+}
+
+// If we have a receiver of pointer type (below: *T) we must ignore
+// the pointer in the implementation of the method lookup because
+// the type bound of T is an interface and pointer to interface types
+// have no methods and then the lookup would fail.
+type C[T any] interface {
+    m()
+}
+
+// using type bound C
+func _[T C[T]](x *T) {
+	x.m()
+}
+
+// using an interface literal as bound
+func _[T interface{ m() }](x *T) {
+	x.m()
+}
+
+func f2[_ interface{ m1(); m2() }]()
+
+type T struct{}
+func (T) m1()
+func (*T) m2()
+
+func _() {
+	f2[T /* ERROR wrong method signature */ ]()
+	f2[*T]()
+}
+
+// When a type parameter is used as an argument to instantiate a parameterized
+// type with a type list constraint, all of the type argument's types in its
+// bound, but at least one (!), must be in the type list of the bound of the
+// corresponding parameterized type's type parameter.
+type T1[P interface{type uint}] struct{}
+
+func _[P any]() {
+    _ = T1[P /* ERROR P has no type constraints */ ]{}
+}
+
+// This is the original (simplified) program causing the same issue.
+type Unsigned interface {
+	type uint
+}
+
+type T2[U Unsigned] struct {
+    s U
+}
+
+func (u T2[U]) Add1() U {
+    return u.s + 1
+}
+
+func NewT2[U any]() T2[U /* ERROR U has no type constraints */ ] {
+    return T2[U /* ERROR U has no type constraints */ ]{}
+}
+
+func _() {
+    u := NewT2[string]()
+    _ = u.Add1()
+}
+
+// When we encounter an instantiated type such as Elem[T] we must
+// not "expand" the instantiation when the type to be instantiated
+// (Elem in this case) is not yet fully set up.
+type Elem[T any] struct {
+	next *Elem[T]
+	list *List[T]
+}
+
+type List[T any] struct {
+	root Elem[T]
+}
+
+func (l *List[T]) Init() {
+	l.root.next = &l.root
+}
+
+// This is the original program causing the same issue.
+type Element2[TElem any] struct {
+	next, prev *Element2[TElem]
+	list *List2[TElem]
+	Value TElem
+}
+
+type List2[TElem any] struct {
+	root Element2[TElem]
+	len  int
+}
+
+func (l *List2[TElem]) Init() *List2[TElem] {
+	l.root.next = &l.root
+	l.root.prev = &l.root
+	l.len = 0
+	return l
+}
+
+// Self-recursive instantiations must work correctly.
+type A[P any] struct { _ *A[P] }
+
+type AB[P any] struct { _ *BA[P] }
+type BA[P any] struct { _ *AB[P] }
+
+// And a variation that also caused a problem with an
+// unresolved underlying type.
+type Element3[TElem any] struct {
+	next, prev *Element3[TElem]
+	list *List3[TElem]
+	Value TElem
+}
+
+func (e *Element3[TElem]) Next() *Element3[TElem] {
+	if p := e.next; e.list != nil && p != &e.list.root {
+		return p
+	}
+	return nil
+}
+
+type List3[TElem any] struct {
+	root Element3[TElem]
+	len  int
+}
+
+// Infinite generic type declarations must lead to an error.
+type inf1[T any] struct{ _ inf1 /* ERROR illegal cycle */ [T] }
+type inf2[T any] struct{ inf2 /* ERROR illegal cycle */ [T] }
+
+// The implementation of conversions T(x) between integers and floating-point
+// numbers checks that both T and x have either integer or floating-point
+// type. When the type of T or x is a type parameter, the respective simple
+// predicate disjunction in the implementation was wrong because if a type list
+// contains both an integer and a floating-point type, the type parameter is
+// neither an integer or a floating-point number.
+func convert[T1, T2 interface{type int, uint, float32}](v T1) T2 {
+	return T2(v)
+}
+
+func _() {
+	convert[int, uint](5)
+}
+
+// When testing binary operators, for +, the operand types must either be
+// both numeric, or both strings. The implementation had the same problem
+// with this check as the conversion issue above (issue #39623).
+
+func issue39623[T interface{type int, string}](x, y T) T {
+	return x + y
+}
+
+// Simplified, from https://go2goplay.golang.org/p/efS6x6s-9NI:
+func Sum[T interface{type int, string}](s []T) (sum T) {
+	for _, v := range s {
+		sum += v
+	}
+	return
+}
+
+// Assignability of an unnamed pointer type to a type parameter that
+// has a matching underlying type.
+func _[T interface{}, PT interface{type *T}] (x T) PT {
+    return &x
+}
+
+// Indexing of generic types containing type parameters in their type list:
+func at[T interface{ type []E }, E interface{}](x T, i int) E {
+        return x[i]
+}
+
+// A generic type inside a function acts like a named type. Its underlying
+// type is itself, its "operational type" is defined by the type list in
+// the tybe bound, if any.
+func _[T interface{type int}](x T) {
+	type myint int
+	var _ int = int(x)
+	var _ T = 42
+	var _ T = T(myint(42))
+}
+
+// Indexing a generic type with an array type bound checks length.
+// (Example by mdempsky@.)
+func _[T interface { type [10]int }](x T) {
+	_ = x[9] // ok
+	_ = x[20 /* ERROR out of bounds */ ]
+}
+
+// Pointer indirection of a generic type.
+func _[T interface{ type *int }](p T) int {
+	return *p
+}
+
+// Channel sends and receives on generic types.
+func _[T interface{ type chan int }](ch T) int {
+	ch <- 0
+	return <- ch
+}
+
+// Calling of a generic variable.
+func _[T interface{ type func() }](f T) {
+	f()
+	go f()
+}
+
+// We must compare against the underlying type of type list entries
+// when checking if a constraint is satisfied by a type. The under-
+// lying type of each type list entry must be computed after the
+// interface has been instantiated as its typelist may contain a
+// type parameter that was substituted with a defined type.
+// Test case from an (originally) failing example.
+
+type sliceOf[E any] interface{ type []E }
+
+func append[T interface{}, S sliceOf[T], T2 interface{ type T }](s S, t ...T2) S
+
+var f           func()
+var cancelSlice []context.CancelFunc
+var _ = append[context.CancelFunc, []context.CancelFunc, context.CancelFunc](cancelSlice, f)
+
+// A generic function must be instantiated with a type, not a value.
+
+func g[T any](T) T
+
+var _ = g[int]
+var _ = g[nil /* ERROR is not a type */ ]
+var _ = g(0)

src/cmd/compile/internal/types2/testdata/linalg.go2

@@ -0,0 +1,83 @@
+// Copyright 2019 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 linalg
+
+import "math"
+
+// Numeric is type bound that matches any numeric type.
+// It would likely be in a constraints package in the standard library.
+type Numeric interface {
+	type int, int8, int16, int32, int64,
+		uint, uint8, uint16, uint32, uint64, uintptr,
+		float32, float64,
+		complex64, complex128
+}
+
+func DotProduct[T Numeric](s1, s2 []T) T {
+	if len(s1) != len(s2) {
+		panic("DotProduct: slices of unequal length")
+	}
+	var r T
+	for i := range s1 {
+		r += s1[i] * s2[i]
+	}
+	return r
+}
+
+// NumericAbs matches numeric types with an Abs method.
+type NumericAbs[T any] interface {
+	Numeric
+
+	Abs() T
+}
+
+// AbsDifference computes the absolute value of the difference of
+// a and b, where the absolute value is determined by the Abs method.
+func AbsDifference[T NumericAbs[T]](a, b T) T {
+	d := a - b
+	return d.Abs()
+}
+
+// OrderedNumeric is a type bound that matches numeric types that support the < operator.
+type OrderedNumeric interface {
+	type int, int8, int16, int32, int64,
+		uint, uint8, uint16, uint32, uint64, uintptr,
+		float32, float64
+}
+
+// Complex is a type bound that matches the two complex types, which do not have a < operator.
+type Complex interface {
+	type complex64, complex128
+}
+
+// OrderedAbs is a helper type that defines an Abs method for
+// ordered numeric types.
+type OrderedAbs[T OrderedNumeric] T
+
+func (a OrderedAbs[T]) Abs() OrderedAbs[T] {
+	if a < 0 {
+		return -a
+	}
+	return a
+}
+
+// ComplexAbs is a helper type that defines an Abs method for
+// complex types.
+type ComplexAbs[T Complex] T
+
+func (a ComplexAbs[T]) Abs() ComplexAbs[T] {
+	r := float64(real(a))
+	i := float64(imag(a))
+	d := math.Sqrt(r * r + i * i)
+	return ComplexAbs[T](complex(d, 0))
+}
+
+func OrderedAbsDifference[T OrderedNumeric](a, b T) T {
+	return T(AbsDifference(OrderedAbs[T](a), OrderedAbs[T](b)))
+}
+
+func ComplexAbsDifference[T Complex](a, b T) T {
+	return T(AbsDifference(ComplexAbs[T](a), ComplexAbs[T](b)))
+}

src/cmd/compile/internal/types2/testdata/map.go2

@@ -0,0 +1,113 @@
+// Copyright 2019 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 orderedmap
+
+// TODO(gri) fix imports for tests
+import "chans" // ERROR could not import
+
+// 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.
+func New[K, V any](compare func(K, K) int) *Map[K, V] {
+        return &Map[K, V]{compare: compare}
+}
+
+// find looks up key in the map, and returns either a pointer
+// to the node holding key, or a pointer to the location where
+// such 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.
+// Returns true if this is a new key, false if already present.
+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 zero 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 // see the discussion of zero values, above
+		return zero, false
+	}
+	return (*pn).val, true
+}
+
+// keyValue is a pair of key and value used when iterating.
+type keyValue[K, V any] struct {
+	key K
+	val V
+}
+
+// InOrder returns an iterator that does an in-order traversal of the map.
+func (m *Map[K, V]) InOrder() *Iterator[K, V] {
+	sender, receiver := chans.Ranger[keyValue[K, V]]()
+	var f func(*node[K, V]) bool
+	f = func(n *node[K, V]) bool {
+		if n == nil {
+			return true
+		}
+		// Stop sending values if sender.Send returns false,
+		// meaning that nothing is listening at the receiver end.
+		return f(n.left) &&
+                        sender.Send(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 *chans.Receiver[keyValue[K, V]]
+}
+
+// Next returns the next key and value pair, and a boolean indicating
+// whether they are valid or whether we have reached the end.
+func (it *Iterator[K, V]) Next() (K, V, bool) {
+	keyval, ok := it.r.Next()
+	if !ok {
+		var zerok K
+		var zerov V
+		return zerok, zerov, false
+	}
+	return keyval.key, keyval.val, true
+}

src/cmd/compile/internal/types2/testdata/map2.go2

@@ -0,0 +1,146 @@
+// Copyright 2019 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.
+
+// This file is like map.go2, but instead if importing chans, it contains
+// the necessary functionality at the end of the file.
+
+// Package orderedmap provides an ordered map, implemented as a binary tree.
+package orderedmap
+
+// 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.
+func New[K, V any](compare func(K, K) int) *Map[K, V] {
+        return &Map[K, V]{compare: compare}
+}
+
+// find looks up key in the map, and returns either a pointer
+// to the node holding key, or a pointer to the location where
+// such 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.
+// Returns true if this is a new key, false if already present.
+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 zero 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 // see the discussion of zero values, above
+		return zero, false
+	}
+	return (*pn).val, true
+}
+
+// keyValue is a pair of key and value used when iterating.
+type keyValue[K, V any] struct {
+	key K
+	val V
+}
+
+// InOrder returns an iterator that does an in-order traversal of the map.
+func (m *Map[K, V]) InOrder() *Iterator[K, V] {
+	sender, receiver := chans_Ranger[keyValue[K, V]]()
+	var f func(*node[K, V]) bool
+	f = func(n *node[K, V]) bool {
+		if n == nil {
+			return true
+		}
+		// Stop sending values if sender.Send returns false,
+		// meaning that nothing is listening at the receiver end.
+		return f(n.left) &&
+                        sender.Send(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 *chans_Receiver[keyValue[K, V]]
+}
+
+// Next returns the next key and value pair, and a boolean indicating
+// whether they are valid or whether we have reached the end.
+func (it *Iterator[K, V]) Next() (K, V, bool) {
+	keyval, ok := it.r.Next()
+	if !ok {
+		var zerok K
+		var zerov V
+		return zerok, zerov, false
+	}
+	return keyval.key, keyval.val, true
+}
+
+// chans
+
+func chans_Ranger[T any]() (*chans_Sender[T], *chans_Receiver[T])
+
+// A sender is used to send values to a Receiver.
+type chans_Sender[T any] struct {
+	values chan<- T
+	done <-chan bool
+}
+
+func (s *chans_Sender[T]) Send(v T) bool {
+	select {
+	case s.values <- v:
+		return true
+	case <-s.done:
+		return false
+	}
+}
+
+func (s *chans_Sender[T]) Close() {
+	close(s.values)
+}
+
+type chans_Receiver[T any] struct {
+	values <-chan T
+	done chan<- bool
+}
+
+func (r *chans_Receiver[T]) Next() (T, bool) {
+	v, ok := <-r.values
+	return v, ok
+}

src/cmd/compile/internal/types2/testdata/mtypeparams.go2

@@ -0,0 +1,52 @@
+// Copyright 2020 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.
+
+// If types2.Config.AcceptMethodTypeParams is set,
+// the type checker accepts methods that have their
+// own type parameter list.
+
+package p
+
+type S struct{}
+
+func (S) m[T any](v T)
+
+// TODO(gri) Once we collect interface method type parameters
+//           in the parser, we can enable these tests again.
+/*
+type I interface {
+   m[T any](v T)
+}
+
+type J interface {
+   m[T any](v T)
+}
+
+var _ I = S{}
+var _ I = J(nil)
+
+type C interface{ n() }
+
+type Sc struct{}
+
+func (Sc) m[T C](v T)
+
+type Ic interface {
+   m[T C](v T)
+}
+
+type Jc interface {
+   m[T C](v T)
+}
+
+var _ Ic = Sc{}
+var _ Ic = Jc(nil)
+
+// TODO(gri) These should fail because the constraints don't match.
+var _ I = Sc{}
+var _ I = Jc(nil)
+
+var _ Ic = S{}
+var _ Ic = J(nil)
+*/

src/cmd/compile/internal/types2/testdata/slices.go2

@@ -0,0 +1,68 @@
+// Copyright 2019 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 implements various slice algorithms.
+package slices
+
+// Map turns a []T1 to a []T2 using a mapping function.
+func Map[T1, T2 any](s []T1, f func(T1) T2) []T2 {
+	r := make([]T2, len(s))
+	for i, v := range s {
+		r[i] = f(v)
+	}
+	return r
+}
+
+// Reduce reduces a []T1 to a single value using a reduction function.
+func Reduce[T1, T2 any](s []T1, initializer T2, f func(T2, T1) T2) T2 {
+	r := initializer
+	for _, v := range s {
+		r = f(r, v)
+	}
+	return r
+}
+
+// Filter filters values from a slice using a filter function.
+func Filter[T any](s []T, f func(T) bool) []T {
+	var r []T
+	for _, v := range s {
+		if f(v) {
+			r = append(r, v)
+		}
+	}
+	return r
+}
+
+// Example uses
+
+func limiter(x int) byte {
+	switch {
+	case x < 0:
+		return 0
+	default:
+		return byte(x)
+	case x > 255:
+		return 255
+	}
+}
+
+var input = []int{-4, 68954, 7, 44, 0, -555, 6945}
+var limited1 = Map[int, byte](input, limiter)
+var limited2 = Map(input, limiter) // using type inference
+
+func reducer(x float64, y int) float64 {
+	return x + float64(y)
+}
+
+var reduced1 = Reduce[int, float64](input, 0, reducer)
+var reduced2 = Reduce(input, 1i /* ERROR overflows */, reducer) // using type inference
+var reduced3 = Reduce(input, 1, reducer) // using type inference
+
+func filter(x int) bool {
+	return x&1 != 0
+}
+
+var filtered1 = Filter[int](input, filter)
+var filtered2 = Filter(input, filter) // using type inference
+

src/cmd/compile/internal/types2/testdata/tinference.go2

@@ -0,0 +1,105 @@
+// Copyright 2020 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 tinferenceB
+
+import "strconv"
+
+type any interface{}
+
+func f0[A any, B interface{type C}, C interface{type D}, D interface{type A}](A, B, C, D)
+func _() {
+	f := f0[string]
+	f("a", "b", "c", "d")
+	f0("a", "b", "c", "d")
+}
+
+func f1[A any, B interface{type A}](A, B)
+func _() {
+	f := f1[int]
+	f(int(0), int(0))
+	f1(int(0), int(0))
+}
+
+func f2[A any, B interface{type []A}](A, B)
+func _() {
+	f := f2[byte]
+	f(byte(0), []byte{})
+	f2(byte(0), []byte{})
+}
+
+func f3[A any, B interface{type C}, C interface{type *A}](A, B, C)
+func _() {
+	f := f3[int]
+	var x int
+	f(x, &x, &x)
+	f3(x, &x, &x)
+}
+
+func f4[A any, B interface{type []C}, C interface{type *A}](A, B, C)
+func _() {
+	f := f4[int]
+	var x int
+	f(x, []*int{}, &x)
+	f4(x, []*int{}, &x)
+}
+
+func f5[A interface{type struct{b B; c C}}, B any, C interface{type *B}](x B) A
+func _() {
+	x := f5(1.2)
+	var _ float64 = x.b
+	var _ float64 = *x.c
+}
+
+func f6[A any, B interface{type struct{f []A}}](B) A
+func _() {
+	x := f6(struct{f []string}{})
+	var _ string = x
+}
+
+// TODO(gri) Need to flag invalid recursive constraints. At the
+// moment these cause infinite recursions and stack overflow.
+// func f7[A interface{type B}, B interface{type A}]()
+
+// More realistic examples
+
+func Double[S interface{ type []E }, E interface{ type int, int8, int16, int32, int64 }](s S) S {
+	r := make(S, len(s))
+	for i, v := range s {
+		r[i] = v + v
+	}
+	return r
+}
+
+type MySlice []int
+
+var _ = Double(MySlice{1})
+
+// From the draft design.
+
+type Setter[B any] interface {
+	Set(string)
+	type *B
+}
+
+func FromStrings[T interface{}, PT Setter[T]](s []string) []T {
+	result := make([]T, len(s))
+	for i, v := range s {
+		// The type of &result[i] is *T which is in the type list
+		// of Setter2, so we can convert it to PT.
+		p := PT(&result[i])
+		// PT has a Set method.
+		p.Set(v)
+	}
+	return result
+}
+
+type Settable int
+
+func (p *Settable) Set(s string) {
+	i, _ := strconv.Atoi(s) // real code should not ignore the error
+	*p = Settable(i)
+}
+
+var _ = FromStrings[Settable]([]string{"1", "2"})

src/cmd/compile/internal/types2/testdata/tmp.go2

@@ -0,0 +1,17 @@
+// Copyright 2020 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.
+
+// This file is meant as "dumping ground" for debugging code.
+
+package p
+
+// fun test case
+type C[P interface{m()}] P
+
+func (r C[P]) m() { r.m() }
+
+func f[T interface{m(); n()}](x T) {
+	y := C[T](x)
+	y.m()
+}

src/cmd/compile/internal/types2/testdata/typeinst.go2

@@ -0,0 +1,59 @@
+// Copyright 2019 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 p
+
+type myInt int
+
+// Parameterized type declarations
+
+type T1[P any] P
+
+type T2[P any] struct {
+        f P
+        g int // int should still be in scope chain
+}
+
+type List[P any] []P
+
+// Alias type declarations cannot have type parameters. Syntax error.
+type A1[P any] = /* ERROR cannot be alias */ P
+
+// But an alias may refer to a generic, uninstantiated type.
+type A2 = List
+var _ A2[int]
+var _ A2 /* ERROR without instantiation */
+
+type A3 = List[int]
+var _ A3
+
+// Parameterized type instantiations
+
+var x int
+type _ x /* ERROR not a type */ [int]
+
+type _ int[] // ERROR expecting type
+type _ myInt[] // ERROR expecting type
+
+// TODO(gri) better error messages
+type _ T1 /* ERROR without instantiation */ [] // ERROR expecting type
+type _ T1[x /* ERROR not a type */ ]
+type _ T1 /* ERROR got 2 arguments but 1 type parameters */ [int, float32]
+
+var _ T2[int] = T2[int]{}
+
+var _ List[int] = []int{1, 2, 3}
+var _ List[[]int] = [][]int{{1, 2, 3}}
+var _ List[List[List[int]]]
+
+// Parameterized types containing parameterized types
+
+type T3[P any] List[P]
+
+var _ T3[int] = T3[int](List[int]{1, 2, 3})
+
+// Self-recursive generic types are not permitted
+
+type self1[P any] self1 /* ERROR illegal cycle */ [P]
+type self2[P any] *self2[P] // this is ok

src/cmd/compile/internal/types2/testdata/typeinst2.go2

@@ -0,0 +1,256 @@
+// Copyright 2019 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 p
+
+type List[E any] []E
+var _ List[List[List[int]]]
+var _ List[List[List[int]]] = []List[List[int]]{}
+
+type (
+	T1[P1 any] struct {
+		f1 T2[P1, float32]
+	}
+
+	T2[P2, P3 any] struct {
+		f2 P2
+		f3 P3
+	}
+)
+
+func _() {
+	var x1 T1[int]
+	var x2 T2[int, float32]
+
+	x1.f1.f2 = 0
+	x1.f1 = x2
+}
+
+type T3[P any] T1[T2[P, P]]
+
+func _() {
+	var x1 T3[int]
+	var x2 T2[int, int]
+	x1.f1.f2 = x2
+}
+
+func f[P any] (x P) List[P] {
+	return List[P]{x}
+}
+
+var (
+	_ []int = f(0)
+	_ []float32 = f[float32](10)
+	_ List[complex128] = f(1i)
+	_ []List[int] = f(List[int]{})
+        _ List[List[int]] = []List[int]{}
+        _ = []List[int]{}
+)
+
+// Parameterized types with methods
+
+func (l List[E]) Head() (_ E, _ bool) {
+	if len(l) > 0 {
+		return l[0], true
+	}
+	return
+}
+
+// A test case for instantiating types with other types (extracted from map.go2)
+
+type Pair[K any] struct {
+	key K
+}
+
+type Receiver[T any] struct {
+	values T
+}
+
+type Iterator[K any] struct {
+	r Receiver[Pair[K]]
+}
+
+func Values [T any] (r Receiver[T]) T {
+        return r.values
+}
+
+func (it Iterator[K]) Next() K {
+        return Values[Pair[K]](it.r).key
+}
+
+// A more complex test case testing type bounds (extracted from linalg.go2 and reduced to essence)
+
+type NumericAbs[T any] interface {
+	Abs() T
+}
+
+func AbsDifference[T NumericAbs[T]](x T)
+
+type OrderedAbs[T any] T
+
+func (a OrderedAbs[T]) Abs() OrderedAbs[T]
+
+func OrderedAbsDifference[T any](x T) {
+	AbsDifference(OrderedAbs[T](x))
+}
+
+// same code, reduced to essence
+
+func g[P interface{ m() P }](x P)
+
+type T4[P any] P
+
+func (_ T4[P]) m() T4[P]
+
+func _[Q any](x Q) {
+	g(T4[Q](x))
+}
+
+// Another test case that caused  problems in the past
+
+type T5[_ interface { a() }, _ interface{}] struct{}
+
+type A[P any] struct{ x P }
+
+func (_ A[P]) a() {}
+
+var _ T5[A[int], int]
+
+// Invoking methods with parameterized receiver types uses
+// type inference to determine the actual type arguments matching
+// the receiver type parameters from the actual receiver argument.
+// Go does implicit address-taking and dereferenciation depending
+// on the actual receiver and the method's receiver type. To make
+// type inference work, the type-checker matches "pointer-ness"
+// of the actual receiver and the method's receiver type.
+// The following code tests this mechanism.
+
+type R1[A any] struct{}
+func (_ R1[A]) vm()
+func (_ *R1[A]) pm()
+
+func _[T any](r R1[T], p *R1[T]) {
+	r.vm()
+	r.pm()
+	p.vm()
+	p.pm()
+}
+
+type R2[A, B any] struct{}
+func (_ R2[A, B]) vm()
+func (_ *R2[A, B]) pm()
+
+func _[T any](r R2[T, int], p *R2[string, T]) {
+	r.vm()
+	r.pm()
+	p.vm()
+	p.pm()
+}
+
+// An interface can (explicitly) declare at most one type list.
+type _ interface {
+	m0()
+	type int, string, bool
+	type /* ERROR multiple type lists */ float32, float64
+	m1()
+	m2()
+	type /* ERROR multiple type lists */ complex64, complex128
+	type /* ERROR multiple type lists */ rune
+}
+
+// Interface type lists may contain each type at most once.
+// (If there are multiple lists, we assume the author intended
+// for them to be all in a single list, and we report the error
+// as well.)
+type _ interface {
+	type int, int /* ERROR duplicate type int */
+	type /* ERROR multiple type lists */ int /* ERROR duplicate type int */
+}
+
+type _ interface {
+	type struct{f int}, struct{g int}, struct /* ERROR duplicate type */ {f int}
+}
+
+// Interface type lists can contain any type, incl. *Named types.
+// Verify that we use the underlying type to compute the operational type.
+type MyInt int
+func add1[T interface{type MyInt}](x T) T {
+	return x + 1
+}
+
+type MyString string
+func double[T interface{type MyInt, MyString}](x T) T {
+	return x + x
+}
+
+// Embedding of interfaces with type lists leads to interfaces
+// with type lists that are the intersection of the embedded
+// type lists.
+
+type E0 interface {
+	type int, bool, string
+}
+
+type E1 interface {
+	type int, float64, string
+}
+
+type E2 interface {
+	type float64
+}
+
+type I0 interface {
+	E0
+}
+
+func f0[T I0]()
+var _ = f0[int]
+var _ = f0[bool]
+var _ = f0[string]
+var _ = f0[float64 /* ERROR does not satisfy I0 */ ]
+
+type I01 interface {
+	E0
+	E1
+}
+
+func f01[T I01]()
+var _ = f01[int]
+var _ = f01[bool /* ERROR does not satisfy I0 */ ]
+var _ = f01[string]
+var _ = f01[float64 /* ERROR does not satisfy I0 */ ]
+
+type I012 interface {
+	E0
+	E1
+	E2
+}
+
+func f012[T I012]()
+var _ = f012[int /* ERROR does not satisfy I012 */ ]
+var _ = f012[bool /* ERROR does not satisfy I012 */ ]
+var _ = f012[string /* ERROR does not satisfy I012 */ ]
+var _ = f012[float64 /* ERROR does not satisfy I012 */ ]
+
+type I12 interface {
+	E1
+	E2
+}
+
+func f12[T I12]()
+var _ = f12[int /* ERROR does not satisfy I12 */ ]
+var _ = f12[bool /* ERROR does not satisfy I12 */ ]
+var _ = f12[string /* ERROR does not satisfy I12 */ ]
+var _ = f12[float64]
+
+type I0_ interface {
+	E0
+	type int
+}
+
+func f0_[T I0_]()
+var _ = f0_[int]
+var _ = f0_[bool /* ERROR does not satisfy I0_ */ ]
+var _ = f0_[string /* ERROR does not satisfy I0_ */ ]
+var _ = f0_[float64 /* ERROR does not satisfy I0_ */ ]

src/cmd/compile/internal/types2/testdata/typeparams.go2

@@ -0,0 +1,422 @@
+// Copyright 2020 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 p
+
+// import "io" // for type assertion tests
+
+// The predeclared identifier "any" is only visible as a constraint
+// in a type parameter list.
+var _ any // ERROR undeclared
+func _[_ any /* ok here */ , _ interface{any /* ERROR undeclared */ }](any /* ERROR undeclared */ ) {
+        var _ any /* ERROR undeclared */
+}
+
+func identity[T any](x T) T { return x }
+
+func _[_ any](x int) int
+func _[T any](T /* ERROR redeclared */ T)()
+func _[T, T /* ERROR redeclared */ any]()
+
+func reverse[T any](list []T) []T {
+        rlist := make([]T, len(list))
+        i := len(list)
+        for _, x := range list {
+                i--
+                rlist[i] = x
+        }
+        return rlist
+}
+
+var _ = reverse /* ERROR cannot use generic function reverse */
+var _ = reverse[int, float32 /* ERROR got 2 type arguments */ ] ([]int{1, 2, 3})
+var _ = reverse[int]([ /* ERROR cannot use */ ]float32{1, 2, 3})
+var f = reverse[chan int]
+var _ = f(0 /* ERROR cannot convert 0 .* to \[\]chan int */ )
+
+func swap[A, B any](a A, b B) (B, A) { return b, a }
+
+var _ = swap /* ERROR single value is expected */ [int, float32](1, 2)
+var f32, i = swap[int, float32](swap[float32, int](1, 2))
+var _ float32 = f32
+var _ int = i
+
+func swapswap[A, B any](a A, b B) (A, B) {
+        return swap[B, A](b, a)
+}
+
+type F[A, B any] func(A, B) (B, A)
+
+func min[T interface{ type int }](x, y T) T {
+        if x < y {
+                return x
+        }
+        return y
+}
+
+func _[T interface{type int, float32}](x, y T) bool { return x < y }
+func _[T any](x, y T) bool { return x /* ERROR cannot compare */ < y }
+func _[T interface{type int, float32, bool}](x, y T) bool { return x /* ERROR cannot compare */ < y }
+
+func _[T C1[T]](x, y T) bool { return x /* ERROR cannot compare */ < y }
+func _[T C2[T]](x, y T) bool { return x < y }
+
+type C1[T any] interface{}
+type C2[T any] interface{ type int, float32 }
+
+func new[T any]() *T {
+        var x T
+        return &x
+}
+
+var _ = new /* ERROR cannot use generic function new */
+var _ *int = new[int]()
+
+func _[T any](map[T /* ERROR invalid map key type T \(missing comparable constraint\) */]int) // w/o constraint we don't know if T is comparable
+
+func f1[T1 any](struct{T1}) int
+var _ = f1[int](struct{T1}{})
+type T1 = int
+
+func f2[t1 any](struct{t1; x float32}) int
+var _ = f2[t1](struct{t1; x float32}{})
+type t1 = int
+
+
+func f3[A, B, C any](A, struct{x B}, func(A, struct{x B}, *C)) int
+
+var _ = f3[int, rune, bool](1, struct{x rune}{}, nil)
+
+// indexing
+
+func _[T any] (x T, i int) { _ = x /* ERROR "cannot index" */ [i] }
+func _[T interface{ type int }] (x T, i int) { _ = x /* ERROR "cannot index" */ [i] }
+func _[T interface{ type string }] (x T, i int) { _ = x[i] }
+func _[T interface{ type []int }] (x T, i int) { _ = x[i] }
+func _[T interface{ type [10]int, *[20]int, map[string]int }] (x T, i int) { _ = x[i] }
+func _[T interface{ type string, []byte }] (x T, i int) { _ = x[i] }
+func _[T interface{ type []int, [1]rune }] (x T, i int) { _ = x /* ERROR "cannot index" */ [i] }
+func _[T interface{ type string, []rune }] (x T, i int) { _ = x /* ERROR "cannot index" */ [i] }
+
+// slicing
+// TODO(gri) implement this
+
+func _[T interface{ type string }] (x T, i, j, k int) { _ = x /* ERROR invalid operation */ [i:j:k] }
+
+// len/cap built-ins
+
+func _[T any](x T) { _ = len(x /* ERROR invalid argument */ ) }
+func _[T interface{ type int }](x T) { _ = len(x /* ERROR invalid argument */ ) }
+func _[T interface{ type string, []byte, int }](x T) { _ = len(x /* ERROR invalid argument */ ) }
+func _[T interface{ type string }](x T) { _ = len(x) }
+func _[T interface{ type [10]int }](x T) { _ = len(x) }
+func _[T interface{ type []byte }](x T) { _ = len(x) }
+func _[T interface{ type map[int]int }](x T) { _ = len(x) }
+func _[T interface{ type chan int }](x T) { _ = len(x) }
+func _[T interface{ type string, []byte, chan int }](x T) { _ = len(x) }
+
+func _[T any](x T) { _ = cap(x /* ERROR invalid argument */ ) }
+func _[T interface{ type int }](x T) { _ = cap(x /* ERROR invalid argument */ ) }
+func _[T interface{ type string, []byte, int }](x T) { _ = cap(x /* ERROR invalid argument */ ) }
+func _[T interface{ type string }](x T) { _ = cap(x /* ERROR invalid argument */ ) }
+func _[T interface{ type [10]int }](x T) { _ = cap(x) }
+func _[T interface{ type []byte }](x T) { _ = cap(x) }
+func _[T interface{ type map[int]int }](x T) { _ = cap(x /* ERROR invalid argument */ ) }
+func _[T interface{ type chan int }](x T) { _ = cap(x) }
+func _[T interface{ type []byte, chan int }](x T) { _ = cap(x) }
+
+// range iteration
+
+func _[T interface{}](x T) {
+        for range x /* ERROR cannot range */ {}
+}
+
+func _[T interface{ type string, []string }](x T) {
+        for range x {}
+        for i := range x { _ = i }
+        for i, _ := range x { _ = i }
+        for i, e := range x /* ERROR must have the same element type */ { _ = i }
+        for _, e := range x /* ERROR must have the same element type */ {}
+        var e rune
+        _ = e
+        for _, (e) = range x /* ERROR must have the same element type */ {}
+}
+
+
+func _[T interface{ type string, []rune, map[int]rune }](x T) {
+        for _, e := range x { _ = e }
+        for i, e := range x { _ = i; _ = e }
+}
+
+func _[T interface{ type string, []rune, map[string]rune }](x T) {
+        for _, e := range x { _ = e }
+        for i, e := range x /* ERROR must have the same key type */ { _ = e }
+}
+
+func _[T interface{ type string, chan int }](x T) {
+        for range x {}
+        for i := range x { _ = i }
+        for i, _ := range x { _ = i } // TODO(gri) should get an error here: channels only return one value
+}
+
+func _[T interface{ type string, chan<-int }](x T) {
+        for i := range x /* ERROR send-only channel */ { _ = i }
+}
+
+// type inference checks
+
+var _ = new() /* ERROR cannot infer T */
+
+func f4[A, B, C any](A, B) C
+
+var _ = f4(1, 2) /* ERROR cannot infer C */
+var _ = f4[int, float32, complex128](1, 2)
+
+func f5[A, B, C any](A, []*B, struct{f []C}) int
+
+var _ = f5[int, float32, complex128](0, nil, struct{f []complex128}{})
+var _ = f5(0, nil, struct{f []complex128}{}) // ERROR cannot infer
+var _ = f5(0, []*float32{new[float32]()}, struct{f []complex128}{})
+
+func f6[A any](A, []A) int
+
+var _ = f6(0, nil)
+
+func f6nil[A any](A) int
+
+var _ = f6nil(nil) // ERROR cannot infer
+
+// type inference with variadic functions
+
+func f7[T any](...T) T
+
+var _ int = f7() /* ERROR cannot infer T */
+var _ int = f7(1)
+var _ int = f7(1, 2)
+var _ int = f7([]int{}...)
+var _ int = f7 /* ERROR cannot use */ ([]float64{}...)
+var _ float64 = f7([]float64{}...)
+var _ = f7[float64](1, 2.3)
+var _ = f7(float64(1), 2.3)
+var _ = f7(1, 2.3 /* ERROR does not match */ )
+var _ = f7(1.2, 3 /* ERROR does not match */ )
+
+func f8[A, B any](A, B, ...B) int
+
+var _ = f8(1) /* ERROR not enough arguments */
+var _ = f8(1, 2.3)
+var _ = f8(1, 2.3, 3.4, 4.5)
+var _ = f8(1, 2.3, 3.4, 4 /* ERROR does not match */ )
+var _ = f8[int, float64](1, 2.3, 3.4, 4)
+
+var _ = f8[int, float64](0, 0, nil...) // test case for #18268
+
+// init functions cannot have type parameters
+
+func init() {}
+func init[/* ERROR func init must have no type parameters */ _ any]() {}
+func init[/* ERROR func init must have no type parameters */ P any]() {}
+
+type T struct {}
+
+func (T) m1() {}
+// The type checker accepts method type parameters if configured accordingly.
+func (T) m2[_ any]() {}
+func (T) m3[P any]() {}
+
+// type inference across parameterized types
+
+type S1[P any] struct { f P }
+
+func f9[P any](x S1[P])
+
+func _() {
+        f9[int](S1[int]{42})
+	f9(S1[int]{42})
+}
+
+type S2[A, B, C any] struct{}
+
+func f10[X, Y, Z any](a S2[X, int, Z], b S2[X, Y, bool])
+
+func _[P any]() {
+        f10[int, float32, string](S2[int, int, string]{}, S2[int, float32, bool]{})
+        f10(S2[int, int, string]{}, S2[int, float32, bool]{})
+        f10(S2[P, int, P]{}, S2[P, float32, bool]{})
+}
+
+// corner case for type inference
+// (was bug: after instanting f11, the type-checker didn't mark f11 as non-generic)
+
+func f11[T any]()
+
+func _() {
+	f11[int]()
+}
+
+// the previous example was extracted from
+
+func f12[T interface{m() T}]()
+
+type A[T any] T
+
+func (a A[T]) m() A[T]
+
+func _[T any]() {
+	f12[A[T]]()
+}
+
+// method expressions
+
+func (_ S1[P]) m()
+
+func _() {
+	m := S1[int].m
+	m(struct { f int }{42})
+}
+
+func _[T any] (x T) {
+        m := S1[T].m
+        m(S1[T]{x})
+}
+
+// type parameters in methods (generalization)
+
+type R0 struct{}
+
+func (R0) _[T any](x T)
+func (R0 /* ERROR invalid receiver */ ) _[R0 any]() // scope of type parameters starts at "func"
+
+type R1[A, B any] struct{}
+
+func (_ R1[A, B]) m0(A, B)
+func (_ R1[A, B]) m1[T any](A, B, T) T
+func (_ R1 /* ERROR not a generic type */ [R1, _]) _()
+func (_ R1[A, B]) _[A /* ERROR redeclared */ any](B)
+
+func _() {
+        var r R1[int, string]
+        r.m1[rune](42, "foo", 'a')
+        r.m1[rune](42, "foo", 1.2 /* ERROR truncated to rune */)
+        r.m1(42, "foo", 1.2) // using type inference
+        var _ float64 = r.m1(42, "foo", 1.2)
+}
+
+type I1[A any] interface {
+        m1(A)
+}
+
+var _ I1[int] = r1[int]{}
+
+type r1[T any] struct{}
+
+func (_ r1[T]) m1(T)
+
+type I2[A, B any] interface {
+        m1(A)
+        m2(A) B
+}
+
+var _ I2[int, float32] = R2[int, float32]{}
+
+type R2[P, Q any] struct{}
+
+func (_ R2[X, Y]) m1(X)
+func (_ R2[X, Y]) m2(X) Y
+
+// type assertions and type switches over generic types
+// NOTE: These are currently disabled because it's unclear what the correct
+// approach is, and one can always work around by assigning the variable to
+// an interface first.
+
+// // ReadByte1 corresponds to the ReadByte example in the draft design.
+// func ReadByte1[T io.Reader](r T) (byte, error) {
+// 	if br, ok := r.(io.ByteReader); ok {
+// 		return br.ReadByte()
+// 	}
+// 	var b [1]byte
+// 	_, err := r.Read(b[:])
+// 	return b[0], err
+// }
+//
+// // ReadBytes2 is like ReadByte1 but uses a type switch instead.
+// func ReadByte2[T io.Reader](r T) (byte, error) {
+//         switch br := r.(type) {
+//         case io.ByteReader:
+//                 return br.ReadByte()
+//         }
+// 	var b [1]byte
+// 	_, err := r.Read(b[:])
+// 	return b[0], err
+// }
+//
+// // type assertions and type switches over generic types are strict
+// type I3 interface {
+//         m(int)
+// }
+//
+// type I4 interface {
+//         m() int // different signature from I3.m
+// }
+//
+// func _[T I3](x I3, p T) {
+//         // type assertions and type switches over interfaces are not strict
+//         _ = x.(I4)
+//         switch x.(type) {
+//         case I4:
+//         }
+//
+//         // type assertions and type switches over generic types are strict
+//         _ = p /* ERROR cannot have dynamic type I4 */.(I4)
+//         switch p.(type) {
+//         case I4 /* ERROR cannot have dynamic type I4 */ :
+//         }
+// }
+
+// type assertions and type switches over generic types lead to errors for now
+
+func _[T any](x T) {
+	_ = x /* ERROR not an interface */ .(int)
+	switch x /* ERROR not an interface */ .(type) {
+	}
+
+	// work-around
+	var t interface{} = x
+	_ = t.(int)
+	switch t.(type) {
+	}
+}
+
+func _[T interface{type int}](x T) {
+	_ = x /* ERROR not an interface */ .(int)
+	switch x /* ERROR not an interface */ .(type) {
+	}
+
+	// work-around
+	var t interface{} = x
+	_ = t.(int)
+	switch t.(type) {
+	}
+}
+
+// error messages related to type bounds mention those bounds
+type C[P any] interface{}
+
+func _[P C[P]] (x P) {
+	x.m /* ERROR x.m undefined */ ()
+}
+
+type I interface {}
+
+func _[P I] (x P) {
+	x.m /* ERROR interface I has no method m */ ()
+}
+
+func _[P interface{}] (x P) {
+	x.m /* ERROR type bound for P has no method m */ ()
+}
+
+func _[P any] (x P) {
+	x.m /* ERROR type bound for P has no method m */ ()
+}