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// Copyright 2011 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 types
import (
// Note: This is a uint32 rather than a uint64 because the
// respective 64 bit atomic instructions are not available
// on all platforms.
var lastID uint32
// nextID returns a value increasing monotonically by 1 with
// each call, starting with 1. It may be called concurrently.
func nextID() uint64 { return uint64(atomic.AddUint32(&lastID, 1)) }
// A TypeParam represents a type parameter type.
type TypeParam struct {
check *Checker // for lazy type bound completion
id uint64 // unique id, for debugging only
obj *TypeName // corresponding type name
index int // type parameter index in source order, starting at 0
// TODO(rfindley): this could also be Typ[Invalid]. Verify that this is handled correctly.
bound Type // *Named or *Interface; underlying type is always *Interface
// NewTypeParam returns a new TypeParam. Type parameters may be set on a Named
// or Signature type by calling SetTParams. Setting a type parameter on more
// than one type will result in a panic.
// The bound argument can be nil, and set later via SetBound.
func (check *Checker) NewTypeParam(obj *TypeName, bound Type) *TypeParam {
// Always increment lastID, even if it is not used.
id := nextID()
if check != nil {
id = check.nextID
typ := &TypeParam{check: check, id: id, obj: obj, index: -1, bound: bound}
if obj.typ == nil {
obj.typ = typ
return typ
// TODO(rfindley): remove or export these placeholder APIs.
// Index returns the index of the type param within its param list.
func (t *TypeParam) _Index() int {
return t.index
// SetId sets the unique id of a type param. Should only be used for type params
// in imported generic types.
func (t *TypeParam) _SetId(id uint64) { = id
// Obj returns the type name for t.
func (t *TypeParam) Obj() *TypeName { return t.obj }
// Constraint returns the type constraint specified for t.
func (t *TypeParam) Constraint() Type {
// compute the type set if possible (we may not have an interface)
if iface, _ := under(t.bound).(*Interface); iface != nil {
// use the type bound position if we have one
pos := token.NoPos
if n, _ := t.bound.(*Named); n != nil {
pos = n.obj.pos
computeInterfaceTypeSet(t.check, pos, iface)
return t.bound
// SetConstraint sets the type constraint for t.
func (t *TypeParam) SetConstraint(bound Type) {
if bound == nil {
panic("nil constraint")
t.bound = bound
func (t *TypeParam) Underlying() Type { return t }
func (t *TypeParam) String() string { return TypeString(t, nil) }
// ----------------------------------------------------------------------------
// Implementation
// iface returns the constraint interface of t.
func (t *TypeParam) iface() *Interface {
if iface, _ := under(t.Constraint()).(*Interface); iface != nil {
return iface
return &emptyInterface
// structuralType returns the structural type of the type parameter's constraint; or nil.
func (t *TypeParam) structuralType() Type {
return t.iface().typeSet().structuralType()
func (t *TypeParam) is(f func(*term) bool) bool {
return t.iface().typeSet().is(f)
func (t *TypeParam) underIs(f func(Type) bool) bool {
return t.iface().typeSet().underIs(f)