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// Copyright 2023 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 types2
import "fmt"
// An Alias represents an alias type.
// Whether or not Alias types are created is controlled by the
// gotypesalias setting with the GODEBUG environment variable.
// For gotypesalias=1, alias declarations produce an Alias type.
// Otherwise, the alias information is only in the type name,
// which points directly to the actual (aliased) type.
type Alias struct {
obj *TypeName // corresponding declared alias object
fromRHS Type // RHS of type alias declaration; may be an alias
actual Type // actual (aliased) type; never an alias
}
// NewAlias creates a new Alias type with the given type name and rhs.
// rhs must not be nil.
func NewAlias(obj *TypeName, rhs Type) *Alias {
alias := (*Checker)(nil).newAlias(obj, rhs)
// Ensure that alias.actual is set (#65455).
unalias(alias)
return alias
}
func (a *Alias) Obj() *TypeName { return a.obj }
func (a *Alias) Underlying() Type { return unalias(a).Underlying() }
func (a *Alias) String() string { return TypeString(a, nil) }
// Type accessors
// Unalias returns t if it is not an alias type;
// otherwise it follows t's alias chain until it
// reaches a non-alias type which is then returned.
// Consequently, the result is never an alias type.
func Unalias(t Type) Type {
if a0, _ := t.(*Alias); a0 != nil {
return unalias(a0)
}
return t
}
func unalias(a0 *Alias) Type {
if a0.actual != nil {
return a0.actual
}
var t Type
for a := a0; a != nil; a, _ = t.(*Alias) {
t = a.fromRHS
}
if t == nil {
panic(fmt.Sprintf("non-terminated alias %s", a0.obj.name))
}
a0.actual = t
return t
}
// asNamed returns t as *Named if that is t's
// actual type. It returns nil otherwise.
func asNamed(t Type) *Named {
n, _ := Unalias(t).(*Named)
return n
}
// newAlias creates a new Alias type with the given type name and rhs.
// rhs must not be nil.
func (check *Checker) newAlias(obj *TypeName, rhs Type) *Alias {
assert(rhs != nil)
a := &Alias{obj, rhs, nil}
if obj.typ == nil {
obj.typ = a
}
// Ensure that a.actual is set at the end of type checking.
if check != nil {
check.needsCleanup(a)
}
return a
}
func (a *Alias) cleanup() {
Unalias(a)
}
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