alias.go

     1  // Copyright 2023 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package types2
     6  
     7  import "fmt"
     8  
     9  // An Alias represents an alias type.
    10  // Whether or not Alias types are created is controlled by the
    11  // gotypesalias setting with the GODEBUG environment variable.
    12  // For gotypesalias=1, alias declarations produce an Alias type.
    13  // Otherwise, the alias information is only in the type name,
    14  // which points directly to the actual (aliased) type.
    15  type Alias struct {
    16  	obj     *TypeName      // corresponding declared alias object
    17  	orig    *Alias         // original, uninstantiated alias
    18  	tparams *TypeParamList // type parameters, or nil
    19  	targs   *TypeList      // type arguments, or nil
    20  	fromRHS Type           // RHS of type alias declaration; may be an alias
    21  	actual  Type           // actual (aliased) type; never an alias
    22  }
    23  
    24  // NewAlias creates a new Alias type with the given type name and rhs.
    25  // rhs must not be nil.
    26  func NewAlias(obj *TypeName, rhs Type) *Alias {
    27  	alias := (*Checker)(nil).newAlias(obj, rhs)
    28  	// Ensure that alias.actual is set (#65455).
    29  	alias.cleanup()
    30  	return alias
    31  }
    32  
    33  func (a *Alias) Obj() *TypeName { return a.obj }
    34  func (a *Alias) String() string { return TypeString(a, nil) }
    35  
    36  // Underlying returns the [underlying type] of the alias type a, which is the
    37  // underlying type of the aliased type. Underlying types are never Named,
    38  // TypeParam, or Alias types.
    39  //
    40  // [underlying type]: https://go.dev/ref/spec#Underlying_types.
    41  func (a *Alias) Underlying() Type { return unalias(a).Underlying() }
    42  
    43  // Origin returns the generic Alias type of which a is an instance.
    44  // If a is not an instance of a generic alias, Origin returns a.
    45  func (a *Alias) Origin() *Alias { return a.orig }
    46  
    47  // TypeParams returns the type parameters of the alias type a, or nil.
    48  // A generic Alias and its instances have the same type parameters.
    49  func (a *Alias) TypeParams() *TypeParamList { return a.tparams }
    50  
    51  // SetTypeParams sets the type parameters of the alias type a.
    52  // The alias a must not have type arguments.
    53  func (a *Alias) SetTypeParams(tparams []*TypeParam) {
    54  	assert(a.targs == nil)
    55  	a.tparams = bindTParams(tparams)
    56  }
    57  
    58  // TypeArgs returns the type arguments used to instantiate the Alias type.
    59  // If a is not an instance of a generic alias, the result is nil.
    60  func (a *Alias) TypeArgs() *TypeList { return a.targs }
    61  
    62  // Rhs returns the type R on the right-hand side of an alias
    63  // declaration "type A = R", which may be another alias.
    64  func (a *Alias) Rhs() Type { return a.fromRHS }
    65  
    66  // Unalias returns t if it is not an alias type;
    67  // otherwise it follows t's alias chain until it
    68  // reaches a non-alias type which is then returned.
    69  // Consequently, the result is never an alias type.
    70  func Unalias(t Type) Type {
    71  	if a0, _ := t.(*Alias); a0 != nil {
    72  		return unalias(a0)
    73  	}
    74  	return t
    75  }
    76  
    77  func unalias(a0 *Alias) Type {
    78  	if a0.actual != nil {
    79  		return a0.actual
    80  	}
    81  	var t Type
    82  	for a := a0; a != nil; a, _ = t.(*Alias) {
    83  		t = a.fromRHS
    84  	}
    85  	if t == nil {
    86  		panic(fmt.Sprintf("non-terminated alias %s", a0.obj.name))
    87  	}
    88  
    89  	// Memoize the type only if valid.
    90  	// In the presence of unfinished cyclic declarations, Unalias
    91  	// would otherwise latch the invalid value (#66704).
    92  	// TODO(adonovan): rethink, along with checker.typeDecl's use
    93  	// of Invalid to mark unfinished aliases.
    94  	if t != Typ[Invalid] {
    95  		a0.actual = t
    96  	}
    97  
    98  	return t
    99  }
   100  
   101  // asNamed returns t as *Named if that is t's
   102  // actual type. It returns nil otherwise.
   103  func asNamed(t Type) *Named {
   104  	n, _ := Unalias(t).(*Named)
   105  	return n
   106  }
   107  
   108  // newAlias creates a new Alias type with the given type name and rhs.
   109  // rhs must not be nil.
   110  func (check *Checker) newAlias(obj *TypeName, rhs Type) *Alias {
   111  	assert(rhs != nil)
   112  	a := new(Alias)
   113  	a.obj = obj
   114  	a.orig = a
   115  	a.fromRHS = rhs
   116  	if obj.typ == nil {
   117  		obj.typ = a
   118  	}
   119  
   120  	// Ensure that a.actual is set at the end of type checking.
   121  	if check != nil {
   122  		check.needsCleanup(a)
   123  	}
   124  
   125  	return a
   126  }
   127  
   128  func (a *Alias) cleanup() {
   129  	// Ensure a.actual is set before types are published,
   130  	// so Unalias is a pure "getter", not a "setter".
   131  	actual := Unalias(a)
   132  
   133  	if actual == Typ[Invalid] {
   134  		// We don't set a.actual to Typ[Invalid] during type checking,
   135  		// as it may indicate that the RHS is not fully set up.
   136  		a.actual = actual
   137  	}
   138  }
   139
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