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 (
     8  	"cmd/compile/internal/syntax"
     9  	"fmt"
    10  )
    11  
    12  // An Alias represents an alias type.
    13  //
    14  // Alias types are created by alias declarations such as:
    15  //
    16  //	type A = int
    17  //
    18  // The type on the right-hand side of the declaration can be accessed
    19  // using [Alias.Rhs]. This type may itself be an alias.
    20  // Call [Unalias] to obtain the first non-alias type in a chain of
    21  // alias type declarations.
    22  //
    23  // Like a defined ([Named]) type, an alias type has a name.
    24  // Use the [Alias.Obj] method to access its [TypeName] object.
    25  //
    26  // Historically, Alias types were not materialized so that, in the example
    27  // above, A's type was represented by a Basic (int), not an Alias
    28  // whose [Alias.Rhs] is int. But Go 1.24 allows you to declare an
    29  // alias type with type parameters or arguments:
    30  //
    31  //	type Set[K comparable] = map[K]bool
    32  //	s := make(Set[String])
    33  //
    34  // and this requires that Alias types be materialized. Use the
    35  // [Alias.TypeParams] and [Alias.TypeArgs] methods to access them.
    36  //
    37  // To ease the transition, the Alias type was introduced in go1.22,
    38  // but the type-checker would not construct values of this type unless
    39  // the GODEBUG=gotypesalias=1 environment variable was provided.
    40  // Starting in go1.23, this variable is enabled by default.
    41  // This setting also causes the predeclared type "any" to be
    42  // represented as an Alias, not a bare [Interface].
    43  type Alias struct {
    44  	obj     *TypeName      // corresponding declared alias object
    45  	orig    *Alias         // original, uninstantiated alias
    46  	tparams *TypeParamList // type parameters, or nil
    47  	targs   *TypeList      // type arguments, or nil
    48  	fromRHS Type           // RHS of type alias declaration; may be an alias
    49  	actual  Type           // actual (aliased) type; never an alias
    50  }
    51  
    52  // NewAlias creates a new Alias type with the given type name and rhs.
    53  // rhs must not be nil.
    54  func NewAlias(obj *TypeName, rhs Type) *Alias {
    55  	alias := (*Checker)(nil).newAlias(obj, rhs)
    56  	// Ensure that alias.actual is set (#65455).
    57  	alias.cleanup()
    58  	return alias
    59  }
    60  
    61  // Obj returns the type name for the declaration defining the alias type a.
    62  // For instantiated types, this is same as the type name of the origin type.
    63  func (a *Alias) Obj() *TypeName { return a.orig.obj }
    64  
    65  func (a *Alias) String() string { return TypeString(a, nil) }
    66  
    67  // Underlying returns the [underlying type] of the alias type a, which is the
    68  // underlying type of the aliased type. Underlying types are never Named,
    69  // TypeParam, or Alias types.
    70  //
    71  // [underlying type]: https://go.dev/ref/spec#Underlying_types.
    72  func (a *Alias) Underlying() Type { return unalias(a).Underlying() }
    73  
    74  // Origin returns the generic Alias type of which a is an instance.
    75  // If a is not an instance of a generic alias, Origin returns a.
    76  func (a *Alias) Origin() *Alias { return a.orig }
    77  
    78  // TypeParams returns the type parameters of the alias type a, or nil.
    79  // A generic Alias and its instances have the same type parameters.
    80  func (a *Alias) TypeParams() *TypeParamList { return a.tparams }
    81  
    82  // SetTypeParams sets the type parameters of the alias type a.
    83  // The alias a must not have type arguments.
    84  func (a *Alias) SetTypeParams(tparams []*TypeParam) {
    85  	assert(a.targs == nil)
    86  	a.tparams = bindTParams(tparams)
    87  }
    88  
    89  // TypeArgs returns the type arguments used to instantiate the Alias type.
    90  // If a is not an instance of a generic alias, the result is nil.
    91  func (a *Alias) TypeArgs() *TypeList { return a.targs }
    92  
    93  // Rhs returns the type R on the right-hand side of an alias
    94  // declaration "type A = R", which may be another alias.
    95  func (a *Alias) Rhs() Type { return a.fromRHS }
    96  
    97  // Unalias returns t if it is not an alias type;
    98  // otherwise it follows t's alias chain until it
    99  // reaches a non-alias type which is then returned.
   100  // Consequently, the result is never an alias type.
   101  func Unalias(t Type) Type {
   102  	if a0, _ := t.(*Alias); a0 != nil {
   103  		return unalias(a0)
   104  	}
   105  	return t
   106  }
   107  
   108  func unalias(a0 *Alias) Type {
   109  	if a0.actual != nil {
   110  		return a0.actual
   111  	}
   112  	var t Type
   113  	for a := a0; a != nil; a, _ = t.(*Alias) {
   114  		t = a.fromRHS
   115  	}
   116  	if t == nil {
   117  		panic(fmt.Sprintf("non-terminated alias %s", a0.obj.name))
   118  	}
   119  
   120  	// Memoize the type only if valid.
   121  	// In the presence of unfinished cyclic declarations, Unalias
   122  	// would otherwise latch the invalid value (#66704).
   123  	// TODO(adonovan): rethink, along with checker.typeDecl's use
   124  	// of Invalid to mark unfinished aliases.
   125  	if t != Typ[Invalid] {
   126  		a0.actual = t
   127  	}
   128  
   129  	return t
   130  }
   131  
   132  // asNamed returns t as *Named if that is t's
   133  // actual type. It returns nil otherwise.
   134  func asNamed(t Type) *Named {
   135  	n, _ := Unalias(t).(*Named)
   136  	return n
   137  }
   138  
   139  // newAlias creates a new Alias type with the given type name and rhs.
   140  // rhs must not be nil.
   141  func (check *Checker) newAlias(obj *TypeName, rhs Type) *Alias {
   142  	assert(rhs != nil)
   143  	a := new(Alias)
   144  	a.obj = obj
   145  	a.orig = a
   146  	a.fromRHS = rhs
   147  	if obj.typ == nil {
   148  		obj.typ = a
   149  	}
   150  
   151  	// Ensure that a.actual is set at the end of type checking.
   152  	if check != nil {
   153  		check.needsCleanup(a)
   154  	}
   155  
   156  	return a
   157  }
   158  
   159  // newAliasInstance creates a new alias instance for the given origin and type
   160  // arguments, recording pos as the position of its synthetic object (for error
   161  // reporting).
   162  func (check *Checker) newAliasInstance(pos syntax.Pos, orig *Alias, targs []Type, expanding *Named, ctxt *Context) *Alias {
   163  	assert(len(targs) > 0)
   164  	obj := NewTypeName(pos, orig.obj.pkg, orig.obj.name, nil)
   165  	rhs := check.subst(pos, orig.fromRHS, makeSubstMap(orig.TypeParams().list(), targs), expanding, ctxt)
   166  	res := check.newAlias(obj, rhs)
   167  	res.orig = orig
   168  	res.tparams = orig.tparams
   169  	res.targs = newTypeList(targs)
   170  	return res
   171  }
   172  
   173  func (a *Alias) cleanup() {
   174  	// Ensure a.actual is set before types are published,
   175  	// so Unalias is a pure "getter", not a "setter".
   176  	actual := Unalias(a)
   177  
   178  	if actual == Typ[Invalid] {
   179  		// We don't set a.actual to Typ[Invalid] during type checking,
   180  		// as it may indicate that the RHS is not fully set up.
   181  		a.actual = actual
   182  	}
   183  }
   184
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