typexpr.go

Documentation: go/types

     1  // Copyright 2013 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  // This file implements type-checking of identifiers and type expressions.
     6  
     7  package types
     8  
     9  import (
    10  	"fmt"
    11  	"go/ast"
    12  	"go/constant"
    13  	"go/internal/typeparams"
    14  	"strings"
    15  )
    16  
    17  // ident type-checks identifier e and initializes x with the value or type of e.
    18  // If an error occurred, x.mode is set to invalid.
    19  // For the meaning of def, see Checker.definedType, below.
    20  // If wantType is set, the identifier e is expected to denote a type.
    21  func (check *Checker) ident(x *operand, e *ast.Ident, def *Named, wantType bool) {
    22  	x.mode = invalid
    23  	x.expr = e
    24  
    25  	// Note that we cannot use check.lookup here because the returned scope
    26  	// may be different from obj.Parent(). See also Scope.LookupParent doc.
    27  	scope, obj := check.scope.LookupParent(e.Name, check.pos)
    28  	switch obj {
    29  	case nil:
    30  		if e.Name == "_" {
    31  			// Blank identifiers are never declared, but the current identifier may
    32  			// be a placeholder for a receiver type parameter. In this case we can
    33  			// resolve its type and object from Checker.recvTParamMap.
    34  			if tpar := check.recvTParamMap[e]; tpar != nil {
    35  				x.mode = typexpr
    36  				x.typ = tpar
    37  			} else {
    38  				check.error(e, _InvalidBlank, "cannot use _ as value or type")
    39  			}
    40  		} else {
    41  			check.errorf(e, _UndeclaredName, "undeclared name: %s", e.Name)
    42  		}
    43  		return
    44  	case universeAny, universeComparable:
    45  		if !check.allowVersion(check.pkg, 1, 18) {
    46  			check.versionErrorf(e, _UndeclaredName, "go1.18", "predeclared %s", e.Name)
    47  			return // avoid follow-on errors
    48  		}
    49  	}
    50  	check.recordUse(e, obj)
    51  
    52  	// Type-check the object.
    53  	// Only call Checker.objDecl if the object doesn't have a type yet
    54  	// (in which case we must actually determine it) or the object is a
    55  	// TypeName and we also want a type (in which case we might detect
    56  	// a cycle which needs to be reported). Otherwise we can skip the
    57  	// call and avoid a possible cycle error in favor of the more
    58  	// informative "not a type/value" error that this function's caller
    59  	// will issue (see issue #25790).
    60  	typ := obj.Type()
    61  	if _, gotType := obj.(*TypeName); typ == nil || gotType && wantType {
    62  		check.objDecl(obj, def)
    63  		typ = obj.Type() // type must have been assigned by Checker.objDecl
    64  	}
    65  	assert(typ != nil)
    66  
    67  	// The object may have been dot-imported.
    68  	// If so, mark the respective package as used.
    69  	// (This code is only needed for dot-imports. Without them,
    70  	// we only have to mark variables, see *Var case below).
    71  	if pkgName := check.dotImportMap[dotImportKey{scope, obj.Name()}]; pkgName != nil {
    72  		pkgName.used = true
    73  	}
    74  
    75  	switch obj := obj.(type) {
    76  	case *PkgName:
    77  		check.errorf(e, _InvalidPkgUse, "use of package %s not in selector", obj.name)
    78  		return
    79  
    80  	case *Const:
    81  		check.addDeclDep(obj)
    82  		if typ == Typ[Invalid] {
    83  			return
    84  		}
    85  		if obj == universeIota {
    86  			if check.iota == nil {
    87  				check.errorf(e, _InvalidIota, "cannot use iota outside constant declaration")
    88  				return
    89  			}
    90  			x.val = check.iota
    91  		} else {
    92  			x.val = obj.val
    93  		}
    94  		assert(x.val != nil)
    95  		x.mode = constant_
    96  
    97  	case *TypeName:
    98  		if check.isBrokenAlias(obj) {
    99  			check.errorf(e, _InvalidDeclCycle, "invalid use of type alias %s in recursive type (see issue #50729)", obj.name)
   100  			return
   101  		}
   102  		x.mode = typexpr
   103  
   104  	case *Var:
   105  		// It's ok to mark non-local variables, but ignore variables
   106  		// from other packages to avoid potential race conditions with
   107  		// dot-imported variables.
   108  		if obj.pkg == check.pkg {
   109  			obj.used = true
   110  		}
   111  		check.addDeclDep(obj)
   112  		if typ == Typ[Invalid] {
   113  			return
   114  		}
   115  		x.mode = variable
   116  
   117  	case *Func:
   118  		check.addDeclDep(obj)
   119  		x.mode = value
   120  
   121  	case *Builtin:
   122  		x.id = obj.id
   123  		x.mode = builtin
   124  
   125  	case *Nil:
   126  		x.mode = value
   127  
   128  	default:
   129  		unreachable()
   130  	}
   131  
   132  	x.typ = typ
   133  }
   134  
   135  // typ type-checks the type expression e and returns its type, or Typ[Invalid].
   136  // The type must not be an (uninstantiated) generic type.
   137  func (check *Checker) typ(e ast.Expr) Type {
   138  	return check.definedType(e, nil)
   139  }
   140  
   141  // varType type-checks the type expression e and returns its type, or Typ[Invalid].
   142  // The type must not be an (uninstantiated) generic type and it must not be a
   143  // constraint interface.
   144  func (check *Checker) varType(e ast.Expr) Type {
   145  	typ := check.definedType(e, nil)
   146  	check.validVarType(e, typ)
   147  	return typ
   148  }
   149  
   150  // validVarType reports an error if typ is a constraint interface.
   151  // The expression e is used for error reporting, if any.
   152  func (check *Checker) validVarType(e ast.Expr, typ Type) {
   153  	// If we have a type parameter there's nothing to do.
   154  	if isTypeParam(typ) {
   155  		return
   156  	}
   157  
   158  	// We don't want to call under() or complete interfaces while we are in
   159  	// the middle of type-checking parameter declarations that might belong
   160  	// to interface methods. Delay this check to the end of type-checking.
   161  	check.later(func() {
   162  		if t, _ := under(typ).(*Interface); t != nil {
   163  			tset := computeInterfaceTypeSet(check, e.Pos(), t) // TODO(gri) is this the correct position?
   164  			if !tset.IsMethodSet() {
   165  				if tset.comparable {
   166  					check.softErrorf(e, _MisplacedConstraintIface, "cannot use type %s outside a type constraint: interface is (or embeds) comparable", typ)
   167  				} else {
   168  					check.softErrorf(e, _MisplacedConstraintIface, "cannot use type %s outside a type constraint: interface contains type constraints", typ)
   169  				}
   170  			}
   171  		}
   172  	}).describef(e, "check var type %s", typ)
   173  }
   174  
   175  // definedType is like typ but also accepts a type name def.
   176  // If def != nil, e is the type specification for the defined type def, declared
   177  // in a type declaration, and def.underlying will be set to the type of e before
   178  // any components of e are type-checked.
   179  func (check *Checker) definedType(e ast.Expr, def *Named) Type {
   180  	typ := check.typInternal(e, def)
   181  	assert(isTyped(typ))
   182  	if isGeneric(typ) {
   183  		check.errorf(e, _WrongTypeArgCount, "cannot use generic type %s without instantiation", typ)
   184  		typ = Typ[Invalid]
   185  	}
   186  	check.recordTypeAndValue(e, typexpr, typ, nil)
   187  	return typ
   188  }
   189  
   190  // genericType is like typ but the type must be an (uninstantiated) generic
   191  // type. If reason is non-nil and the type expression was a valid type but not
   192  // generic, reason will be populated with a message describing the error.
   193  func (check *Checker) genericType(e ast.Expr, reason *string) Type {
   194  	typ := check.typInternal(e, nil)
   195  	assert(isTyped(typ))
   196  	if typ != Typ[Invalid] && !isGeneric(typ) {
   197  		if reason != nil {
   198  			*reason = check.sprintf("%s is not a generic type", typ)
   199  		}
   200  		typ = Typ[Invalid]
   201  	}
   202  	// TODO(gri) what is the correct call below?
   203  	check.recordTypeAndValue(e, typexpr, typ, nil)
   204  	return typ
   205  }
   206  
   207  // goTypeName returns the Go type name for typ and
   208  // removes any occurrences of "types." from that name.
   209  func goTypeName(typ Type) string {
   210  	return strings.ReplaceAll(fmt.Sprintf("%T", typ), "types.", "")
   211  }
   212  
   213  // typInternal drives type checking of types.
   214  // Must only be called by definedType or genericType.
   215  func (check *Checker) typInternal(e0 ast.Expr, def *Named) (T Type) {
   216  	if trace {
   217  		check.trace(e0.Pos(), "-- type %s", e0)
   218  		check.indent++
   219  		defer func() {
   220  			check.indent--
   221  			var under Type
   222  			if T != nil {
   223  				// Calling under() here may lead to endless instantiations.
   224  				// Test case: type T[P any] *T[P]
   225  				under = safeUnderlying(T)
   226  			}
   227  			if T == under {
   228  				check.trace(e0.Pos(), "=> %s // %s", T, goTypeName(T))
   229  			} else {
   230  				check.trace(e0.Pos(), "=> %s (under = %s) // %s", T, under, goTypeName(T))
   231  			}
   232  		}()
   233  	}
   234  
   235  	switch e := e0.(type) {
   236  	case *ast.BadExpr:
   237  		// ignore - error reported before
   238  
   239  	case *ast.Ident:
   240  		var x operand
   241  		check.ident(&x, e, def, true)
   242  
   243  		switch x.mode {
   244  		case typexpr:
   245  			typ := x.typ
   246  			def.setUnderlying(typ)
   247  			return typ
   248  		case invalid:
   249  			// ignore - error reported before
   250  		case novalue:
   251  			check.errorf(&x, _NotAType, "%s used as type", &x)
   252  		default:
   253  			check.errorf(&x, _NotAType, "%s is not a type", &x)
   254  		}
   255  
   256  	case *ast.SelectorExpr:
   257  		var x operand
   258  		check.selector(&x, e, def)
   259  
   260  		switch x.mode {
   261  		case typexpr:
   262  			typ := x.typ
   263  			def.setUnderlying(typ)
   264  			return typ
   265  		case invalid:
   266  			// ignore - error reported before
   267  		case novalue:
   268  			check.errorf(&x, _NotAType, "%s used as type", &x)
   269  		default:
   270  			check.errorf(&x, _NotAType, "%s is not a type", &x)
   271  		}
   272  
   273  	case *ast.IndexExpr, *ast.IndexListExpr:
   274  		ix := typeparams.UnpackIndexExpr(e)
   275  		if !check.allowVersion(check.pkg, 1, 18) {
   276  			check.softErrorf(inNode(e, ix.Lbrack), _UnsupportedFeature, "type instantiation requires go1.18 or later")
   277  		}
   278  		return check.instantiatedType(ix, def)
   279  
   280  	case *ast.ParenExpr:
   281  		// Generic types must be instantiated before they can be used in any form.
   282  		// Consequently, generic types cannot be parenthesized.
   283  		return check.definedType(e.X, def)
   284  
   285  	case *ast.ArrayType:
   286  		if e.Len == nil {
   287  			typ := new(Slice)
   288  			def.setUnderlying(typ)
   289  			typ.elem = check.varType(e.Elt)
   290  			return typ
   291  		}
   292  
   293  		typ := new(Array)
   294  		def.setUnderlying(typ)
   295  		typ.len = check.arrayLength(e.Len)
   296  		typ.elem = check.varType(e.Elt)
   297  		if typ.len >= 0 {
   298  			return typ
   299  		}
   300  
   301  	case *ast.Ellipsis:
   302  		// dots are handled explicitly where they are legal
   303  		// (array composite literals and parameter lists)
   304  		check.error(e, _InvalidDotDotDot, "invalid use of '...'")
   305  		check.use(e.Elt)
   306  
   307  	case *ast.StructType:
   308  		typ := new(Struct)
   309  		def.setUnderlying(typ)
   310  		check.structType(typ, e)
   311  		return typ
   312  
   313  	case *ast.StarExpr:
   314  		typ := new(Pointer)
   315  		typ.base = Typ[Invalid] // avoid nil base in invalid recursive type declaration
   316  		def.setUnderlying(typ)
   317  		typ.base = check.varType(e.X)
   318  		return typ
   319  
   320  	case *ast.FuncType:
   321  		typ := new(Signature)
   322  		def.setUnderlying(typ)
   323  		check.funcType(typ, nil, e)
   324  		return typ
   325  
   326  	case *ast.InterfaceType:
   327  		typ := check.newInterface()
   328  		def.setUnderlying(typ)
   329  		check.interfaceType(typ, e, def)
   330  		return typ
   331  
   332  	case *ast.MapType:
   333  		typ := new(Map)
   334  		def.setUnderlying(typ)
   335  
   336  		typ.key = check.varType(e.Key)
   337  		typ.elem = check.varType(e.Value)
   338  
   339  		// spec: "The comparison operators == and != must be fully defined
   340  		// for operands of the key type; thus the key type must not be a
   341  		// function, map, or slice."
   342  		//
   343  		// Delay this check because it requires fully setup types;
   344  		// it is safe to continue in any case (was issue 6667).
   345  		check.later(func() {
   346  			if !Comparable(typ.key) {
   347  				var why string
   348  				if isTypeParam(typ.key) {
   349  					why = " (missing comparable constraint)"
   350  				}
   351  				check.errorf(e.Key, _IncomparableMapKey, "incomparable map key type %s%s", typ.key, why)
   352  			}
   353  		}).describef(e.Key, "check map key %s", typ.key)
   354  
   355  		return typ
   356  
   357  	case *ast.ChanType:
   358  		typ := new(Chan)
   359  		def.setUnderlying(typ)
   360  
   361  		dir := SendRecv
   362  		switch e.Dir {
   363  		case ast.SEND | ast.RECV:
   364  			// nothing to do
   365  		case ast.SEND:
   366  			dir = SendOnly
   367  		case ast.RECV:
   368  			dir = RecvOnly
   369  		default:
   370  			check.invalidAST(e, "unknown channel direction %d", e.Dir)
   371  			// ok to continue
   372  		}
   373  
   374  		typ.dir = dir
   375  		typ.elem = check.varType(e.Value)
   376  		return typ
   377  
   378  	default:
   379  		check.errorf(e0, _NotAType, "%s is not a type", e0)
   380  	}
   381  
   382  	typ := Typ[Invalid]
   383  	def.setUnderlying(typ)
   384  	return typ
   385  }
   386  
   387  func (check *Checker) instantiatedType(ix *typeparams.IndexExpr, def *Named) (res Type) {
   388  	if trace {
   389  		check.trace(ix.Pos(), "-- instantiating type %s with %s", ix.X, ix.Indices)
   390  		check.indent++
   391  		defer func() {
   392  			check.indent--
   393  			// Don't format the underlying here. It will always be nil.
   394  			check.trace(ix.Pos(), "=> %s", res)
   395  		}()
   396  	}
   397  
   398  	var reason string
   399  	gtyp := check.genericType(ix.X, &reason)
   400  	if reason != "" {
   401  		check.invalidOp(ix.Orig, _NotAGenericType, "%s (%s)", ix.Orig, reason)
   402  	}
   403  	if gtyp == Typ[Invalid] {
   404  		return gtyp // error already reported
   405  	}
   406  
   407  	orig, _ := gtyp.(*Named)
   408  	if orig == nil {
   409  		panic(fmt.Sprintf("%v: cannot instantiate %v", ix.Pos(), gtyp))
   410  	}
   411  
   412  	// evaluate arguments
   413  	targs := check.typeList(ix.Indices)
   414  	if targs == nil {
   415  		def.setUnderlying(Typ[Invalid]) // avoid errors later due to lazy instantiation
   416  		return Typ[Invalid]
   417  	}
   418  
   419  	// create the instance
   420  	inst := check.instance(ix.Pos(), orig, targs, nil, check.context()).(*Named)
   421  	def.setUnderlying(inst)
   422  
   423  	// orig.tparams may not be set up, so we need to do expansion later.
   424  	check.later(func() {
   425  		// This is an instance from the source, not from recursive substitution,
   426  		// and so it must be resolved during type-checking so that we can report
   427  		// errors.
   428  		check.recordInstance(ix.Orig, inst.TypeArgs().list(), inst)
   429  
   430  		if check.validateTArgLen(ix.Pos(), inst.TypeParams().Len(), inst.TypeArgs().Len()) {
   431  			if i, err := check.verify(ix.Pos(), inst.TypeParams().list(), inst.TypeArgs().list(), check.context()); err != nil {
   432  				// best position for error reporting
   433  				pos := ix.Pos()
   434  				if i < len(ix.Indices) {
   435  					pos = ix.Indices[i].Pos()
   436  				}
   437  				check.softErrorf(atPos(pos), _InvalidTypeArg, err.Error())
   438  			} else {
   439  				check.mono.recordInstance(check.pkg, ix.Pos(), inst.TypeParams().list(), inst.TypeArgs().list(), ix.Indices)
   440  			}
   441  		}
   442  
   443  		// TODO(rfindley): remove this call: we don't need to call validType here,
   444  		// as cycles can only occur for types used inside a Named type declaration,
   445  		// and so it suffices to call validType from declared types.
   446  		check.validType(inst)
   447  	}).describef(ix, "resolve instance %s", inst)
   448  
   449  	return inst
   450  }
   451  
   452  // arrayLength type-checks the array length expression e
   453  // and returns the constant length >= 0, or a value < 0
   454  // to indicate an error (and thus an unknown length).
   455  func (check *Checker) arrayLength(e ast.Expr) int64 {
   456  	// If e is an identifier, the array declaration might be an
   457  	// attempt at a parameterized type declaration with missing
   458  	// constraint. Provide an error message that mentions array
   459  	// length.
   460  	if name, _ := e.(*ast.Ident); name != nil {
   461  		obj := check.lookup(name.Name)
   462  		if obj == nil {
   463  			check.errorf(name, _InvalidArrayLen, "undeclared name %s for array length", name.Name)
   464  			return -1
   465  		}
   466  		if _, ok := obj.(*Const); !ok {
   467  			check.errorf(name, _InvalidArrayLen, "invalid array length %s", name.Name)
   468  			return -1
   469  		}
   470  	}
   471  
   472  	var x operand
   473  	check.expr(&x, e)
   474  	if x.mode != constant_ {
   475  		if x.mode != invalid {
   476  			check.errorf(&x, _InvalidArrayLen, "array length %s must be constant", &x)
   477  		}
   478  		return -1
   479  	}
   480  
   481  	if isUntyped(x.typ) || isInteger(x.typ) {
   482  		if val := constant.ToInt(x.val); val.Kind() == constant.Int {
   483  			if representableConst(val, check, Typ[Int], nil) {
   484  				if n, ok := constant.Int64Val(val); ok && n >= 0 {
   485  					return n
   486  				}
   487  				check.errorf(&x, _InvalidArrayLen, "invalid array length %s", &x)
   488  				return -1
   489  			}
   490  		}
   491  	}
   492  
   493  	check.errorf(&x, _InvalidArrayLen, "array length %s must be integer", &x)
   494  	return -1
   495  }
   496  
   497  // typeList provides the list of types corresponding to the incoming expression list.
   498  // If an error occurred, the result is nil, but all list elements were type-checked.
   499  func (check *Checker) typeList(list []ast.Expr) []Type {
   500  	res := make([]Type, len(list)) // res != nil even if len(list) == 0
   501  	for i, x := range list {
   502  		t := check.varType(x)
   503  		if t == Typ[Invalid] {
   504  			res = nil
   505  		}
   506  		if res != nil {
   507  			res[i] = t
   508  		}
   509  	}
   510  	return res
   511  }
   512
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