Source file src/go/types/expr.go

     1  // Copyright 2012 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 typechecking of expressions.
     6  
     7  package types
     8  
     9  import (
    10  	"fmt"
    11  	"go/ast"
    12  	"go/constant"
    13  	"go/token"
    14  	. "internal/types/errors"
    15  )
    16  
    17  /*
    18  Basic algorithm:
    19  
    20  Expressions are checked recursively, top down. Expression checker functions
    21  are generally of the form:
    22  
    23    func f(x *operand, e *ast.Expr, ...)
    24  
    25  where e is the expression to be checked, and x is the result of the check.
    26  The check performed by f may fail in which case x.mode_ == invalid, and
    27  related error messages will have been issued by f.
    28  
    29  If a hint argument is present, it is the composite literal element type
    30  of an outer composite literal; it is used to type-check composite literal
    31  elements that have no explicit type specification in the source
    32  (e.g.: []T{{...}, {...}}, the hint is the type T in this case).
    33  
    34  All expressions are checked via rawExpr, which dispatches according
    35  to expression kind. Upon returning, rawExpr is recording the types and
    36  constant values for all expressions that have an untyped type (those types
    37  may change on the way up in the expression tree). Usually these are constants,
    38  but the results of comparisons or non-constant shifts of untyped constants
    39  may also be untyped, but not constant.
    40  
    41  Untyped expressions may eventually become fully typed (i.e., not untyped),
    42  typically when the value is assigned to a variable, or is used otherwise.
    43  The updateExprType method is used to record this final type and update
    44  the recorded types: the type-checked expression tree is again traversed down,
    45  and the new type is propagated as needed. Untyped constant expression values
    46  that become fully typed must now be representable by the full type (constant
    47  sub-expression trees are left alone except for their roots). This mechanism
    48  ensures that a client sees the actual (run-time) type an untyped value would
    49  have. It also permits type-checking of lhs shift operands "as if the shift
    50  were not present": when updateExprType visits an untyped lhs shift operand
    51  and assigns it its final type, that type must be an integer type, and a
    52  constant lhs must be representable as an integer.
    53  
    54  When an expression gets its final type, either on the way out from rawExpr,
    55  on the way down in updateExprType, or at the end of the type checker run,
    56  the type (and constant value, if any) is recorded via Info.Types, if present.
    57  */
    58  
    59  type opPredicates map[token.Token]func(Type) bool
    60  
    61  var unaryOpPredicates opPredicates
    62  
    63  func init() {
    64  	// Setting unaryOpPredicates in init avoids declaration cycles.
    65  	unaryOpPredicates = opPredicates{
    66  		token.ADD: allNumeric,
    67  		token.SUB: allNumeric,
    68  		token.XOR: allInteger,
    69  		token.NOT: allBoolean,
    70  	}
    71  }
    72  
    73  func (check *Checker) op(m opPredicates, x *operand, op token.Token) bool {
    74  	if pred := m[op]; pred != nil {
    75  		if !pred(x.typ()) {
    76  			check.errorf(x, UndefinedOp, invalidOp+"operator %s not defined on %s", op, x)
    77  			return false
    78  		}
    79  	} else {
    80  		check.errorf(x, InvalidSyntaxTree, "unknown operator %s", op)
    81  		return false
    82  	}
    83  	return true
    84  }
    85  
    86  // opPos returns the position of the operator if x is an operation;
    87  // otherwise it returns the start position of x.
    88  func opPos(x ast.Expr) token.Pos {
    89  	switch op := x.(type) {
    90  	case nil:
    91  		return nopos // don't crash
    92  	case *ast.BinaryExpr:
    93  		return op.OpPos
    94  	default:
    95  		return x.Pos()
    96  	}
    97  }
    98  
    99  // opName returns the name of the operation if x is an operation
   100  // that might overflow; otherwise it returns the empty string.
   101  func opName(e ast.Expr) string {
   102  	switch e := e.(type) {
   103  	case *ast.BinaryExpr:
   104  		if int(e.Op) < len(op2str2) {
   105  			return op2str2[e.Op]
   106  		}
   107  	case *ast.UnaryExpr:
   108  		if int(e.Op) < len(op2str1) {
   109  			return op2str1[e.Op]
   110  		}
   111  	}
   112  	return ""
   113  }
   114  
   115  var op2str1 = [...]string{
   116  	token.XOR: "bitwise complement",
   117  }
   118  
   119  // This is only used for operations that may cause overflow.
   120  var op2str2 = [...]string{
   121  	token.ADD: "addition",
   122  	token.SUB: "subtraction",
   123  	token.XOR: "bitwise XOR",
   124  	token.MUL: "multiplication",
   125  	token.SHL: "shift",
   126  }
   127  
   128  // The unary expression e may be nil. It's passed in for better error messages only.
   129  func (check *Checker) unary(x *operand, e *ast.UnaryExpr) {
   130  	check.expr(nil, x, e.X)
   131  	if !x.isValid() {
   132  		return
   133  	}
   134  
   135  	op := e.Op
   136  	switch op {
   137  	case token.AND:
   138  		// spec: "As an exception to the addressability
   139  		// requirement x may also be a composite literal."
   140  		if _, ok := ast.Unparen(e.X).(*ast.CompositeLit); !ok && x.mode() != variable {
   141  			check.errorf(x, UnaddressableOperand, invalidOp+"cannot take address of %s", x)
   142  			x.invalidate()
   143  			return
   144  		}
   145  		x.mode_ = value
   146  		x.typ_ = &Pointer{base: x.typ()}
   147  		return
   148  
   149  	case token.ARROW:
   150  		// We cannot receive a value with an incomplete type; make sure it's complete.
   151  		if elem := check.chanElem(x, x, true); elem != nil && check.isComplete(elem) {
   152  			x.mode_ = commaok
   153  			x.typ_ = elem
   154  			check.hasCallOrRecv = true
   155  			return
   156  		}
   157  		x.invalidate()
   158  		return
   159  
   160  	case token.TILDE:
   161  		// Provide a better error position and message than what check.op below would do.
   162  		if !allInteger(x.typ()) {
   163  			check.error(e, UndefinedOp, "cannot use ~ outside of interface or type constraint")
   164  			x.invalidate()
   165  			return
   166  		}
   167  		check.error(e, UndefinedOp, "cannot use ~ outside of interface or type constraint (use ^ for bitwise complement)")
   168  		op = token.XOR
   169  	}
   170  
   171  	if !check.op(unaryOpPredicates, x, op) {
   172  		x.invalidate()
   173  		return
   174  	}
   175  
   176  	if x.mode() == constant_ {
   177  		if x.val.Kind() == constant.Unknown {
   178  			// nothing to do (and don't cause an error below in the overflow check)
   179  			return
   180  		}
   181  		var prec uint
   182  		if isUnsigned(x.typ()) {
   183  			prec = uint(check.conf.sizeof(x.typ()) * 8)
   184  		}
   185  		x.val = constant.UnaryOp(op, x.val, prec)
   186  		x.expr = e
   187  		check.overflow(x, opPos(x.expr))
   188  		return
   189  	}
   190  
   191  	x.mode_ = value
   192  	// x.typ remains unchanged
   193  }
   194  
   195  // chanElem returns the channel element type of x for a receive from x (recv == true)
   196  // or send to x (recv == false) operation. If the operation is not valid, chanElem
   197  // reports an error and returns nil.
   198  func (check *Checker) chanElem(pos positioner, x *operand, recv bool) Type {
   199  	u, err := commonUnder(x.typ(), func(t, u Type) *typeError {
   200  		if u == nil {
   201  			return typeErrorf("no specific channel type")
   202  		}
   203  		ch, _ := u.(*Chan)
   204  		if ch == nil {
   205  			return typeErrorf("non-channel %s", t)
   206  		}
   207  		if recv && ch.dir == SendOnly {
   208  			return typeErrorf("send-only channel %s", t)
   209  		}
   210  		if !recv && ch.dir == RecvOnly {
   211  			return typeErrorf("receive-only channel %s", t)
   212  		}
   213  		return nil
   214  	})
   215  
   216  	if u != nil {
   217  		return u.(*Chan).elem
   218  	}
   219  
   220  	cause := err.format(check)
   221  	if recv {
   222  		if isTypeParam(x.typ()) {
   223  			check.errorf(pos, InvalidReceive, invalidOp+"cannot receive from %s: %s", x, cause)
   224  		} else {
   225  			// In this case, only the non-channel and send-only channel error are possible.
   226  			check.errorf(pos, InvalidReceive, invalidOp+"cannot receive from %s %s", cause, x)
   227  		}
   228  	} else {
   229  		if isTypeParam(x.typ()) {
   230  			check.errorf(pos, InvalidSend, invalidOp+"cannot send to %s: %s", x, cause)
   231  		} else {
   232  			// In this case, only the non-channel and receive-only channel error are possible.
   233  			check.errorf(pos, InvalidSend, invalidOp+"cannot send to %s %s", cause, x)
   234  		}
   235  	}
   236  	return nil
   237  }
   238  
   239  func isShift(op token.Token) bool {
   240  	return op == token.SHL || op == token.SHR
   241  }
   242  
   243  func isComparison(op token.Token) bool {
   244  	// Note: tokens are not ordered well to make this much easier
   245  	switch op {
   246  	case token.EQL, token.NEQ, token.LSS, token.LEQ, token.GTR, token.GEQ:
   247  		return true
   248  	}
   249  	return false
   250  }
   251  
   252  // updateExprType updates the type of x to typ and invokes itself
   253  // recursively for the operands of x, depending on expression kind.
   254  // If typ is still an untyped and not the final type, updateExprType
   255  // only updates the recorded untyped type for x and possibly its
   256  // operands. Otherwise (i.e., typ is not an untyped type anymore,
   257  // or it is the final type for x), the type and value are recorded.
   258  // Also, if x is a constant, it must be representable as a value of typ,
   259  // and if x is the (formerly untyped) lhs operand of a non-constant
   260  // shift, it must be an integer value.
   261  func (check *Checker) updateExprType(x ast.Expr, typ Type, final bool) {
   262  	old, found := check.untyped[x]
   263  	if !found {
   264  		return // nothing to do
   265  	}
   266  
   267  	// update operands of x if necessary
   268  	switch x := x.(type) {
   269  	case *ast.BadExpr,
   270  		*ast.FuncLit,
   271  		*ast.CompositeLit,
   272  		*ast.IndexExpr,
   273  		*ast.SliceExpr,
   274  		*ast.TypeAssertExpr,
   275  		*ast.StarExpr,
   276  		*ast.KeyValueExpr,
   277  		*ast.ArrayType,
   278  		*ast.StructType,
   279  		*ast.FuncType,
   280  		*ast.InterfaceType,
   281  		*ast.MapType,
   282  		*ast.ChanType:
   283  		// These expression are never untyped - nothing to do.
   284  		// The respective sub-expressions got their final types
   285  		// upon assignment or use.
   286  		if debug {
   287  			check.dump("%v: found old type(%s): %s (new: %s)", x.Pos(), x, old.typ, typ)
   288  			panic("unreachable")
   289  		}
   290  		return
   291  
   292  	case *ast.CallExpr:
   293  		// Resulting in an untyped constant (e.g., built-in complex).
   294  		// The respective calls take care of calling updateExprType
   295  		// for the arguments if necessary.
   296  
   297  	case *ast.Ident, *ast.BasicLit, *ast.SelectorExpr:
   298  		// An identifier denoting a constant, a constant literal,
   299  		// or a qualified identifier (imported untyped constant).
   300  		// No operands to take care of.
   301  
   302  	case *ast.ParenExpr:
   303  		check.updateExprType(x.X, typ, final)
   304  
   305  	case *ast.UnaryExpr:
   306  		// If x is a constant, the operands were constants.
   307  		// The operands don't need to be updated since they
   308  		// never get "materialized" into a typed value. If
   309  		// left in the untyped map, they will be processed
   310  		// at the end of the type check.
   311  		if old.val != nil {
   312  			break
   313  		}
   314  		check.updateExprType(x.X, typ, final)
   315  
   316  	case *ast.BinaryExpr:
   317  		if old.val != nil {
   318  			break // see comment for unary expressions
   319  		}
   320  		if isComparison(x.Op) {
   321  			// The result type is independent of operand types
   322  			// and the operand types must have final types.
   323  		} else if isShift(x.Op) {
   324  			// The result type depends only on lhs operand.
   325  			// The rhs type was updated when checking the shift.
   326  			check.updateExprType(x.X, typ, final)
   327  		} else {
   328  			// The operand types match the result type.
   329  			check.updateExprType(x.X, typ, final)
   330  			check.updateExprType(x.Y, typ, final)
   331  		}
   332  
   333  	default:
   334  		panic("unreachable")
   335  	}
   336  
   337  	// If the new type is not final and still untyped, just
   338  	// update the recorded type.
   339  	if !final && isUntyped(typ) {
   340  		old.typ = typ.Underlying().(*Basic)
   341  		check.untyped[x] = old
   342  		return
   343  	}
   344  
   345  	// Otherwise we have the final (typed or untyped type).
   346  	// Remove it from the map of yet untyped expressions.
   347  	delete(check.untyped, x)
   348  
   349  	if old.isLhs {
   350  		// If x is the lhs of a shift, its final type must be integer.
   351  		// We already know from the shift check that it is representable
   352  		// as an integer if it is a constant.
   353  		if !allInteger(typ) {
   354  			check.errorf(x, InvalidShiftOperand, invalidOp+"shifted operand %s (type %s) must be integer", x, typ)
   355  			return
   356  		}
   357  		// Even if we have an integer, if the value is a constant we
   358  		// still must check that it is representable as the specific
   359  		// int type requested (was go.dev/issue/22969). Fall through here.
   360  	}
   361  	if old.val != nil {
   362  		// If x is a constant, it must be representable as a value of typ.
   363  		c := operand{old.mode, x, old.typ, old.val, 0}
   364  		check.convertUntyped(&c, typ)
   365  		if !c.isValid() {
   366  			return
   367  		}
   368  	}
   369  
   370  	// Everything's fine, record final type and value for x.
   371  	check.recordTypeAndValue(x, old.mode, typ, old.val)
   372  }
   373  
   374  // updateExprVal updates the value of x to val.
   375  func (check *Checker) updateExprVal(x ast.Expr, val constant.Value) {
   376  	if info, ok := check.untyped[x]; ok {
   377  		info.val = val
   378  		check.untyped[x] = info
   379  	}
   380  }
   381  
   382  // implicitTypeAndValue returns the implicit type of x when used in a context
   383  // where the target type is expected. If no such implicit conversion is
   384  // possible, it returns a nil Type and non-zero error code.
   385  //
   386  // If x is a constant operand, the returned constant.Value will be the
   387  // representation of x in this context.
   388  func (check *Checker) implicitTypeAndValue(x *operand, target Type) (Type, constant.Value, Code) {
   389  	if !x.isValid() || isTyped(x.typ()) || !isValid(target) {
   390  		return x.typ(), nil, 0
   391  	}
   392  	// x is untyped
   393  
   394  	if isUntyped(target) {
   395  		// both x and target are untyped
   396  		if m := maxType(x.typ(), target); m != nil {
   397  			return m, nil, 0
   398  		}
   399  		return nil, nil, InvalidUntypedConversion
   400  	}
   401  
   402  	switch u := target.Underlying().(type) {
   403  	case *Basic:
   404  		if x.mode() == constant_ {
   405  			v, code := check.representation(x, u)
   406  			if code != 0 {
   407  				return nil, nil, code
   408  			}
   409  			return target, v, code
   410  		}
   411  		// Non-constant untyped values may appear as the
   412  		// result of comparisons (untyped bool), intermediate
   413  		// (delayed-checked) rhs operands of shifts, and as
   414  		// the value nil.
   415  		switch x.typ().(*Basic).kind {
   416  		case UntypedBool:
   417  			if !isBoolean(target) {
   418  				return nil, nil, InvalidUntypedConversion
   419  			}
   420  		case UntypedInt, UntypedRune, UntypedFloat, UntypedComplex:
   421  			if !isNumeric(target) {
   422  				return nil, nil, InvalidUntypedConversion
   423  			}
   424  		case UntypedString:
   425  			// Non-constant untyped string values are not permitted by the spec and
   426  			// should not occur during normal typechecking passes, but this path is
   427  			// reachable via the AssignableTo API.
   428  			if !isString(target) {
   429  				return nil, nil, InvalidUntypedConversion
   430  			}
   431  		case UntypedNil:
   432  			// Unsafe.Pointer is a basic type that includes nil.
   433  			if !hasNil(target) {
   434  				return nil, nil, InvalidUntypedConversion
   435  			}
   436  			// Preserve the type of nil as UntypedNil: see go.dev/issue/13061.
   437  			return Typ[UntypedNil], nil, 0
   438  		default:
   439  			return nil, nil, InvalidUntypedConversion
   440  		}
   441  	case *Interface:
   442  		if isTypeParam(target) {
   443  			if !underIs(target, func(u Type) bool {
   444  				if u == nil {
   445  					return false
   446  				}
   447  				t, _, _ := check.implicitTypeAndValue(x, u)
   448  				return t != nil
   449  			}) {
   450  				return nil, nil, InvalidUntypedConversion
   451  			}
   452  			// keep nil untyped (was bug go.dev/issue/39755)
   453  			if x.isNil() {
   454  				return Typ[UntypedNil], nil, 0
   455  			}
   456  			break
   457  		}
   458  		// Values must have concrete dynamic types. If the value is nil,
   459  		// keep it untyped (this is important for tools such as go vet which
   460  		// need the dynamic type for argument checking of say, print
   461  		// functions)
   462  		if x.isNil() {
   463  			return Typ[UntypedNil], nil, 0
   464  		}
   465  		// cannot assign untyped values to non-empty interfaces
   466  		if !u.Empty() {
   467  			return nil, nil, InvalidUntypedConversion
   468  		}
   469  		return Default(x.typ()), nil, 0
   470  	case *Pointer, *Signature, *Slice, *Map, *Chan:
   471  		if !x.isNil() {
   472  			return nil, nil, InvalidUntypedConversion
   473  		}
   474  		// Keep nil untyped - see comment for interfaces, above.
   475  		return Typ[UntypedNil], nil, 0
   476  	default:
   477  		return nil, nil, InvalidUntypedConversion
   478  	}
   479  	return target, nil, 0
   480  }
   481  
   482  // If switchCase is true, the operator op is ignored.
   483  func (check *Checker) comparison(x, y *operand, op token.Token, switchCase bool) {
   484  	// Avoid spurious errors if any of the operands has an invalid type (go.dev/issue/54405).
   485  	if !isValid(x.typ()) || !isValid(y.typ()) {
   486  		x.invalidate()
   487  		return
   488  	}
   489  
   490  	if switchCase {
   491  		op = token.EQL
   492  	}
   493  
   494  	errOp := x  // operand for which error is reported, if any
   495  	cause := "" // specific error cause, if any
   496  
   497  	// spec: "In any comparison, the first operand must be assignable
   498  	// to the type of the second operand, or vice versa."
   499  	code := MismatchedTypes
   500  	ok, _ := x.assignableTo(check, y.typ(), nil)
   501  	if !ok {
   502  		ok, _ = y.assignableTo(check, x.typ(), nil)
   503  	}
   504  	if !ok {
   505  		// Report the error on the 2nd operand since we only
   506  		// know after seeing the 2nd operand whether we have
   507  		// a type mismatch.
   508  		errOp = y
   509  		cause = check.sprintf("mismatched types %s and %s", x.typ(), y.typ())
   510  		goto Error
   511  	}
   512  
   513  	// check if comparison is defined for operands
   514  	code = UndefinedOp
   515  	switch op {
   516  	case token.EQL, token.NEQ:
   517  		// spec: "The equality operators == and != apply to operands that are comparable."
   518  		switch {
   519  		case x.isNil() || y.isNil():
   520  			// Comparison against nil requires that the other operand type has nil.
   521  			typ := x.typ()
   522  			if x.isNil() {
   523  				typ = y.typ()
   524  			}
   525  			if !hasNil(typ) {
   526  				// This case should only be possible for "nil == nil".
   527  				// Report the error on the 2nd operand since we only
   528  				// know after seeing the 2nd operand whether we have
   529  				// an invalid comparison.
   530  				errOp = y
   531  				goto Error
   532  			}
   533  
   534  		case !Comparable(x.typ()):
   535  			errOp = x
   536  			cause = check.incomparableCause(x.typ())
   537  			goto Error
   538  
   539  		case !Comparable(y.typ()):
   540  			errOp = y
   541  			cause = check.incomparableCause(y.typ())
   542  			goto Error
   543  		}
   544  
   545  	case token.LSS, token.LEQ, token.GTR, token.GEQ:
   546  		// spec: The ordering operators <, <=, >, and >= apply to operands that are ordered."
   547  		switch {
   548  		case !allOrdered(x.typ()):
   549  			errOp = x
   550  			goto Error
   551  		case !allOrdered(y.typ()):
   552  			errOp = y
   553  			goto Error
   554  		}
   555  
   556  	default:
   557  		panic("unreachable")
   558  	}
   559  
   560  	// comparison is ok
   561  	if x.mode() == constant_ && y.mode() == constant_ {
   562  		x.val = constant.MakeBool(constant.Compare(x.val, op, y.val))
   563  		// The operands are never materialized; no need to update
   564  		// their types.
   565  	} else {
   566  		x.mode_ = value
   567  		// The operands have now their final types, which at run-
   568  		// time will be materialized. Update the expression trees.
   569  		// If the current types are untyped, the materialized type
   570  		// is the respective default type.
   571  		check.updateExprType(x.expr, Default(x.typ()), true)
   572  		check.updateExprType(y.expr, Default(y.typ()), true)
   573  	}
   574  
   575  	// spec: "Comparison operators compare two operands and yield
   576  	//        an untyped boolean value."
   577  	x.typ_ = Typ[UntypedBool]
   578  	return
   579  
   580  Error:
   581  	// We have an offending operand errOp and possibly an error cause.
   582  	if cause == "" {
   583  		if isTypeParam(x.typ()) || isTypeParam(y.typ()) {
   584  			// TODO(gri) should report the specific type causing the problem, if any
   585  			if !isTypeParam(x.typ()) {
   586  				errOp = y
   587  			}
   588  			cause = check.sprintf("type parameter %s cannot use operator %s", errOp.typ(), op)
   589  		} else {
   590  			// catch-all neither x nor y is a type parameter
   591  			what := compositeKind(errOp.typ())
   592  			if what == "" {
   593  				what = check.sprintf("%s", errOp.typ())
   594  			}
   595  			cause = check.sprintf("operator %s not defined on %s", op, what)
   596  		}
   597  	}
   598  	if switchCase {
   599  		check.errorf(x, code, "invalid case %s in switch on %s (%s)", x.expr, y.expr, cause) // error position always at 1st operand
   600  	} else {
   601  		check.errorf(errOp, code, invalidOp+"%s %s %s (%s)", x.expr, op, y.expr, cause)
   602  	}
   603  	x.invalidate()
   604  }
   605  
   606  // incomparableCause returns a more specific cause why typ is not comparable.
   607  // If there is no more specific cause, the result is "".
   608  func (check *Checker) incomparableCause(typ Type) string {
   609  	switch typ.Underlying().(type) {
   610  	case *Slice, *Signature, *Map:
   611  		return compositeKind(typ) + " can only be compared to nil"
   612  	}
   613  	// see if we can extract a more specific error
   614  	return comparableType(typ, true, nil).format(check)
   615  }
   616  
   617  // If e != nil, it must be the shift expression; it may be nil for non-constant shifts.
   618  func (check *Checker) shift(x, y *operand, e ast.Expr, op token.Token) {
   619  	// TODO(gri) This function seems overly complex. Revisit.
   620  
   621  	var xval constant.Value
   622  	if x.mode() == constant_ {
   623  		xval = constant.ToInt(x.val)
   624  	}
   625  
   626  	if allInteger(x.typ()) || isUntyped(x.typ()) && xval != nil && xval.Kind() == constant.Int {
   627  		// The lhs is of integer type or an untyped constant representable
   628  		// as an integer. Nothing to do.
   629  	} else {
   630  		// shift has no chance
   631  		check.errorf(x, InvalidShiftOperand, invalidOp+"shifted operand %s must be integer", x)
   632  		x.invalidate()
   633  		return
   634  	}
   635  
   636  	// spec: "The right operand in a shift expression must have integer type
   637  	// or be an untyped constant representable by a value of type uint."
   638  
   639  	// Check that constants are representable by uint, but do not convert them
   640  	// (see also go.dev/issue/47243).
   641  	var yval constant.Value
   642  	if y.mode() == constant_ {
   643  		// Provide a good error message for negative shift counts.
   644  		yval = constant.ToInt(y.val) // consider -1, 1.0, but not -1.1
   645  		if yval.Kind() == constant.Int && constant.Sign(yval) < 0 {
   646  			check.errorf(y, InvalidShiftCount, invalidOp+"negative shift count %s", y)
   647  			x.invalidate()
   648  			return
   649  		}
   650  
   651  		if isUntyped(y.typ()) {
   652  			// Caution: Check for representability here, rather than in the switch
   653  			// below, because isInteger includes untyped integers (was bug go.dev/issue/43697).
   654  			check.representable(y, Typ[Uint])
   655  			if !y.isValid() {
   656  				x.invalidate()
   657  				return
   658  			}
   659  		}
   660  	} else {
   661  		// Check that RHS is otherwise at least of integer type.
   662  		switch {
   663  		case allInteger(y.typ()):
   664  			if !allUnsigned(y.typ()) && !check.verifyVersionf(y, go1_13, invalidOp+"signed shift count %s", y) {
   665  				x.invalidate()
   666  				return
   667  			}
   668  		case isUntyped(y.typ()):
   669  			// This is incorrect, but preserves pre-existing behavior.
   670  			// See also go.dev/issue/47410.
   671  			check.convertUntyped(y, Typ[Uint])
   672  			if !y.isValid() {
   673  				x.invalidate()
   674  				return
   675  			}
   676  		default:
   677  			check.errorf(y, InvalidShiftCount, invalidOp+"shift count %s must be integer", y)
   678  			x.invalidate()
   679  			return
   680  		}
   681  	}
   682  
   683  	if x.mode() == constant_ {
   684  		if y.mode() == constant_ {
   685  			// if either x or y has an unknown value, the result is unknown
   686  			if x.val.Kind() == constant.Unknown || y.val.Kind() == constant.Unknown {
   687  				x.val = constant.MakeUnknown()
   688  				// ensure the correct type - see comment below
   689  				if !isInteger(x.typ()) {
   690  					x.typ_ = Typ[UntypedInt]
   691  				}
   692  				return
   693  			}
   694  			// rhs must be within reasonable bounds in constant shifts
   695  			const shiftBound = 1023 - 1 + 52 // so we can express smallestFloat64 (see go.dev/issue/44057)
   696  			s, ok := constant.Uint64Val(yval)
   697  			if !ok || s > shiftBound {
   698  				check.errorf(y, InvalidShiftCount, invalidOp+"invalid shift count %s", y)
   699  				x.invalidate()
   700  				return
   701  			}
   702  			// The lhs is representable as an integer but may not be an integer
   703  			// (e.g., 2.0, an untyped float) - this can only happen for untyped
   704  			// non-integer numeric constants. Correct the type so that the shift
   705  			// result is of integer type.
   706  			if !isInteger(x.typ()) {
   707  				x.typ_ = Typ[UntypedInt]
   708  			}
   709  			// x is a constant so xval != nil and it must be of Int kind.
   710  			x.val = constant.Shift(xval, op, uint(s))
   711  			x.expr = e
   712  			check.overflow(x, opPos(x.expr))
   713  			return
   714  		}
   715  
   716  		// non-constant shift with constant lhs
   717  		if isUntyped(x.typ()) {
   718  			// spec: "If the left operand of a non-constant shift
   719  			// expression is an untyped constant, the type of the
   720  			// constant is what it would be if the shift expression
   721  			// were replaced by its left operand alone.".
   722  			//
   723  			// Delay operand checking until we know the final type
   724  			// by marking the lhs expression as lhs shift operand.
   725  			//
   726  			// Usually (in correct programs), the lhs expression
   727  			// is in the untyped map. However, it is possible to
   728  			// create incorrect programs where the same expression
   729  			// is evaluated twice (via a declaration cycle) such
   730  			// that the lhs expression type is determined in the
   731  			// first round and thus deleted from the map, and then
   732  			// not found in the second round (double insertion of
   733  			// the same expr node still just leads to one entry for
   734  			// that node, and it can only be deleted once).
   735  			// Be cautious and check for presence of entry.
   736  			// Example: var e, f = int(1<<""[f]) // go.dev/issue/11347
   737  			if info, found := check.untyped[x.expr]; found {
   738  				info.isLhs = true
   739  				check.untyped[x.expr] = info
   740  			}
   741  			// keep x's type
   742  			x.mode_ = value
   743  			return
   744  		}
   745  	}
   746  
   747  	// non-constant shift - lhs must be an integer
   748  	if !allInteger(x.typ()) {
   749  		check.errorf(x, InvalidShiftOperand, invalidOp+"shifted operand %s must be integer", x)
   750  		x.invalidate()
   751  		return
   752  	}
   753  
   754  	x.mode_ = value
   755  }
   756  
   757  var binaryOpPredicates opPredicates
   758  
   759  func init() {
   760  	// Setting binaryOpPredicates in init avoids declaration cycles.
   761  	binaryOpPredicates = opPredicates{
   762  		token.ADD: allNumericOrString,
   763  		token.SUB: allNumeric,
   764  		token.MUL: allNumeric,
   765  		token.QUO: allNumeric,
   766  		token.REM: allInteger,
   767  
   768  		token.AND:     allInteger,
   769  		token.OR:      allInteger,
   770  		token.XOR:     allInteger,
   771  		token.AND_NOT: allInteger,
   772  
   773  		token.LAND: allBoolean,
   774  		token.LOR:  allBoolean,
   775  	}
   776  }
   777  
   778  // If e != nil, it must be the binary expression; it may be nil for non-constant expressions
   779  // (when invoked for an assignment operation where the binary expression is implicit).
   780  func (check *Checker) binary(x *operand, e ast.Expr, lhs, rhs ast.Expr, op token.Token, opPos token.Pos) {
   781  	var y operand
   782  
   783  	check.expr(nil, x, lhs)
   784  	check.expr(nil, &y, rhs)
   785  
   786  	if !x.isValid() {
   787  		return
   788  	}
   789  	if !y.isValid() {
   790  		x.invalidate()
   791  		x.expr = y.expr
   792  		return
   793  	}
   794  
   795  	if isShift(op) {
   796  		check.shift(x, &y, e, op)
   797  		return
   798  	}
   799  
   800  	check.matchTypes(x, &y)
   801  	if !x.isValid() {
   802  		return
   803  	}
   804  
   805  	if isComparison(op) {
   806  		check.comparison(x, &y, op, false)
   807  		return
   808  	}
   809  
   810  	if !Identical(x.typ(), y.typ()) {
   811  		// only report an error if we have valid types
   812  		// (otherwise we had an error reported elsewhere already)
   813  		if isValid(x.typ()) && isValid(y.typ()) {
   814  			var posn positioner = x
   815  			if e != nil {
   816  				posn = e
   817  			}
   818  			if e != nil {
   819  				check.errorf(posn, MismatchedTypes, invalidOp+"%s (mismatched types %s and %s)", e, x.typ(), y.typ())
   820  			} else {
   821  				check.errorf(posn, MismatchedTypes, invalidOp+"%s %s= %s (mismatched types %s and %s)", lhs, op, rhs, x.typ(), y.typ())
   822  			}
   823  		}
   824  		x.invalidate()
   825  		return
   826  	}
   827  
   828  	if !check.op(binaryOpPredicates, x, op) {
   829  		x.invalidate()
   830  		return
   831  	}
   832  
   833  	if op == token.QUO || op == token.REM {
   834  		// check for zero divisor
   835  		if (x.mode() == constant_ || allInteger(x.typ())) && y.mode() == constant_ && constant.Sign(y.val) == 0 {
   836  			check.error(&y, DivByZero, invalidOp+"division by zero")
   837  			x.invalidate()
   838  			return
   839  		}
   840  
   841  		// check for divisor underflow in complex division (see go.dev/issue/20227)
   842  		if x.mode() == constant_ && y.mode() == constant_ && isComplex(x.typ()) {
   843  			re, im := constant.Real(y.val), constant.Imag(y.val)
   844  			re2, im2 := constant.BinaryOp(re, token.MUL, re), constant.BinaryOp(im, token.MUL, im)
   845  			if constant.Sign(re2) == 0 && constant.Sign(im2) == 0 {
   846  				check.error(&y, DivByZero, invalidOp+"division by zero")
   847  				x.invalidate()
   848  				return
   849  			}
   850  		}
   851  	}
   852  
   853  	if x.mode() == constant_ && y.mode() == constant_ {
   854  		// if either x or y has an unknown value, the result is unknown
   855  		if x.val.Kind() == constant.Unknown || y.val.Kind() == constant.Unknown {
   856  			x.val = constant.MakeUnknown()
   857  			// x.typ is unchanged
   858  			return
   859  		}
   860  		// force integer division of integer operands
   861  		if op == token.QUO && isInteger(x.typ()) {
   862  			op = token.QUO_ASSIGN
   863  		}
   864  		x.val = constant.BinaryOp(x.val, op, y.val)
   865  		x.expr = e
   866  		check.overflow(x, opPos)
   867  		return
   868  	}
   869  
   870  	x.mode_ = value
   871  	// x.typ is unchanged
   872  }
   873  
   874  // matchTypes attempts to convert any untyped types x and y such that they match.
   875  // If an error occurs, x.mode is set to invalid.
   876  func (check *Checker) matchTypes(x, y *operand) {
   877  	// mayConvert reports whether the operands x and y may
   878  	// possibly have matching types after converting one
   879  	// untyped operand to the type of the other.
   880  	// If mayConvert returns true, we try to convert the
   881  	// operands to each other's types, and if that fails
   882  	// we report a conversion failure.
   883  	// If mayConvert returns false, we continue without an
   884  	// attempt at conversion, and if the operand types are
   885  	// not compatible, we report a type mismatch error.
   886  	mayConvert := func(x, y *operand) bool {
   887  		// If both operands are typed, there's no need for an implicit conversion.
   888  		if isTyped(x.typ()) && isTyped(y.typ()) {
   889  			return false
   890  		}
   891  		// A numeric type can only convert to another numeric type.
   892  		if allNumeric(x.typ()) != allNumeric(y.typ()) {
   893  			return false
   894  		}
   895  		// An untyped operand may convert to its default type when paired with an empty interface
   896  		// TODO(gri) This should only matter for comparisons (the only binary operation that is
   897  		//           valid with interfaces), but in that case the assignability check should take
   898  		//           care of the conversion. Verify and possibly eliminate this extra test.
   899  		if isNonTypeParamInterface(x.typ()) || isNonTypeParamInterface(y.typ()) {
   900  			return true
   901  		}
   902  		// A boolean type can only convert to another boolean type.
   903  		if allBoolean(x.typ()) != allBoolean(y.typ()) {
   904  			return false
   905  		}
   906  		// A string type can only convert to another string type.
   907  		if allString(x.typ()) != allString(y.typ()) {
   908  			return false
   909  		}
   910  		// Untyped nil can only convert to a type that has a nil.
   911  		if x.isNil() {
   912  			return hasNil(y.typ())
   913  		}
   914  		if y.isNil() {
   915  			return hasNil(x.typ())
   916  		}
   917  		// An untyped operand cannot convert to a pointer.
   918  		// TODO(gri) generalize to type parameters
   919  		if isPointer(x.typ()) || isPointer(y.typ()) {
   920  			return false
   921  		}
   922  		return true
   923  	}
   924  
   925  	if mayConvert(x, y) {
   926  		check.convertUntyped(x, y.typ())
   927  		if !x.isValid() {
   928  			return
   929  		}
   930  		check.convertUntyped(y, x.typ())
   931  		if !y.isValid() {
   932  			x.invalidate()
   933  			return
   934  		}
   935  	}
   936  }
   937  
   938  // exprKind describes the kind of an expression; the kind
   939  // determines if an expression is valid in 'statement context'.
   940  type exprKind int
   941  
   942  const (
   943  	conversion exprKind = iota
   944  	expression
   945  	statement
   946  )
   947  
   948  // target represent the (signature) type and description of the LHS
   949  // variable of an assignment, or of a function result variable.
   950  type target struct {
   951  	sig  *Signature
   952  	desc string
   953  }
   954  
   955  // newTarget creates a new target for the given type and description.
   956  // The result is nil if typ is not a signature.
   957  func newTarget(typ Type, desc string) *target {
   958  	if typ != nil {
   959  		if sig, _ := typ.Underlying().(*Signature); sig != nil {
   960  			return &target{sig, desc}
   961  		}
   962  	}
   963  	return nil
   964  }
   965  
   966  // rawExpr typechecks expression e and initializes x with the expression
   967  // value or type. If an error occurred, x.mode is set to invalid.
   968  // If a non-nil target T is given and e is a generic function,
   969  // T is used to infer the type arguments for e.
   970  // If hint != nil, it is the type of a composite literal element.
   971  // If allowGeneric is set, the operand type may be an uninstantiated
   972  // parameterized type or function value.
   973  func (check *Checker) rawExpr(T *target, x *operand, e ast.Expr, hint Type, allowGeneric bool) exprKind {
   974  	if check.conf._Trace {
   975  		check.trace(e.Pos(), "-- expr %s", e)
   976  		check.indent++
   977  		defer func() {
   978  			check.indent--
   979  			check.trace(e.Pos(), "=> %s", x)
   980  		}()
   981  	}
   982  
   983  	kind := check.exprInternal(T, x, e, hint)
   984  
   985  	if !allowGeneric {
   986  		check.nonGeneric(T, x)
   987  	}
   988  
   989  	check.record(x)
   990  
   991  	return kind
   992  }
   993  
   994  // If x is a generic type, or a generic function whose type arguments cannot be inferred
   995  // from a non-nil target T, nonGeneric reports an error and invalidates x.mode and x.typ.
   996  // Otherwise it leaves x alone.
   997  func (check *Checker) nonGeneric(T *target, x *operand) {
   998  	if !x.isValid() || x.mode() == novalue {
   999  		return
  1000  	}
  1001  	var what string
  1002  	switch t := x.typ().(type) {
  1003  	case *Alias, *Named:
  1004  		if isGeneric(t) {
  1005  			what = "type"
  1006  		}
  1007  	case *Signature:
  1008  		if t.tparams != nil {
  1009  			if enableReverseTypeInference && T != nil {
  1010  				check.funcInst(T, x.Pos(), x, nil, true)
  1011  				return
  1012  			}
  1013  			what = "function"
  1014  		}
  1015  	}
  1016  	if what != "" {
  1017  		check.errorf(x.expr, WrongTypeArgCount, "cannot use generic %s %s without instantiation", what, x.expr)
  1018  		x.invalidate()
  1019  		x.typ_ = Typ[Invalid]
  1020  	}
  1021  }
  1022  
  1023  // exprInternal contains the core of type checking of expressions.
  1024  // Must only be called by rawExpr.
  1025  // (See rawExpr for an explanation of the parameters.)
  1026  func (check *Checker) exprInternal(T *target, x *operand, e ast.Expr, hint Type) exprKind {
  1027  	// make sure x has a valid state in case of bailout
  1028  	// (was go.dev/issue/5770)
  1029  	x.invalidate()
  1030  	x.typ_ = Typ[Invalid]
  1031  
  1032  	switch e := e.(type) {
  1033  	case *ast.BadExpr:
  1034  		goto Error // error was reported before
  1035  
  1036  	case *ast.Ident:
  1037  		check.ident(x, e, false)
  1038  
  1039  	case *ast.Ellipsis:
  1040  		// ellipses are handled explicitly where they are valid
  1041  		check.error(e, InvalidSyntaxTree, "invalid use of ...")
  1042  		goto Error
  1043  
  1044  	case *ast.BasicLit:
  1045  		check.basicLit(x, e)
  1046  		if !x.isValid() {
  1047  			goto Error
  1048  		}
  1049  
  1050  	case *ast.FuncLit:
  1051  		check.funcLit(x, e)
  1052  		if !x.isValid() {
  1053  			goto Error
  1054  		}
  1055  
  1056  	case *ast.CompositeLit:
  1057  		check.compositeLit(x, e, hint)
  1058  		if !x.isValid() {
  1059  			goto Error
  1060  		}
  1061  
  1062  	case *ast.ParenExpr:
  1063  		// type inference doesn't go past parentheses (target type T = nil)
  1064  		kind := check.rawExpr(nil, x, e.X, nil, false)
  1065  		x.expr = e
  1066  		return kind
  1067  
  1068  	case *ast.SelectorExpr:
  1069  		check.selector(x, e, false)
  1070  
  1071  	case *ast.IndexExpr, *ast.IndexListExpr:
  1072  		ix := unpackIndexedExpr(e)
  1073  		if check.indexExpr(x, ix) {
  1074  			if !enableReverseTypeInference {
  1075  				T = nil
  1076  			}
  1077  			check.funcInst(T, e.Pos(), x, ix, true)
  1078  		}
  1079  		if !x.isValid() {
  1080  			goto Error
  1081  		}
  1082  
  1083  	case *ast.SliceExpr:
  1084  		check.sliceExpr(x, e)
  1085  		if !x.isValid() {
  1086  			goto Error
  1087  		}
  1088  
  1089  	case *ast.TypeAssertExpr:
  1090  		check.expr(nil, x, e.X)
  1091  		if !x.isValid() {
  1092  			goto Error
  1093  		}
  1094  		// x.(type) expressions are handled explicitly in type switches
  1095  		if e.Type == nil {
  1096  			// Don't use InvalidSyntaxTree because this can occur in the AST produced by
  1097  			// go/parser.
  1098  			check.error(e, BadTypeKeyword, "use of .(type) outside type switch")
  1099  			goto Error
  1100  		}
  1101  		if isTypeParam(x.typ()) {
  1102  			check.errorf(x, InvalidAssert, invalidOp+"cannot use type assertion on type parameter value %s", x)
  1103  			goto Error
  1104  		}
  1105  		if _, ok := x.typ().Underlying().(*Interface); !ok {
  1106  			check.errorf(x, InvalidAssert, invalidOp+"%s is not an interface", x)
  1107  			goto Error
  1108  		}
  1109  		T := check.varType(e.Type)
  1110  		if !isValid(T) {
  1111  			goto Error
  1112  		}
  1113  		// We cannot assert to an incomplete type; make sure it's complete.
  1114  		if !check.isComplete(T) {
  1115  			goto Error
  1116  		}
  1117  		check.typeAssertion(e, x, T, false)
  1118  		x.mode_ = commaok
  1119  		x.typ_ = T
  1120  
  1121  	case *ast.CallExpr:
  1122  		return check.callExpr(x, e)
  1123  
  1124  	case *ast.StarExpr:
  1125  		check.exprOrType(x, e.X, false)
  1126  		switch x.mode() {
  1127  		case invalid:
  1128  			goto Error
  1129  		case typexpr:
  1130  			check.validVarType(e.X, x.typ())
  1131  			x.typ_ = &Pointer{base: x.typ()}
  1132  		default:
  1133  			var base Type
  1134  			if !underIs(x.typ(), func(u Type) bool {
  1135  				p, _ := u.(*Pointer)
  1136  				if p == nil {
  1137  					check.errorf(x, InvalidIndirection, invalidOp+"cannot indirect %s", x)
  1138  					return false
  1139  				}
  1140  				if base != nil && !Identical(p.base, base) {
  1141  					check.errorf(x, InvalidIndirection, invalidOp+"pointers of %s must have identical base types", x)
  1142  					return false
  1143  				}
  1144  				base = p.base
  1145  				return true
  1146  			}) {
  1147  				goto Error
  1148  			}
  1149  			// We cannot dereference a pointer with an incomplete base type; make sure it's complete.
  1150  			if !check.isComplete(base) {
  1151  				goto Error
  1152  			}
  1153  			x.mode_ = variable
  1154  			x.typ_ = base
  1155  		}
  1156  
  1157  	case *ast.UnaryExpr:
  1158  		check.unary(x, e)
  1159  		if !x.isValid() {
  1160  			goto Error
  1161  		}
  1162  		if e.Op == token.ARROW {
  1163  			x.expr = e
  1164  			return statement // receive operations may appear in statement context
  1165  		}
  1166  
  1167  	case *ast.BinaryExpr:
  1168  		check.binary(x, e, e.X, e.Y, e.Op, e.OpPos)
  1169  		if !x.isValid() {
  1170  			goto Error
  1171  		}
  1172  
  1173  	case *ast.KeyValueExpr:
  1174  		// key:value expressions are handled in composite literals
  1175  		check.error(e, InvalidSyntaxTree, "no key:value expected")
  1176  		goto Error
  1177  
  1178  	case *ast.ArrayType, *ast.StructType, *ast.FuncType,
  1179  		*ast.InterfaceType, *ast.MapType, *ast.ChanType:
  1180  		x.mode_ = typexpr
  1181  		x.typ_ = check.typ(e)
  1182  		// Note: rawExpr (caller of exprInternal) will call check.recordTypeAndValue
  1183  		// even though check.typ has already called it. This is fine as both
  1184  		// times the same expression and type are recorded. It is also not a
  1185  		// performance issue because we only reach here for composite literal
  1186  		// types, which are comparatively rare.
  1187  
  1188  	default:
  1189  		panic(fmt.Sprintf("%s: unknown expression type %T", check.fset.Position(e.Pos()), e))
  1190  	}
  1191  
  1192  	// everything went well
  1193  	x.expr = e
  1194  	return expression
  1195  
  1196  Error:
  1197  	x.invalidate()
  1198  	x.expr = e
  1199  	return statement // avoid follow-up errors
  1200  }
  1201  
  1202  // keyVal maps a complex, float, integer, string or boolean constant value
  1203  // to the corresponding complex128, float64, int64, uint64, string, or bool
  1204  // Go value if possible; otherwise it returns x.
  1205  // A complex constant that can be represented as a float (such as 1.2 + 0i)
  1206  // is returned as a floating point value; if a floating point value can be
  1207  // represented as an integer (such as 1.0) it is returned as an integer value.
  1208  // This ensures that constants of different kind but equal value (such as
  1209  // 1.0 + 0i, 1.0, 1) result in the same value.
  1210  func keyVal(x constant.Value) any {
  1211  	switch x.Kind() {
  1212  	case constant.Complex:
  1213  		f := constant.ToFloat(x)
  1214  		if f.Kind() != constant.Float {
  1215  			r, _ := constant.Float64Val(constant.Real(x))
  1216  			i, _ := constant.Float64Val(constant.Imag(x))
  1217  			return complex(r, i)
  1218  		}
  1219  		x = f
  1220  		fallthrough
  1221  	case constant.Float:
  1222  		i := constant.ToInt(x)
  1223  		if i.Kind() != constant.Int {
  1224  			v, _ := constant.Float64Val(x)
  1225  			return v
  1226  		}
  1227  		x = i
  1228  		fallthrough
  1229  	case constant.Int:
  1230  		if v, ok := constant.Int64Val(x); ok {
  1231  			return v
  1232  		}
  1233  		if v, ok := constant.Uint64Val(x); ok {
  1234  			return v
  1235  		}
  1236  	case constant.String:
  1237  		return constant.StringVal(x)
  1238  	case constant.Bool:
  1239  		return constant.BoolVal(x)
  1240  	}
  1241  	return x
  1242  }
  1243  
  1244  // typeAssertion checks x.(T). The type of x must be an interface.
  1245  func (check *Checker) typeAssertion(e ast.Expr, x *operand, T Type, typeSwitch bool) {
  1246  	var cause string
  1247  	if check.assertableTo(x.typ(), T, &cause) {
  1248  		return // success
  1249  	}
  1250  
  1251  	if typeSwitch {
  1252  		check.errorf(e, ImpossibleAssert, "impossible type switch case: %s\n\t%s cannot have dynamic type %s %s", e, x, T, cause)
  1253  		return
  1254  	}
  1255  
  1256  	check.errorf(e, ImpossibleAssert, "impossible type assertion: %s\n\t%s does not implement %s %s", e, T, x.typ(), cause)
  1257  }
  1258  
  1259  // expr typechecks expression e and initializes x with the expression value.
  1260  // If a non-nil target T is given and e is a generic function or
  1261  // a function call, T is used to infer the type arguments for e.
  1262  // The result must be a single value.
  1263  // If an error occurred, x.mode is set to invalid.
  1264  func (check *Checker) expr(T *target, x *operand, e ast.Expr) {
  1265  	check.rawExpr(T, x, e, nil, false)
  1266  	check.exclude(x, 1<<novalue|1<<builtin|1<<typexpr)
  1267  	check.singleValue(x)
  1268  }
  1269  
  1270  // genericExpr is like expr but the result may also be generic.
  1271  func (check *Checker) genericExpr(x *operand, e ast.Expr) {
  1272  	check.rawExpr(nil, x, e, nil, true)
  1273  	check.exclude(x, 1<<novalue|1<<builtin|1<<typexpr)
  1274  	check.singleValue(x)
  1275  }
  1276  
  1277  // multiExpr typechecks e and returns its value (or values) in list.
  1278  // If allowCommaOk is set and e is a map index, comma-ok, or comma-err
  1279  // expression, the result is a two-element list containing the value
  1280  // of e, and an untyped bool value or an error value, respectively.
  1281  // If an error occurred, list[0] is not valid.
  1282  func (check *Checker) multiExpr(e ast.Expr, allowCommaOk bool) (list []*operand, commaOk bool) {
  1283  	var x operand
  1284  	check.rawExpr(nil, &x, e, nil, false)
  1285  	check.exclude(&x, 1<<novalue|1<<builtin|1<<typexpr)
  1286  
  1287  	if t, ok := x.typ().(*Tuple); ok && x.isValid() {
  1288  		// multiple values
  1289  		list = make([]*operand, t.Len())
  1290  		for i, v := range t.vars {
  1291  			list[i] = &operand{mode_: value, expr: e, typ_: v.typ}
  1292  		}
  1293  		return
  1294  	}
  1295  
  1296  	// exactly one (possibly invalid or comma-ok) value
  1297  	list = []*operand{&x}
  1298  	if allowCommaOk && (x.mode() == mapindex || x.mode() == commaok || x.mode() == commaerr) {
  1299  		x2 := &operand{mode_: value, expr: e, typ_: Typ[UntypedBool]}
  1300  		if x.mode() == commaerr {
  1301  			x2.typ_ = universeError
  1302  		}
  1303  		list = append(list, x2)
  1304  		commaOk = true
  1305  	}
  1306  
  1307  	return
  1308  }
  1309  
  1310  // exprWithHint typechecks expression e and initializes x with the expression value;
  1311  // hint is the type of a composite literal element.
  1312  // If an error occurred, x.mode is set to invalid.
  1313  func (check *Checker) exprWithHint(x *operand, e ast.Expr, hint Type) {
  1314  	assert(hint != nil)
  1315  	check.rawExpr(nil, x, e, hint, false)
  1316  	check.exclude(x, 1<<novalue|1<<builtin|1<<typexpr)
  1317  	check.singleValue(x)
  1318  }
  1319  
  1320  // exprOrType typechecks expression or type e and initializes x with the expression value or type.
  1321  // If allowGeneric is set, the operand type may be an uninstantiated parameterized type or function
  1322  // value.
  1323  // If an error occurred, x.mode is set to invalid.
  1324  func (check *Checker) exprOrType(x *operand, e ast.Expr, allowGeneric bool) {
  1325  	check.rawExpr(nil, x, e, nil, allowGeneric)
  1326  	check.exclude(x, 1<<novalue)
  1327  	check.singleValue(x)
  1328  }
  1329  
  1330  // exclude reports an error if x.mode is in modeset and sets x.mode to invalid.
  1331  // The modeset may contain any of 1<<novalue, 1<<builtin, 1<<typexpr.
  1332  func (check *Checker) exclude(x *operand, modeset uint) {
  1333  	if modeset&(1<<x.mode()) != 0 {
  1334  		var msg string
  1335  		var code Code
  1336  		switch x.mode() {
  1337  		case novalue:
  1338  			if modeset&(1<<typexpr) != 0 {
  1339  				msg = "%s used as value"
  1340  			} else {
  1341  				msg = "%s used as value or type"
  1342  			}
  1343  			code = TooManyValues
  1344  		case builtin:
  1345  			msg = "%s must be called"
  1346  			code = UncalledBuiltin
  1347  		case typexpr:
  1348  			msg = "%s is not an expression"
  1349  			code = NotAnExpr
  1350  		default:
  1351  			panic("unreachable")
  1352  		}
  1353  		check.errorf(x, code, msg, x)
  1354  		x.invalidate()
  1355  	}
  1356  }
  1357  
  1358  // singleValue reports an error if x describes a tuple and sets x.mode to invalid.
  1359  func (check *Checker) singleValue(x *operand) {
  1360  	if x.mode() == value {
  1361  		// tuple types are never named - no need for underlying type below
  1362  		if t, ok := x.typ().(*Tuple); ok {
  1363  			assert(t.Len() != 1)
  1364  			check.errorf(x, TooManyValues, "multiple-value %s in single-value context", x)
  1365  			x.invalidate()
  1366  		}
  1367  	}
  1368  }
  1369  

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