Exemplo n.º 1
0
// constValue returns the value of the constant with the
// dynamic type tag appropriate for c.Type().
func constValue(c *ssa.Const) value {
	if c.IsNil() {
		return zero(c.Type()) // typed nil
	}

	if t, ok := c.Type().Underlying().(*types.Basic); ok {
		// TODO(adonovan): eliminate untyped constants from SSA form.
		switch t.Kind() {
		case types.Bool, types.UntypedBool:
			return exact.BoolVal(c.Value)
		case types.Int, types.UntypedInt:
			// Assume sizeof(int) is same on host and target.
			return int(c.Int64())
		case types.Int8:
			return int8(c.Int64())
		case types.Int16:
			return int16(c.Int64())
		case types.Int32, types.UntypedRune:
			return int32(c.Int64())
		case types.Int64:
			return c.Int64()
		case types.Uint:
			// Assume sizeof(uint) is same on host and target.
			return uint(c.Uint64())
		case types.Uint8:
			return uint8(c.Uint64())
		case types.Uint16:
			return uint16(c.Uint64())
		case types.Uint32:
			return uint32(c.Uint64())
		case types.Uint64:
			return c.Uint64()
		case types.Uintptr:
			// Assume sizeof(uintptr) is same on host and target.
			return uintptr(c.Uint64())
		case types.Float32:
			return float32(c.Float64())
		case types.Float64, types.UntypedFloat:
			return c.Float64()
		case types.Complex64:
			return complex64(c.Complex128())
		case types.Complex128, types.UntypedComplex:
			return c.Complex128()
		case types.String, types.UntypedString:
			if c.Value.Kind() == exact.String {
				return exact.StringVal(c.Value)
			}
			return string(rune(c.Int64()))
		}
	}

	panic(fmt.Sprintf("constValue: %s", c))
}
Exemplo n.º 2
0
func (c *Const) RelString(from *types.Package) string {
	var s string
	if c.Value == nil {
		s = "nil"
	} else if c.Value.Kind() == exact.String {
		s = exact.StringVal(c.Value)
		const max = 20
		// TODO(adonovan): don't cut a rune in half.
		if len(s) > max {
			s = s[:max-3] + "..." // abbreviate
		}
		s = strconv.Quote(s)
	} else {
		s = c.Value.String()
	}
	return s + ":" + relType(c.Type(), from)
}
Exemplo n.º 3
0
// builtin type-checks a call to the built-in specified by id and
// returns true if the call is valid, with *x holding the result;
// but x.expr is not set. If the call is invalid, the result is
// false, and *x is undefined.
//
func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ bool) {
	// append is the only built-in that permits the use of ... for the last argument
	bin := predeclaredFuncs[id]
	if call.Ellipsis.IsValid() && id != _Append {
		check.invalidOp(call.Ellipsis, "invalid use of ... with built-in %s", bin.name)
		check.use(call.Args...)
		return
	}

	// For len(x) and cap(x) we need to know if x contains any function calls or
	// receive operations. Save/restore current setting and set hasCallOrRecv to
	// false for the evaluation of x so that we can check it afterwards.
	// Note: We must do this _before_ calling unpack because unpack evaluates the
	//       first argument before we even call arg(x, 0)!
	if id == _Len || id == _Cap {
		defer func(b bool) {
			check.hasCallOrRecv = b
		}(check.hasCallOrRecv)
		check.hasCallOrRecv = false
	}

	// determine actual arguments
	var arg getter
	nargs := len(call.Args)
	switch id {
	default:
		// make argument getter
		arg, nargs, _ = unpack(func(x *operand, i int) { check.expr(x, call.Args[i]) }, nargs, false)
		if arg == nil {
			return
		}
		// evaluate first argument, if present
		if nargs > 0 {
			arg(x, 0)
			if x.mode == invalid {
				return
			}
		}
	case _Make, _New, _Offsetof, _Trace:
		// arguments require special handling
	}

	// check argument count
	{
		msg := ""
		if nargs < bin.nargs {
			msg = "not enough"
		} else if !bin.variadic && nargs > bin.nargs {
			msg = "too many"
		}
		if msg != "" {
			check.invalidOp(call.Rparen, "%s arguments for %s (expected %d, found %d)", msg, call, bin.nargs, nargs)
			return
		}
	}

	switch id {
	case _Append:
		// append(s S, x ...T) S, where T is the element type of S
		// spec: "The variadic function append appends zero or more values x to s of type
		// S, which must be a slice type, and returns the resulting slice, also of type S.
		// The values x are passed to a parameter of type ...T where T is the element type
		// of S and the respective parameter passing rules apply."
		S := x.typ
		var T Type
		if s, _ := S.Underlying().(*Slice); s != nil {
			T = s.elem
		} else {
			check.invalidArg(x.pos(), "%s is not a slice", x)
			return
		}

		// remember arguments that have been evaluated already
		alist := []operand{*x}

		// spec: "As a special case, append also accepts a first argument assignable
		// to type []byte with a second argument of string type followed by ... .
		// This form appends the bytes of the string.
		if nargs == 2 && call.Ellipsis.IsValid() && x.assignableTo(check.conf, NewSlice(universeByte)) {
			arg(x, 1)
			if x.mode == invalid {
				return
			}
			if isString(x.typ) {
				if check.Types != nil {
					sig := makeSig(S, S, x.typ)
					sig.variadic = true
					check.recordBuiltinType(call.Fun, sig)
				}
				x.mode = value
				x.typ = S
				break
			}
			alist = append(alist, *x)
			// fallthrough
		}

		// check general case by creating custom signature
		sig := makeSig(S, S, NewSlice(T)) // []T required for variadic signature
		sig.variadic = true
		check.arguments(x, call, sig, func(x *operand, i int) {
			// only evaluate arguments that have not been evaluated before
			if i < len(alist) {
				*x = alist[i]
				return
			}
			arg(x, i)
		}, nargs)
		// ok to continue even if check.arguments reported errors

		x.mode = value
		x.typ = S
		if check.Types != nil {
			check.recordBuiltinType(call.Fun, sig)
		}

	case _Cap, _Len:
		// cap(x)
		// len(x)
		mode := invalid
		var typ Type
		var val exact.Value
		switch typ = implicitArrayDeref(x.typ.Underlying()); t := typ.(type) {
		case *Basic:
			if isString(t) && id == _Len {
				if x.mode == constant {
					mode = constant
					val = exact.MakeInt64(int64(len(exact.StringVal(x.val))))
				} else {
					mode = value
				}
			}

		case *Array:
			mode = value
			// spec: "The expressions len(s) and cap(s) are constants
			// if the type of s is an array or pointer to an array and
			// the expression s does not contain channel receives or
			// function calls; in this case s is not evaluated."
			if !check.hasCallOrRecv {
				mode = constant
				val = exact.MakeInt64(t.len)
			}

		case *Slice, *Chan:
			mode = value

		case *Map:
			if id == _Len {
				mode = value
			}
		}

		if mode == invalid {
			check.invalidArg(x.pos(), "%s for %s", x, bin.name)
			return
		}

		x.mode = mode
		x.typ = Typ[Int]
		x.val = val
		if check.Types != nil && mode != constant {
			check.recordBuiltinType(call.Fun, makeSig(x.typ, typ))
		}

	case _Close:
		// close(c)
		c, _ := x.typ.Underlying().(*Chan)
		if c == nil {
			check.invalidArg(x.pos(), "%s is not a channel", x)
			return
		}
		if c.dir == RecvOnly {
			check.invalidArg(x.pos(), "%s must not be a receive-only channel", x)
			return
		}

		x.mode = novalue
		if check.Types != nil {
			check.recordBuiltinType(call.Fun, makeSig(nil, c))
		}

	case _Complex:
		// complex(x, y realT) complexT
		if !check.complexArg(x) {
			return
		}

		var y operand
		arg(&y, 1)
		if y.mode == invalid {
			return
		}
		if !check.complexArg(&y) {
			return
		}

		check.convertUntyped(x, y.typ)
		if x.mode == invalid {
			return
		}
		check.convertUntyped(&y, x.typ)
		if y.mode == invalid {
			return
		}

		if !Identical(x.typ, y.typ) {
			check.invalidArg(x.pos(), "mismatched types %s and %s", x.typ, y.typ)
			return
		}

		if x.mode == constant && y.mode == constant {
			x.val = exact.BinaryOp(x.val, token.ADD, exact.MakeImag(y.val))
		} else {
			x.mode = value
		}

		realT := x.typ
		complexT := Typ[Invalid]
		switch realT.Underlying().(*Basic).kind {
		case Float32:
			complexT = Typ[Complex64]
		case Float64:
			complexT = Typ[Complex128]
		case UntypedInt, UntypedRune, UntypedFloat:
			if x.mode == constant {
				realT = defaultType(realT).(*Basic)
				complexT = Typ[UntypedComplex]
			} else {
				// untyped but not constant; probably because one
				// operand is a non-constant shift of untyped lhs
				realT = Typ[Float64]
				complexT = Typ[Complex128]
			}
		default:
			check.invalidArg(x.pos(), "float32 or float64 arguments expected")
			return
		}

		x.typ = complexT
		if check.Types != nil && x.mode != constant {
			check.recordBuiltinType(call.Fun, makeSig(complexT, realT, realT))
		}

		if x.mode != constant {
			// The arguments have now their final types, which at run-
			// time will be materialized. Update the expression trees.
			// If the current types are untyped, the materialized type
			// is the respective default type.
			// (If the result is constant, the arguments are never
			// materialized and there is nothing to do.)
			check.updateExprType(x.expr, realT, true)
			check.updateExprType(y.expr, realT, true)
		}

	case _Copy:
		// copy(x, y []T) int
		var dst Type
		if t, _ := x.typ.Underlying().(*Slice); t != nil {
			dst = t.elem
		}

		var y operand
		arg(&y, 1)
		if y.mode == invalid {
			return
		}
		var src Type
		switch t := y.typ.Underlying().(type) {
		case *Basic:
			if isString(y.typ) {
				src = universeByte
			}
		case *Slice:
			src = t.elem
		}

		if dst == nil || src == nil {
			check.invalidArg(x.pos(), "copy expects slice arguments; found %s and %s", x, &y)
			return
		}

		if !Identical(dst, src) {
			check.invalidArg(x.pos(), "arguments to copy %s and %s have different element types %s and %s", x, &y, dst, src)
			return
		}

		if check.Types != nil {
			check.recordBuiltinType(call.Fun, makeSig(Typ[Int], x.typ, y.typ))
		}
		x.mode = value
		x.typ = Typ[Int]

	case _Delete:
		// delete(m, k)
		m, _ := x.typ.Underlying().(*Map)
		if m == nil {
			check.invalidArg(x.pos(), "%s is not a map", x)
			return
		}
		arg(x, 1) // k
		if x.mode == invalid {
			return
		}

		if !x.assignableTo(check.conf, m.key) {
			check.invalidArg(x.pos(), "%s is not assignable to %s", x, m.key)
			return
		}

		x.mode = novalue
		if check.Types != nil {
			check.recordBuiltinType(call.Fun, makeSig(nil, m, m.key))
		}

	case _Imag, _Real:
		// imag(complexT) realT
		// real(complexT) realT
		if !isComplex(x.typ) {
			check.invalidArg(x.pos(), "%s must be a complex number", x)
			return
		}
		if x.mode == constant {
			if id == _Real {
				x.val = exact.Real(x.val)
			} else {
				x.val = exact.Imag(x.val)
			}
		} else {
			x.mode = value
		}
		var k BasicKind
		switch x.typ.Underlying().(*Basic).kind {
		case Complex64:
			k = Float32
		case Complex128:
			k = Float64
		case UntypedComplex:
			k = UntypedFloat
		default:
			unreachable()
		}

		if check.Types != nil && x.mode != constant {
			check.recordBuiltinType(call.Fun, makeSig(Typ[k], x.typ))
		}
		x.typ = Typ[k]

	case _Make:
		// make(T, n)
		// make(T, n, m)
		// (no argument evaluated yet)
		arg0 := call.Args[0]
		T := check.typ(arg0)
		if T == Typ[Invalid] {
			return
		}

		var min int // minimum number of arguments
		switch T.Underlying().(type) {
		case *Slice:
			min = 2
		case *Map, *Chan:
			min = 1
		default:
			check.invalidArg(arg0.Pos(), "cannot make %s; type must be slice, map, or channel", arg0)
			return
		}
		if nargs < min || min+1 < nargs {
			check.errorf(call.Pos(), "%s expects %d or %d arguments; found %d", call, min, min+1, nargs)
			return
		}
		var sizes []int64 // constant integer arguments, if any
		for _, arg := range call.Args[1:] {
			if s, ok := check.index(arg, -1); ok && s >= 0 {
				sizes = append(sizes, s)
			}
		}
		if len(sizes) == 2 && sizes[0] > sizes[1] {
			check.invalidArg(call.Args[1].Pos(), "length and capacity swapped")
			// safe to continue
		}
		x.mode = value
		x.typ = T
		if check.Types != nil {
			params := [...]Type{T, Typ[Int], Typ[Int]}
			check.recordBuiltinType(call.Fun, makeSig(x.typ, params[:1+len(sizes)]...))
		}

	case _New:
		// new(T)
		// (no argument evaluated yet)
		T := check.typ(call.Args[0])
		if T == Typ[Invalid] {
			return
		}

		x.mode = value
		x.typ = &Pointer{base: T}
		if check.Types != nil {
			check.recordBuiltinType(call.Fun, makeSig(x.typ, T))
		}

	case _Panic:
		// panic(x)
		T := new(Interface)
		if !check.assignment(x, T) {
			assert(x.mode == invalid)
			return
		}

		x.mode = novalue
		if check.Types != nil {
			check.recordBuiltinType(call.Fun, makeSig(nil, T))
		}

	case _Print, _Println:
		// print(x, y, ...)
		// println(x, y, ...)
		var params []Type
		if nargs > 0 {
			params = make([]Type, nargs)
			for i := 0; i < nargs; i++ {
				if i > 0 {
					arg(x, i) // first argument already evaluated
				}
				if !check.assignment(x, nil) {
					assert(x.mode == invalid)
					return
				}
				params[i] = x.typ
			}
		}

		x.mode = novalue
		if check.Types != nil {
			check.recordBuiltinType(call.Fun, makeSig(nil, params...))
		}

	case _Recover:
		// recover() interface{}
		x.mode = value
		x.typ = new(Interface)
		if check.Types != nil {
			check.recordBuiltinType(call.Fun, makeSig(x.typ))
		}

	case _Alignof:
		// unsafe.Alignof(x T) uintptr
		if !check.assignment(x, nil) {
			assert(x.mode == invalid)
			return
		}

		x.mode = constant
		x.val = exact.MakeInt64(check.conf.alignof(x.typ))
		x.typ = Typ[Uintptr]
		// result is constant - no need to record signature

	case _Offsetof:
		// unsafe.Offsetof(x T) uintptr, where x must be a selector
		// (no argument evaluated yet)
		arg0 := call.Args[0]
		selx, _ := unparen(arg0).(*ast.SelectorExpr)
		if selx == nil {
			check.invalidArg(arg0.Pos(), "%s is not a selector expression", arg0)
			check.use(arg0)
			return
		}

		check.expr(x, selx.X)
		if x.mode == invalid {
			return
		}

		base := derefStructPtr(x.typ)
		sel := selx.Sel.Name
		obj, index, indirect := LookupFieldOrMethod(base, false, check.pkg, sel)
		switch obj.(type) {
		case nil:
			check.invalidArg(x.pos(), "%s has no single field %s", base, sel)
			return
		case *Func:
			// TODO(gri) Using derefStructPtr may result in methods being found
			// that don't actually exist. An error either way, but the error
			// message is confusing. See: http://play.golang.org/p/al75v23kUy ,
			// but go/types reports: "invalid argument: x.m is a method value".
			check.invalidArg(arg0.Pos(), "%s is a method value", arg0)
			return
		}
		if indirect {
			check.invalidArg(x.pos(), "field %s is embedded via a pointer in %s", sel, base)
			return
		}

		// TODO(gri) Should we pass x.typ instead of base (and indirect report if derefStructPtr indirected)?
		check.recordSelection(selx, FieldVal, base, obj, index, false)

		offs := check.conf.offsetof(base, index)
		x.mode = constant
		x.val = exact.MakeInt64(offs)
		x.typ = Typ[Uintptr]
		// result is constant - no need to record signature

	case _Sizeof:
		// unsafe.Sizeof(x T) uintptr
		if !check.assignment(x, nil) {
			assert(x.mode == invalid)
			return
		}

		x.mode = constant
		x.val = exact.MakeInt64(check.conf.sizeof(x.typ))
		x.typ = Typ[Uintptr]
		// result is constant - no need to record signature

	case _Assert:
		// assert(pred) causes a typechecker error if pred is false.
		// The result of assert is the value of pred if there is no error.
		// Note: assert is only available in self-test mode.
		if x.mode != constant || !isBoolean(x.typ) {
			check.invalidArg(x.pos(), "%s is not a boolean constant", x)
			return
		}
		if x.val.Kind() != exact.Bool {
			check.errorf(x.pos(), "internal error: value of %s should be a boolean constant", x)
			return
		}
		if !exact.BoolVal(x.val) {
			check.errorf(call.Pos(), "%s failed", call)
			// compile-time assertion failure - safe to continue
		}
		// result is constant - no need to record signature

	case _Trace:
		// trace(x, y, z, ...) dumps the positions, expressions, and
		// values of its arguments. The result of trace is the value
		// of the first argument.
		// Note: trace is only available in self-test mode.
		// (no argument evaluated yet)
		if nargs == 0 {
			check.dump("%s: trace() without arguments", call.Pos())
			x.mode = novalue
			break
		}
		var t operand
		x1 := x
		for _, arg := range call.Args {
			check.rawExpr(x1, arg, nil) // permit trace for types, e.g.: new(trace(T))
			check.dump("%s: %s", x1.pos(), x1)
			x1 = &t // use incoming x only for first argument
		}
		// trace is only available in test mode - no need to record signature

	default:
		unreachable()
	}

	return true
}
Exemplo n.º 4
0
// exprInternal contains the core of type checking of expressions.
// Must only be called by rawExpr.
//
func (check *Checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
	// make sure x has a valid state in case of bailout
	// (was issue 5770)
	x.mode = invalid
	x.typ = Typ[Invalid]

	switch e := e.(type) {
	case *ast.BadExpr:
		goto Error // error was reported before

	case *ast.Ident:
		check.ident(x, e, nil, nil)

	case *ast.Ellipsis:
		// ellipses are handled explicitly where they are legal
		// (array composite literals and parameter lists)
		check.error(e.Pos(), "invalid use of '...'")
		goto Error

	case *ast.BasicLit:
		x.setConst(e.Kind, e.Value)
		if x.mode == invalid {
			check.invalidAST(e.Pos(), "invalid literal %v", e.Value)
			goto Error
		}

	case *ast.FuncLit:
		if sig, ok := check.typ(e.Type).(*Signature); ok {
			// Anonymous functions are considered part of the
			// init expression/func declaration which contains
			// them: use existing package-level declaration info.
			check.funcBody(check.decl, "", sig, e.Body)
			x.mode = value
			x.typ = sig
		} else {
			check.invalidAST(e.Pos(), "invalid function literal %s", e)
			goto Error
		}

	case *ast.CompositeLit:
		typ := hint
		openArray := false
		if e.Type != nil {
			// [...]T array types may only appear with composite literals.
			// Check for them here so we don't have to handle ... in general.
			typ = nil
			if atyp, _ := e.Type.(*ast.ArrayType); atyp != nil && atyp.Len != nil {
				if ellip, _ := atyp.Len.(*ast.Ellipsis); ellip != nil && ellip.Elt == nil {
					// We have an "open" [...]T array type.
					// Create a new ArrayType with unknown length (-1)
					// and finish setting it up after analyzing the literal.
					typ = &Array{len: -1, elem: check.typ(atyp.Elt)}
					openArray = true
				}
			}
			if typ == nil {
				typ = check.typ(e.Type)
			}
		}
		if typ == nil {
			// TODO(gri) provide better error messages depending on context
			check.error(e.Pos(), "missing type in composite literal")
			goto Error
		}

		switch typ, _ := deref(typ); utyp := typ.Underlying().(type) {
		case *Struct:
			if len(e.Elts) == 0 {
				break
			}
			fields := utyp.fields
			if _, ok := e.Elts[0].(*ast.KeyValueExpr); ok {
				// all elements must have keys
				visited := make([]bool, len(fields))
				for _, e := range e.Elts {
					kv, _ := e.(*ast.KeyValueExpr)
					if kv == nil {
						check.error(e.Pos(), "mixture of field:value and value elements in struct literal")
						continue
					}
					key, _ := kv.Key.(*ast.Ident)
					if key == nil {
						check.errorf(kv.Pos(), "invalid field name %s in struct literal", kv.Key)
						continue
					}
					i := fieldIndex(utyp.fields, check.pkg, key.Name)
					if i < 0 {
						check.errorf(kv.Pos(), "unknown field %s in struct literal", key.Name)
						continue
					}
					fld := fields[i]
					check.recordUse(key, fld)
					// 0 <= i < len(fields)
					if visited[i] {
						check.errorf(kv.Pos(), "duplicate field name %s in struct literal", key.Name)
						continue
					}
					visited[i] = true
					check.expr(x, kv.Value)
					etyp := fld.typ
					if !check.assignment(x, etyp) {
						if x.mode != invalid {
							check.errorf(x.pos(), "cannot use %s as %s value in struct literal", x, etyp)
						}
						continue
					}
				}
			} else {
				// no element must have a key
				for i, e := range e.Elts {
					if kv, _ := e.(*ast.KeyValueExpr); kv != nil {
						check.error(kv.Pos(), "mixture of field:value and value elements in struct literal")
						continue
					}
					check.expr(x, e)
					if i >= len(fields) {
						check.error(x.pos(), "too many values in struct literal")
						break // cannot continue
					}
					// i < len(fields)
					fld := fields[i]
					if !fld.Exported() && fld.pkg != check.pkg {
						check.errorf(x.pos(), "implicit assignment to unexported field %s in %s literal", fld.name, typ)
						continue
					}
					etyp := fld.typ
					if !check.assignment(x, etyp) {
						if x.mode != invalid {
							check.errorf(x.pos(), "cannot use %s as %s value in struct literal", x, etyp)
						}
						continue
					}
				}
				if len(e.Elts) < len(fields) {
					check.error(e.Rbrace, "too few values in struct literal")
					// ok to continue
				}
			}

		case *Array:
			n := check.indexedElts(e.Elts, utyp.elem, utyp.len)
			// if we have an "open" [...]T array, set the length now that we know it
			if openArray {
				utyp.len = n
			}

		case *Slice:
			check.indexedElts(e.Elts, utyp.elem, -1)

		case *Map:
			visited := make(map[interface{}][]Type, len(e.Elts))
			for _, e := range e.Elts {
				kv, _ := e.(*ast.KeyValueExpr)
				if kv == nil {
					check.error(e.Pos(), "missing key in map literal")
					continue
				}
				check.exprWithHint(x, kv.Key, utyp.key)
				if !check.assignment(x, utyp.key) {
					if x.mode != invalid {
						check.errorf(x.pos(), "cannot use %s as %s key in map literal", x, utyp.key)
					}
					continue
				}
				if x.mode == constant {
					duplicate := false
					// if the key is of interface type, the type is also significant when checking for duplicates
					if _, ok := utyp.key.Underlying().(*Interface); ok {
						for _, vtyp := range visited[x.val] {
							if Identical(vtyp, x.typ) {
								duplicate = true
								break
							}
						}
						visited[x.val] = append(visited[x.val], x.typ)
					} else {
						_, duplicate = visited[x.val]
						visited[x.val] = nil
					}
					if duplicate {
						check.errorf(x.pos(), "duplicate key %s in map literal", x.val)
						continue
					}
				}
				check.exprWithHint(x, kv.Value, utyp.elem)
				if !check.assignment(x, utyp.elem) {
					if x.mode != invalid {
						check.errorf(x.pos(), "cannot use %s as %s value in map literal", x, utyp.elem)
					}
					continue
				}
			}

		default:
			// if utyp is invalid, an error was reported before
			if utyp != Typ[Invalid] {
				check.errorf(e.Pos(), "invalid composite literal type %s", typ)
				goto Error
			}
		}

		x.mode = value
		x.typ = typ

	case *ast.ParenExpr:
		kind := check.rawExpr(x, e.X, nil)
		x.expr = e
		return kind

	case *ast.SelectorExpr:
		check.selector(x, e)

	case *ast.IndexExpr:
		check.expr(x, e.X)
		if x.mode == invalid {
			goto Error
		}

		valid := false
		length := int64(-1) // valid if >= 0
		switch typ := x.typ.Underlying().(type) {
		case *Basic:
			if isString(typ) {
				valid = true
				if x.mode == constant {
					length = int64(len(exact.StringVal(x.val)))
				}
				// an indexed string always yields a byte value
				// (not a constant) even if the string and the
				// index are constant
				x.mode = value
				x.typ = universeByte // use 'byte' name
			}

		case *Array:
			valid = true
			length = typ.len
			if x.mode != variable {
				x.mode = value
			}
			x.typ = typ.elem

		case *Pointer:
			if typ, _ := typ.base.Underlying().(*Array); typ != nil {
				valid = true
				length = typ.len
				x.mode = variable
				x.typ = typ.elem
			}

		case *Slice:
			valid = true
			x.mode = variable
			x.typ = typ.elem

		case *Map:
			var key operand
			check.expr(&key, e.Index)
			if !check.assignment(&key, typ.key) {
				if key.mode != invalid {
					check.invalidOp(key.pos(), "cannot use %s as map index of type %s", &key, typ.key)
				}
				goto Error
			}
			x.mode = mapindex
			x.typ = typ.elem
			x.expr = e
			return expression
		}

		if !valid {
			check.invalidOp(x.pos(), "cannot index %s", x)
			goto Error
		}

		if e.Index == nil {
			check.invalidAST(e.Pos(), "missing index for %s", x)
			goto Error
		}

		check.index(e.Index, length)
		// ok to continue

	case *ast.SliceExpr:
		check.expr(x, e.X)
		if x.mode == invalid {
			goto Error
		}

		valid := false
		length := int64(-1) // valid if >= 0
		switch typ := x.typ.Underlying().(type) {
		case *Basic:
			if isString(typ) {
				if e.Slice3 {
					check.invalidOp(x.pos(), "3-index slice of string")
					goto Error
				}
				valid = true
				if x.mode == constant {
					length = int64(len(exact.StringVal(x.val)))
				}
				// spec: "For untyped string operands the result
				// is a non-constant value of type string."
				if typ.kind == UntypedString {
					x.typ = Typ[String]
				}
			}

		case *Array:
			valid = true
			length = typ.len
			if x.mode != variable {
				check.invalidOp(x.pos(), "cannot slice %s (value not addressable)", x)
				goto Error
			}
			x.typ = &Slice{elem: typ.elem}

		case *Pointer:
			if typ, _ := typ.base.Underlying().(*Array); typ != nil {
				valid = true
				length = typ.len
				x.typ = &Slice{elem: typ.elem}
			}

		case *Slice:
			valid = true
			// x.typ doesn't change
		}

		if !valid {
			check.invalidOp(x.pos(), "cannot slice %s", x)
			goto Error
		}

		x.mode = value

		// spec: "Only the first index may be omitted; it defaults to 0."
		if e.Slice3 && (e.High == nil || e.Max == nil) {
			check.error(e.Rbrack, "2nd and 3rd index required in 3-index slice")
			goto Error
		}

		// check indices
		var ind [3]int64
		for i, expr := range []ast.Expr{e.Low, e.High, e.Max} {
			x := int64(-1)
			switch {
			case expr != nil:
				// The "capacity" is only known statically for strings, arrays,
				// and pointers to arrays, and it is the same as the length for
				// those types.
				max := int64(-1)
				if length >= 0 {
					max = length + 1
				}
				if t, ok := check.index(expr, max); ok && t >= 0 {
					x = t
				}
			case i == 0:
				// default is 0 for the first index
				x = 0
			case length >= 0:
				// default is length (== capacity) otherwise
				x = length
			}
			ind[i] = x
		}

		// constant indices must be in range
		// (check.index already checks that existing indices >= 0)
	L:
		for i, x := range ind[:len(ind)-1] {
			if x > 0 {
				for _, y := range ind[i+1:] {
					if y >= 0 && x > y {
						check.errorf(e.Rbrack, "invalid slice indices: %d > %d", x, y)
						break L // only report one error, ok to continue
					}
				}
			}
		}

	case *ast.TypeAssertExpr:
		check.expr(x, e.X)
		if x.mode == invalid {
			goto Error
		}
		xtyp, _ := x.typ.Underlying().(*Interface)
		if xtyp == nil {
			check.invalidOp(x.pos(), "%s is not an interface", x)
			goto Error
		}
		// x.(type) expressions are handled explicitly in type switches
		if e.Type == nil {
			check.invalidAST(e.Pos(), "use of .(type) outside type switch")
			goto Error
		}
		T := check.typ(e.Type)
		if T == Typ[Invalid] {
			goto Error
		}
		check.typeAssertion(x.pos(), x, xtyp, T)
		x.mode = commaok
		x.typ = T

	case *ast.CallExpr:
		return check.call(x, e)

	case *ast.StarExpr:
		check.exprOrType(x, e.X)
		switch x.mode {
		case invalid:
			goto Error
		case typexpr:
			x.typ = &Pointer{base: x.typ}
		default:
			if typ, ok := x.typ.Underlying().(*Pointer); ok {
				x.mode = variable
				x.typ = typ.base
			} else {
				check.invalidOp(x.pos(), "cannot indirect %s", x)
				goto Error
			}
		}

	case *ast.UnaryExpr:
		check.expr(x, e.X)
		if x.mode == invalid {
			goto Error
		}
		check.unary(x, e, e.Op)
		if x.mode == invalid {
			goto Error
		}
		if e.Op == token.ARROW {
			x.expr = e
			return statement // receive operations may appear in statement context
		}

	case *ast.BinaryExpr:
		check.binary(x, e, e.X, e.Y, e.Op)
		if x.mode == invalid {
			goto Error
		}

	case *ast.KeyValueExpr:
		// key:value expressions are handled in composite literals
		check.invalidAST(e.Pos(), "no key:value expected")
		goto Error

	case *ast.ArrayType, *ast.StructType, *ast.FuncType,
		*ast.InterfaceType, *ast.MapType, *ast.ChanType:
		x.mode = typexpr
		x.typ = check.typ(e)
		// Note: rawExpr (caller of exprInternal) will call check.recordTypeAndValue
		// even though check.typ has already called it. This is fine as both
		// times the same expression and type are recorded. It is also not a
		// performance issue because we only reach here for composite literal
		// types, which are comparatively rare.

	default:
		panic(fmt.Sprintf("%s: unknown expression type %T", check.fset.Position(e.Pos()), e))
	}

	// everything went well
	x.expr = e
	return expression

Error:
	x.mode = invalid
	x.expr = e
	return statement // avoid follow-up errors
}