src/go/types/call.go - go - Git at Google

 // Copyright 2013 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 // This file implements typechecking of call and selector expressions.

 package types

 import (
 	"go/ast"
 	"go/token"
 )

 func (check *Checker) call(x *operand, e *ast.CallExpr) exprKind {
 	check.exprOrType(x, e.Fun)

 	switch x.mode {
 	case invalid:
 		check.use(e.Args...)
 		x.mode = invalid
 		x.expr = e
 		return statement

 	case typexpr:
 		// conversion
 		T := x.typ
 		x.mode = invalid
 		switch n := len(e.Args); n {
 		case 0:
 			check.errorf(e.Rparen, "missing argument in conversion to %s", T)
 		case 1:
 			check.expr(x, e.Args[0])
 			if x.mode != invalid {
 				check.conversion(x, T)
 			}
 		default:
 			check.errorf(e.Args[n-1].Pos(), "too many arguments in conversion to %s", T)
 		}
 		x.expr = e
 		return conversion

 	case builtin:
 		id := x.id
 		if !check.builtin(x, e, id) {
 			x.mode = invalid
 		}
 		x.expr = e
 		// a non-constant result implies a function call
 		if x.mode != invalid && x.mode != constant_ {
 			check.hasCallOrRecv = true
 		}
 		return predeclaredFuncs[id].kind

 	default:
 		// function/method call
 		sig, _ := x.typ.Underlying().(*Signature)
 		if sig == nil {
 			check.invalidOp(x.pos(), "cannot call non-function %s", x)
 			x.mode = invalid
 			x.expr = e
 			return statement
 		}

 		arg, n, _ := unpack(func(x *operand, i int) { check.multiExpr(x, e.Args[i]) }, len(e.Args), false)
 		if arg != nil {
 			check.arguments(x, e, sig, arg, n)
 		} else {
 			x.mode = invalid
 		}

 		// determine result
 		switch sig.results.Len() {
 		case 0:
 			x.mode = novalue
 		case 1:
 			x.mode = value
 			x.typ = sig.results.vars[0].typ // unpack tuple
 		default:
 			x.mode = value
 			x.typ = sig.results
 		}

 		x.expr = e
 		check.hasCallOrRecv = true

 		return statement
 	}
 }

 // use type-checks each argument.
 // Useful to make sure expressions are evaluated
 // (and variables are "used") in the presence of other errors.
 func (check *Checker) use(arg ...ast.Expr) {
 	var x operand
 	for _, e := range arg {
 		if e != nil { // be safe
 			check.rawExpr(&x, e, nil)
 		}
 	}
 }

 // useGetter is like use, but takes a getter instead of a list of expressions.
 // It should be called instead of use if a getter is present to avoid repeated
 // evaluation of the first argument (since the getter was likely obtained via
 // unpack, which may have evaluated the first argument already).
 func (check *Checker) useGetter(get getter, n int) {
 	var x operand
 	for i := 0; i < n; i++ {
 		get(&x, i)
 	}
 }

 // A getter sets x as the i'th operand, where 0 <= i < n and n is the total
 // number of operands (context-specific, and maintained elsewhere). A getter
 // type-checks the i'th operand; the details of the actual check are getter-
 // specific.
 type getter func(x *operand, i int)

 // unpack takes a getter get and a number of operands n. If n == 1, unpack
 // calls the incoming getter for the first operand. If that operand is
 // invalid, unpack returns (nil, 0, false). Otherwise, if that operand is a
 // function call, or a comma-ok expression and allowCommaOk is set, the result
 // is a new getter and operand count providing access to the function results,
 // or comma-ok values, respectively. The third result value reports if it
 // is indeed the comma-ok case. In all other cases, the incoming getter and
 // operand count are returned unchanged, and the third result value is false.
 //
 // In other words, if there's exactly one operand that - after type-checking
 // by calling get - stands for multiple operands, the resulting getter provides
 // access to those operands instead.
 //
 // If the returned getter is called at most once for a given operand index i
 // (including i == 0), that operand is guaranteed to cause only one call of
 // the incoming getter with that i.
 //
 func unpack(get getter, n int, allowCommaOk bool) (getter, int, bool) {
 	if n == 1 {
 		// possibly result of an n-valued function call or comma,ok value
 		var x0 operand
 		get(&x0, 0)
 		if x0.mode == invalid {
 			return nil, 0, false
 		}

 		if t, ok := x0.typ.(*Tuple); ok {
 			// result of an n-valued function call
 			return func(x *operand, i int) {
 				x.mode = value
 				x.expr = x0.expr
 				x.typ = t.At(i).typ
 			}, t.Len(), false
 		}

 		if x0.mode == mapindex || x0.mode == commaok {
 			// comma-ok value
 			if allowCommaOk {
 				a := [2]Type{x0.typ, Typ[UntypedBool]}
 				return func(x *operand, i int) {
 					x.mode = value
 					x.expr = x0.expr
 					x.typ = a[i]
 				}, 2, true
 			}
 			x0.mode = value
 		}

 		// single value
 		return func(x *operand, i int) {
 			if i != 0 {
 				unreachable()
 			}
 			*x = x0
 		}, 1, false
 	}

 	// zero or multiple values
 	return get, n, false
 }

 // arguments checks argument passing for the call with the given signature.
 // The arg function provides the operand for the i'th argument.
 func (check *Checker) arguments(x *operand, call *ast.CallExpr, sig *Signature, arg getter, n int) {
 	if call.Ellipsis.IsValid() {
 		// last argument is of the form x...
 		if !sig.variadic {
 			check.errorf(call.Ellipsis, "cannot use ... in call to non-variadic %s", call.Fun)
 			check.useGetter(arg, n)
 			return
 		}
 		if len(call.Args) == 1 && n > 1 {
 			// f()... is not permitted if f() is multi-valued
 			check.errorf(call.Ellipsis, "cannot use ... with %d-valued %s", n, call.Args[0])
 			check.useGetter(arg, n)
 			return
 		}
 	}

 	// evaluate arguments
 	for i := 0; i < n; i++ {
 		arg(x, i)
 		if x.mode != invalid {
 			var ellipsis token.Pos
 			if i == n-1 && call.Ellipsis.IsValid() {
 				ellipsis = call.Ellipsis
 			}
 			check.argument(call.Fun, sig, i, x, ellipsis)
 		}
 	}

 	// check argument count
 	if sig.variadic {
 		// a variadic function accepts an "empty"
 		// last argument: count one extra
 		n++
 	}
 	if n < sig.params.Len() {
 		check.errorf(call.Rparen, "too few arguments in call to %s", call.Fun)
 		// ok to continue
 	}
 }

 // argument checks passing of argument x to the i'th parameter of the given signature.
 // If ellipsis is valid, the argument is followed by ... at that position in the call.
 func (check *Checker) argument(fun ast.Expr, sig *Signature, i int, x *operand, ellipsis token.Pos) {
 	check.singleValue(x)
 	if x.mode == invalid {
 		return
 	}

 	n := sig.params.Len()

 	// determine parameter type
 	var typ Type
 	switch {
 	case i < n:
 		typ = sig.params.vars[i].typ
 	case sig.variadic:
 		typ = sig.params.vars[n-1].typ
 		if debug {
 			if _, ok := typ.(*Slice); !ok {
 				check.dump("%s: expected unnamed slice type, got %s", sig.params.vars[n-1].Pos(), typ)
 			}
 		}
 	default:
 		check.errorf(x.pos(), "too many arguments")
 		return
 	}

 	if ellipsis.IsValid() {
 		// argument is of the form x... and x is single-valued
 		if i != n-1 {
 			check.errorf(ellipsis, "can only use ... with matching parameter")
 			return
 		}
 		if _, ok := x.typ.Underlying().(*Slice); !ok && x.typ != Typ[UntypedNil] { // see issue #18268
 			check.errorf(x.pos(), "cannot use %s as parameter of type %s", x, typ)
 			return
 		}
 	} else if sig.variadic && i >= n-1 {
 		// use the variadic parameter slice's element type
 		typ = typ.(*Slice).elem
 	}

 	check.assignment(x, typ, check.sprintf("argument to %s", fun))
 }

 func (check *Checker) selector(x *operand, e *ast.SelectorExpr) {
 	// these must be declared before the "goto Error" statements
 	var (
 		obj      Object
 		index    []int
 		indirect bool
 	)

 	sel := e.Sel.Name
 	// If the identifier refers to a package, handle everything here
 	// so we don't need a "package" mode for operands: package names
 	// can only appear in qualified identifiers which are mapped to
 	// selector expressions.
 	if ident, ok := e.X.(*ast.Ident); ok {
 		_, obj := check.scope.LookupParent(ident.Name, check.pos)
 		if pname, _ := obj.(*PkgName); pname != nil {
 			assert(pname.pkg == check.pkg)
 			check.recordUse(ident, pname)
 			pname.used = true
 			pkg := pname.imported
 			exp := pkg.scope.Lookup(sel)
 			if exp == nil {
 				if !pkg.fake {
 					check.errorf(e.Pos(), "%s not declared by package %s", sel, pkg.name)
 				}
 				goto Error
 			}
 			if !exp.Exported() {
 				check.errorf(e.Pos(), "%s not exported by package %s", sel, pkg.name)
 				// ok to continue
 			}
 			check.recordUse(e.Sel, exp)

 			// Simplified version of the code for *ast.Idents:
 			// - imported objects are always fully initialized
 			switch exp := exp.(type) {
 			case *Const:
 				assert(exp.Val() != nil)
 				x.mode = constant_
 				x.typ = exp.typ
 				x.val = exp.val
 			case *TypeName:
 				x.mode = typexpr
 				x.typ = exp.typ
 			case *Var:
 				x.mode = variable
 				x.typ = exp.typ
 			case *Func:
 				x.mode = value
 				x.typ = exp.typ
 			case *Builtin:
 				x.mode = builtin
 				x.typ = exp.typ
 				x.id = exp.id
 			default:
 				check.dump("unexpected object %v", exp)
 				unreachable()
 			}
 			x.expr = e
 			return
 		}
 	}

 	check.exprOrType(x, e.X)
 	if x.mode == invalid {
 		goto Error
 	}

 	obj, index, indirect = LookupFieldOrMethod(x.typ, x.mode == variable, check.pkg, sel)
 	if obj == nil {
 		switch {
 		case index != nil:
 			// TODO(gri) should provide actual type where the conflict happens
 			check.invalidOp(e.Pos(), "ambiguous selector %s", sel)
 		case indirect:
 			check.invalidOp(e.Pos(), "%s is not in method set of %s", sel, x.typ)
 		default:
 			check.invalidOp(e.Pos(), "%s has no field or method %s", x, sel)
 		}
 		goto Error
 	}

 	if x.mode == typexpr {
 		// method expression
 		m, _ := obj.(*Func)
 		if m == nil {
 			check.invalidOp(e.Pos(), "%s has no method %s", x, sel)
 			goto Error
 		}

 		check.recordSelection(e, MethodExpr, x.typ, m, index, indirect)

 		// the receiver type becomes the type of the first function
 		// argument of the method expression's function type
 		var params []*Var
 		sig := m.typ.(*Signature)
 		if sig.params != nil {
 			params = sig.params.vars
 		}
 		x.mode = value
 		x.typ = &Signature{
 			params:   NewTuple(append([]*Var{NewVar(token.NoPos, check.pkg, "", x.typ)}, params...)...),
 			results:  sig.results,
 			variadic: sig.variadic,
 		}

 		check.addDeclDep(m)

 	} else {
 		// regular selector
 		switch obj := obj.(type) {
 		case *Var:
 			check.recordSelection(e, FieldVal, x.typ, obj, index, indirect)
 			if x.mode == variable || indirect {
 				x.mode = variable
 			} else {
 				x.mode = value
 			}
 			x.typ = obj.typ

 		case *Func:
 			// TODO(gri) If we needed to take into account the receiver's
 			// addressability, should we report the type &(x.typ) instead?
 			check.recordSelection(e, MethodVal, x.typ, obj, index, indirect)

 			if debug {
 				// Verify that LookupFieldOrMethod and MethodSet.Lookup agree.
 				typ := x.typ
 				if x.mode == variable {
 					// If typ is not an (unnamed) pointer or an interface,
 					// use *typ instead, because the method set of *typ
 					// includes the methods of typ.
 					// Variables are addressable, so we can always take their
 					// address.
 					if _, ok := typ.(*Pointer); !ok && !IsInterface(typ) {
 						typ = &Pointer{base: typ}
 					}
 				}
 				// If we created a synthetic pointer type above, we will throw
 				// away the method set computed here after use.
 				// TODO(gri) Method set computation should probably always compute
 				// both, the value and the pointer receiver method set and represent
 				// them in a single structure.
 				// TODO(gri) Consider also using a method set cache for the lifetime
 				// of checker once we rely on MethodSet lookup instead of individual
 				// lookup.
 				mset := NewMethodSet(typ)
 				if m := mset.Lookup(check.pkg, sel); m == nil || m.obj != obj {
 					check.dump("%s: (%s).%v -> %s", e.Pos(), typ, obj.name, m)
 					check.dump("%s\n", mset)
 					panic("method sets and lookup don't agree")
 				}
 			}

 			x.mode = value

 			// remove receiver
 			sig := *obj.typ.(*Signature)
 			sig.recv = nil
 			x.typ = &sig

 			check.addDeclDep(obj)

 		default:
 			unreachable()
 		}
 	}

 	// everything went well
 	x.expr = e
 	return

 Error:
 	x.mode = invalid
 	x.expr = e
 }
	// Copyright 2013 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	// This file implements typechecking of call and selector expressions.

	package types

	import (
	"go/ast"
	"go/token"
	)

	func (check Checker) call(x operand, e *ast.CallExpr) exprKind {
	check.exprOrType(x, e.Fun)

	switch x.mode {
	case invalid:
	check.use(e.Args...)
	x.mode = invalid
	x.expr = e
	return statement

	case typexpr:
	// conversion
	T := x.typ
	x.mode = invalid
	switch n := len(e.Args); n {
	case 0:
	check.errorf(e.Rparen, "missing argument in conversion to %s", T)
	case 1:
	check.expr(x, e.Args[0])
	if x.mode != invalid {
	check.conversion(x, T)
	}
	default:
	check.errorf(e.Args[n-1].Pos(), "too many arguments in conversion to %s", T)
	}
	x.expr = e
	return conversion

	case builtin:
	id := x.id
	if !check.builtin(x, e, id) {
	x.mode = invalid
	}
	x.expr = e
	// a non-constant result implies a function call
	if x.mode != invalid && x.mode != constant_ {
	check.hasCallOrRecv = true
	}
	return predeclaredFuncs[id].kind

	default:
	// function/method call
	sig, _ := x.typ.Underlying().(*Signature)
	if sig == nil {
	check.invalidOp(x.pos(), "cannot call non-function %s", x)
	x.mode = invalid
	x.expr = e
	return statement
	}

	arg, n, _ := unpack(func(x *operand, i int) { check.multiExpr(x, e.Args[i]) }, len(e.Args), false)
	if arg != nil {
	check.arguments(x, e, sig, arg, n)
	} else {
	x.mode = invalid
	}

	// determine result
	switch sig.results.Len() {
	case 0:
	x.mode = novalue
	case 1:
	x.mode = value
	x.typ = sig.results.vars[0].typ // unpack tuple
	default:
	x.mode = value
	x.typ = sig.results
	}

	x.expr = e
	check.hasCallOrRecv = true

	return statement
	}
	}

	// use type-checks each argument.
	// Useful to make sure expressions are evaluated
	// (and variables are "used") in the presence of other errors.
	func (check *Checker) use(arg ...ast.Expr) {
	var x operand
	for _, e := range arg {
	if e != nil { // be safe
	check.rawExpr(&x, e, nil)
	}
	}
	}

	// useGetter is like use, but takes a getter instead of a list of expressions.
	// It should be called instead of use if a getter is present to avoid repeated
	// evaluation of the first argument (since the getter was likely obtained via
	// unpack, which may have evaluated the first argument already).
	func (check *Checker) useGetter(get getter, n int) {
	var x operand
	for i := 0; i < n; i++ {
	get(&x, i)
	}
	}

	// A getter sets x as the i'th operand, where 0 <= i < n and n is the total
	// number of operands (context-specific, and maintained elsewhere). A getter
	// type-checks the i'th operand; the details of the actual check are getter-
	// specific.
	type getter func(x *operand, i int)

	// unpack takes a getter get and a number of operands n. If n == 1, unpack
	// calls the incoming getter for the first operand. If that operand is
	// invalid, unpack returns (nil, 0, false). Otherwise, if that operand is a
	// function call, or a comma-ok expression and allowCommaOk is set, the result
	// is a new getter and operand count providing access to the function results,
	// or comma-ok values, respectively. The third result value reports if it
	// is indeed the comma-ok case. In all other cases, the incoming getter and
	// operand count are returned unchanged, and the third result value is false.
	//
	// In other words, if there's exactly one operand that - after type-checking
	// by calling get - stands for multiple operands, the resulting getter provides
	// access to those operands instead.
	//
	// If the returned getter is called at most once for a given operand index i
	// (including i == 0), that operand is guaranteed to cause only one call of
	// the incoming getter with that i.
	//
	func unpack(get getter, n int, allowCommaOk bool) (getter, int, bool) {
	if n == 1 {
	// possibly result of an n-valued function call or comma,ok value
	var x0 operand
	get(&x0, 0)
	if x0.mode == invalid {
	return nil, 0, false
	}

	if t, ok := x0.typ.(*Tuple); ok {
	// result of an n-valued function call
	return func(x *operand, i int) {
	x.mode = value
	x.expr = x0.expr
	x.typ = t.At(i).typ
	}, t.Len(), false
	}

	if x0.mode == mapindex \|\| x0.mode == commaok {
	// comma-ok value
	if allowCommaOk {
	a := [2]Type{x0.typ, Typ[UntypedBool]}
	return func(x *operand, i int) {
	x.mode = value
	x.expr = x0.expr
	x.typ = a[i]
	}, 2, true
	}
	x0.mode = value
	}

	// single value
	return func(x *operand, i int) {
	if i != 0 {
	unreachable()
	}
	*x = x0
	}, 1, false
	}

	// zero or multiple values
	return get, n, false
	}

	// arguments checks argument passing for the call with the given signature.
	// The arg function provides the operand for the i'th argument.
	func (check Checker) arguments(x operand, call ast.CallExpr, sig Signature, arg getter, n int) {
	if call.Ellipsis.IsValid() {
	// last argument is of the form x...
	if !sig.variadic {
	check.errorf(call.Ellipsis, "cannot use ... in call to non-variadic %s", call.Fun)
	check.useGetter(arg, n)
	return
	}
	if len(call.Args) == 1 && n > 1 {
	// f()... is not permitted if f() is multi-valued
	check.errorf(call.Ellipsis, "cannot use ... with %d-valued %s", n, call.Args[0])
	check.useGetter(arg, n)
	return
	}
	}

	// evaluate arguments
	for i := 0; i < n; i++ {
	arg(x, i)
	if x.mode != invalid {
	var ellipsis token.Pos
	if i == n-1 && call.Ellipsis.IsValid() {
	ellipsis = call.Ellipsis
	}
	check.argument(call.Fun, sig, i, x, ellipsis)
	}
	}

	// check argument count
	if sig.variadic {
	// a variadic function accepts an "empty"
	// last argument: count one extra
	n++
	}
	if n < sig.params.Len() {
	check.errorf(call.Rparen, "too few arguments in call to %s", call.Fun)
	// ok to continue
	}
	}

	// argument checks passing of argument x to the i'th parameter of the given signature.
	// If ellipsis is valid, the argument is followed by ... at that position in the call.
	func (check Checker) argument(fun ast.Expr, sig Signature, i int, x *operand, ellipsis token.Pos) {
	check.singleValue(x)
	if x.mode == invalid {
	return
	}

	n := sig.params.Len()

	// determine parameter type
	var typ Type
	switch {
	case i < n:
	typ = sig.params.vars[i].typ
	case sig.variadic:
	typ = sig.params.vars[n-1].typ
	if debug {
	if _, ok := typ.(*Slice); !ok {
	check.dump("%s: expected unnamed slice type, got %s", sig.params.vars[n-1].Pos(), typ)
	}
	}
	default:
	check.errorf(x.pos(), "too many arguments")
	return
	}

	if ellipsis.IsValid() {
	// argument is of the form x... and x is single-valued
	if i != n-1 {
	check.errorf(ellipsis, "can only use ... with matching parameter")
	return
	}
	if _, ok := x.typ.Underlying().(*Slice); !ok && x.typ != Typ[UntypedNil] { // see issue #18268
	check.errorf(x.pos(), "cannot use %s as parameter of type %s", x, typ)
	return
	}
	} else if sig.variadic && i >= n-1 {
	// use the variadic parameter slice's element type
	typ = typ.(*Slice).elem
	}

	check.assignment(x, typ, check.sprintf("argument to %s", fun))
	}

	func (check Checker) selector(x operand, e *ast.SelectorExpr) {
	// these must be declared before the "goto Error" statements
	var (
	obj Object
	index []int
	indirect bool
	)

	sel := e.Sel.Name
	// If the identifier refers to a package, handle everything here
	// so we don't need a "package" mode for operands: package names
	// can only appear in qualified identifiers which are mapped to
	// selector expressions.
	if ident, ok := e.X.(*ast.Ident); ok {
	_, obj := check.scope.LookupParent(ident.Name, check.pos)
	if pname, _ := obj.(*PkgName); pname != nil {
	assert(pname.pkg == check.pkg)
	check.recordUse(ident, pname)
	pname.used = true
	pkg := pname.imported
	exp := pkg.scope.Lookup(sel)
	if exp == nil {
	if !pkg.fake {
	check.errorf(e.Pos(), "%s not declared by package %s", sel, pkg.name)
	}
	goto Error
	}
	if !exp.Exported() {
	check.errorf(e.Pos(), "%s not exported by package %s", sel, pkg.name)
	// ok to continue
	}
	check.recordUse(e.Sel, exp)

	// Simplified version of the code for *ast.Idents:
	// - imported objects are always fully initialized
	switch exp := exp.(type) {
	case *Const:
	assert(exp.Val() != nil)
	x.mode = constant_
	x.typ = exp.typ
	x.val = exp.val
	case *TypeName:
	x.mode = typexpr
	x.typ = exp.typ
	case *Var:
	x.mode = variable
	x.typ = exp.typ
	case *Func:
	x.mode = value
	x.typ = exp.typ
	case *Builtin:
	x.mode = builtin
	x.typ = exp.typ
	x.id = exp.id
	default:
	check.dump("unexpected object %v", exp)
	unreachable()
	}
	x.expr = e
	return
	}
	}

	check.exprOrType(x, e.X)
	if x.mode == invalid {
	goto Error
	}

	obj, index, indirect = LookupFieldOrMethod(x.typ, x.mode == variable, check.pkg, sel)
	if obj == nil {
	switch {
	case index != nil:
	// TODO(gri) should provide actual type where the conflict happens
	check.invalidOp(e.Pos(), "ambiguous selector %s", sel)
	case indirect:
	check.invalidOp(e.Pos(), "%s is not in method set of %s", sel, x.typ)
	default:
	check.invalidOp(e.Pos(), "%s has no field or method %s", x, sel)
	}
	goto Error
	}

	if x.mode == typexpr {
	// method expression
	m, _ := obj.(*Func)
	if m == nil {
	check.invalidOp(e.Pos(), "%s has no method %s", x, sel)
	goto Error
	}

	check.recordSelection(e, MethodExpr, x.typ, m, index, indirect)

	// the receiver type becomes the type of the first function
	// argument of the method expression's function type
	var params []*Var
	sig := m.typ.(*Signature)
	if sig.params != nil {
	params = sig.params.vars
	}
	x.mode = value
	x.typ = &Signature{
	params: NewTuple(append([]*Var{NewVar(token.NoPos, check.pkg, "", x.typ)}, params...)...),
	results: sig.results,
	variadic: sig.variadic,
	}

	check.addDeclDep(m)

	} else {
	// regular selector
	switch obj := obj.(type) {
	case *Var:
	check.recordSelection(e, FieldVal, x.typ, obj, index, indirect)
	if x.mode == variable \|\| indirect {
	x.mode = variable
	} else {
	x.mode = value
	}
	x.typ = obj.typ

	case *Func:
	// TODO(gri) If we needed to take into account the receiver's
	// addressability, should we report the type &(x.typ) instead?
	check.recordSelection(e, MethodVal, x.typ, obj, index, indirect)

	if debug {
	// Verify that LookupFieldOrMethod and MethodSet.Lookup agree.
	typ := x.typ
	if x.mode == variable {
	// If typ is not an (unnamed) pointer or an interface,
	// use typ instead, because the method set of typ
	// includes the methods of typ.
	// Variables are addressable, so we can always take their
	// address.
	if _, ok := typ.(*Pointer); !ok && !IsInterface(typ) {
	typ = &Pointer{base: typ}
	}
	}
	// If we created a synthetic pointer type above, we will throw
	// away the method set computed here after use.
	// TODO(gri) Method set computation should probably always compute
	// both, the value and the pointer receiver method set and represent
	// them in a single structure.
	// TODO(gri) Consider also using a method set cache for the lifetime
	// of checker once we rely on MethodSet lookup instead of individual
	// lookup.
	mset := NewMethodSet(typ)
	if m := mset.Lookup(check.pkg, sel); m == nil \|\| m.obj != obj {
	check.dump("%s: (%s).%v -> %s", e.Pos(), typ, obj.name, m)
	check.dump("%s\n", mset)
	panic("method sets and lookup don't agree")
	}
	}

	x.mode = value

	// remove receiver
	sig := obj.typ.(Signature)
	sig.recv = nil
	x.typ = &sig

	check.addDeclDep(obj)

	default:
	unreachable()
	}
	}

	// everything went well
	x.expr = e
	return

	Error:
	x.mode = invalid
	x.expr = e
	}