src/go/types/assignments.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 initialization and assignment checks.

 package types

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

 // assignment reports whether x can be assigned to a variable of type T,
 // if necessary by attempting to convert untyped values to the appropriate
 // type. context describes the context in which the assignment takes place.
 // Use T == nil to indicate assignment to an untyped blank identifier.
 // x.mode is set to invalid if the assignment failed.
 func (check *Checker) assignment(x *operand, T Type, context string) {
 	check.singleValue(x)

 	switch x.mode {
 	case invalid:
 		return // error reported before
 	case constant_, variable, mapindex, value, commaok, commaerr:
 		// ok
 	default:
 		// we may get here because of other problems (issue #39634, crash 12)
 		check.errorf(x, 0, "cannot assign %s to %s in %s", x, T, context)
 		return
 	}

 	if isUntyped(x.typ) {
 		target := T
 		// spec: "If an untyped constant is assigned to a variable of interface
 		// type or the blank identifier, the constant is first converted to type
 		// bool, rune, int, float64, complex128 or string respectively, depending
 		// on whether the value is a boolean, rune, integer, floating-point,
 		// complex, or string constant."
 		if T == nil || IsInterface(T) {
 			if T == nil && x.typ == Typ[UntypedNil] {
 				check.errorf(x, _UntypedNil, "use of untyped nil in %s", context)
 				x.mode = invalid
 				return
 			}
 			target = Default(x.typ)
 		}
 		newType, val, code := check.implicitTypeAndValue(x, target)
 		if code != 0 {
 			msg := check.sprintf("cannot use %s as %s value in %s", x, target, context)
 			switch code {
 			case _TruncatedFloat:
 				msg += " (truncated)"
 			case _NumericOverflow:
 				msg += " (overflows)"
 			default:
 				code = _IncompatibleAssign
 			}
 			check.error(x, code, msg)
 			x.mode = invalid
 			return
 		}
 		if val != nil {
 			x.val = val
 			check.updateExprVal(x.expr, val)
 		}
 		if newType != x.typ {
 			x.typ = newType
 			check.updateExprType(x.expr, newType, false)
 		}
 	}

 	// A generic (non-instantiated) function value cannot be assigned to a variable.
 	if sig := asSignature(x.typ); sig != nil && len(sig.tparams) > 0 {
 		check.errorf(x, _Todo, "cannot use generic function %s without instantiation in %s", x, context)
 	}

 	// spec: "If a left-hand side is the blank identifier, any typed or
 	// non-constant value except for the predeclared identifier nil may
 	// be assigned to it."
 	if T == nil {
 		return
 	}

 	reason := ""
 	if ok, code := x.assignableTo(check, T, &reason); !ok {
 		if reason != "" {
 			check.errorf(x, code, "cannot use %s as %s value in %s: %s", x, T, context, reason)
 		} else {
 			check.errorf(x, code, "cannot use %s as %s value in %s", x, T, context)
 		}
 		x.mode = invalid
 	}
 }

 func (check *Checker) initConst(lhs *Const, x *operand) {
 	if x.mode == invalid || x.typ == Typ[Invalid] || lhs.typ == Typ[Invalid] {
 		if lhs.typ == nil {
 			lhs.typ = Typ[Invalid]
 		}
 		return
 	}

 	// rhs must be a constant
 	if x.mode != constant_ {
 		check.errorf(x, _InvalidConstInit, "%s is not constant", x)
 		if lhs.typ == nil {
 			lhs.typ = Typ[Invalid]
 		}
 		return
 	}
 	assert(isConstType(x.typ))

 	// If the lhs doesn't have a type yet, use the type of x.
 	if lhs.typ == nil {
 		lhs.typ = x.typ
 	}

 	check.assignment(x, lhs.typ, "constant declaration")
 	if x.mode == invalid {
 		return
 	}

 	lhs.val = x.val
 }

 func (check *Checker) initVar(lhs *Var, x *operand, context string) Type {
 	if x.mode == invalid || x.typ == Typ[Invalid] || lhs.typ == Typ[Invalid] {
 		if lhs.typ == nil {
 			lhs.typ = Typ[Invalid]
 		}
 		return nil
 	}

 	// If the lhs doesn't have a type yet, use the type of x.
 	if lhs.typ == nil {
 		typ := x.typ
 		if isUntyped(typ) {
 			// convert untyped types to default types
 			if typ == Typ[UntypedNil] {
 				check.errorf(x, _UntypedNil, "use of untyped nil in %s", context)
 				lhs.typ = Typ[Invalid]
 				return nil
 			}
 			typ = Default(typ)
 		}
 		lhs.typ = typ
 	}

 	check.assignment(x, lhs.typ, context)
 	if x.mode == invalid {
 		return nil
 	}

 	return x.typ
 }

 func (check *Checker) assignVar(lhs ast.Expr, x *operand) Type {
 	if x.mode == invalid || x.typ == Typ[Invalid] {
 		check.useLHS(lhs)
 		return nil
 	}

 	// Determine if the lhs is a (possibly parenthesized) identifier.
 	ident, _ := unparen(lhs).(*ast.Ident)

 	// Don't evaluate lhs if it is the blank identifier.
 	if ident != nil && ident.Name == "_" {
 		check.recordDef(ident, nil)
 		check.assignment(x, nil, "assignment to _ identifier")
 		if x.mode == invalid {
 			return nil
 		}
 		return x.typ
 	}

 	// If the lhs is an identifier denoting a variable v, this assignment
 	// is not a 'use' of v. Remember current value of v.used and restore
 	// after evaluating the lhs via check.expr.
 	var v *Var
 	var v_used bool
 	if ident != nil {
 		if obj := check.lookup(ident.Name); obj != nil {
 			// It's ok to mark non-local variables, but ignore variables
 			// from other packages to avoid potential race conditions with
 			// dot-imported variables.
 			if w, _ := obj.(*Var); w != nil && w.pkg == check.pkg {
 				v = w
 				v_used = v.used
 			}
 		}
 	}

 	var z operand
 	check.expr(&z, lhs)
 	if v != nil {
 		v.used = v_used // restore v.used
 	}

 	if z.mode == invalid || z.typ == Typ[Invalid] {
 		return nil
 	}

 	// spec: "Each left-hand side operand must be addressable, a map index
 	// expression, or the blank identifier. Operands may be parenthesized."
 	switch z.mode {
 	case invalid:
 		return nil
 	case variable, mapindex:
 		// ok
 	default:
 		if sel, ok := z.expr.(*ast.SelectorExpr); ok {
 			var op operand
 			check.expr(&op, sel.X)
 			if op.mode == mapindex {
 				check.errorf(&z, _UnaddressableFieldAssign, "cannot assign to struct field %s in map", ExprString(z.expr))
 				return nil
 			}
 		}
 		check.errorf(&z, _UnassignableOperand, "cannot assign to %s", &z)
 		return nil
 	}

 	check.assignment(x, z.typ, "assignment")
 	if x.mode == invalid {
 		return nil
 	}

 	return x.typ
 }

 // If returnPos is valid, initVars is called to type-check the assignment of
 // return expressions, and returnPos is the position of the return statement.
 func (check *Checker) initVars(lhs []*Var, origRHS []ast.Expr, returnPos token.Pos) {
 	rhs, commaOk := check.exprList(origRHS, len(lhs) == 2 && !returnPos.IsValid())

 	if len(lhs) != len(rhs) {
 		// invalidate lhs
 		for _, obj := range lhs {
 			if obj.typ == nil {
 				obj.typ = Typ[Invalid]
 			}
 		}
 		// don't report an error if we already reported one
 		for _, x := range rhs {
 			if x.mode == invalid {
 				return
 			}
 		}
 		if returnPos.IsValid() {
 			check.errorf(atPos(returnPos), _WrongResultCount, "wrong number of return values (want %d, got %d)", len(lhs), len(rhs))
 			return
 		}
 		check.errorf(rhs[0], _WrongAssignCount, "cannot initialize %d variables with %d values", len(lhs), len(rhs))
 		return
 	}

 	context := "assignment"
 	if returnPos.IsValid() {
 		context = "return statement"
 	}

 	if commaOk {
 		var a [2]Type
 		for i := range a {
 			a[i] = check.initVar(lhs[i], rhs[i], context)
 		}
 		check.recordCommaOkTypes(origRHS[0], a)
 		return
 	}

 	for i, lhs := range lhs {
 		check.initVar(lhs, rhs[i], context)
 	}
 }

 func (check *Checker) assignVars(lhs, origRHS []ast.Expr) {
 	rhs, commaOk := check.exprList(origRHS, len(lhs) == 2)

 	if len(lhs) != len(rhs) {
 		check.useLHS(lhs...)
 		// don't report an error if we already reported one
 		for _, x := range rhs {
 			if x.mode == invalid {
 				return
 			}
 		}
 		check.errorf(rhs[0], _WrongAssignCount, "cannot assign %d values to %d variables", len(rhs), len(lhs))
 		return
 	}

 	if commaOk {
 		var a [2]Type
 		for i := range a {
 			a[i] = check.assignVar(lhs[i], rhs[i])
 		}
 		check.recordCommaOkTypes(origRHS[0], a)
 		return
 	}

 	for i, lhs := range lhs {
 		check.assignVar(lhs, rhs[i])
 	}
 }

 func (check *Checker) shortVarDecl(pos positioner, lhs, rhs []ast.Expr) {
 	top := len(check.delayed)
 	scope := check.scope

 	// collect lhs variables
 	seen := make(map[string]bool, len(lhs))
 	lhsVars := make([]*Var, len(lhs))
 	newVars := make([]*Var, 0, len(lhs))
 	hasErr := false
 	for i, lhs := range lhs {
 		ident, _ := lhs.(*ast.Ident)
 		if ident == nil {
 			check.useLHS(lhs)
 			// TODO(rFindley) this is redundant with a parser error. Consider omitting?
 			check.errorf(lhs, _BadDecl, "non-name %s on left side of :=", lhs)
 			hasErr = true
 			continue
 		}

 		name := ident.Name
 		if name != "_" {
 			if seen[name] {
 				check.errorf(lhs, _RepeatedDecl, "%s repeated on left side of :=", lhs)
 				hasErr = true
 				continue
 			}
 			seen[name] = true
 		}

 		// Use the correct obj if the ident is redeclared. The
 		// variable's scope starts after the declaration; so we
 		// must use Scope.Lookup here and call Scope.Insert
 		// (via check.declare) later.
 		if alt := scope.Lookup(name); alt != nil {
 			check.recordUse(ident, alt)
 			// redeclared object must be a variable
 			if obj, _ := alt.(*Var); obj != nil {
 				lhsVars[i] = obj
 			} else {
 				check.errorf(lhs, _UnassignableOperand, "cannot assign to %s", lhs)
 				hasErr = true
 			}
 			continue
 		}

 		// declare new variable
 		obj := NewVar(ident.Pos(), check.pkg, name, nil)
 		lhsVars[i] = obj
 		if name != "_" {
 			newVars = append(newVars, obj)
 		}
 		check.recordDef(ident, obj)
 	}

 	// create dummy variables where the lhs is invalid
 	for i, obj := range lhsVars {
 		if obj == nil {
 			lhsVars[i] = NewVar(lhs[i].Pos(), check.pkg, "_", nil)
 		}
 	}

 	check.initVars(lhsVars, rhs, token.NoPos)

 	// process function literals in rhs expressions before scope changes
 	check.processDelayed(top)

 	if len(newVars) == 0 && !hasErr {
 		check.softErrorf(pos, _NoNewVar, "no new variables on left side of :=")
 		return
 	}

 	// declare new variables
 	// spec: "The scope of a constant or variable identifier declared inside
 	// a function begins at the end of the ConstSpec or VarSpec (ShortVarDecl
 	// for short variable declarations) and ends at the end of the innermost
 	// containing block."
 	scopePos := rhs[len(rhs)-1].End()
 	for _, obj := range newVars {
 		check.declare(scope, nil, obj, scopePos) // id = nil: recordDef already called
 	}
 }
	// 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 initialization and assignment checks.

	package types

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

	// assignment reports whether x can be assigned to a variable of type T,
	// if necessary by attempting to convert untyped values to the appropriate
	// type. context describes the context in which the assignment takes place.
	// Use T == nil to indicate assignment to an untyped blank identifier.
	// x.mode is set to invalid if the assignment failed.
	func (check Checker) assignment(x operand, T Type, context string) {
	check.singleValue(x)

	switch x.mode {
	case invalid:
	return // error reported before
	case constant_, variable, mapindex, value, commaok, commaerr:
	// ok
	default:
	// we may get here because of other problems (issue #39634, crash 12)
	check.errorf(x, 0, "cannot assign %s to %s in %s", x, T, context)
	return
	}

	if isUntyped(x.typ) {
	target := T
	// spec: "If an untyped constant is assigned to a variable of interface
	// type or the blank identifier, the constant is first converted to type
	// bool, rune, int, float64, complex128 or string respectively, depending
	// on whether the value is a boolean, rune, integer, floating-point,
	// complex, or string constant."
	if T == nil \|\| IsInterface(T) {
	if T == nil && x.typ == Typ[UntypedNil] {
	check.errorf(x, _UntypedNil, "use of untyped nil in %s", context)
	x.mode = invalid
	return
	}
	target = Default(x.typ)
	}
	newType, val, code := check.implicitTypeAndValue(x, target)
	if code != 0 {
	msg := check.sprintf("cannot use %s as %s value in %s", x, target, context)
	switch code {
	case _TruncatedFloat:
	msg += " (truncated)"
	case _NumericOverflow:
	msg += " (overflows)"
	default:
	code = _IncompatibleAssign
	}
	check.error(x, code, msg)
	x.mode = invalid
	return
	}
	if val != nil {
	x.val = val
	check.updateExprVal(x.expr, val)
	}
	if newType != x.typ {
	x.typ = newType
	check.updateExprType(x.expr, newType, false)
	}
	}

	// A generic (non-instantiated) function value cannot be assigned to a variable.
	if sig := asSignature(x.typ); sig != nil && len(sig.tparams) > 0 {
	check.errorf(x, _Todo, "cannot use generic function %s without instantiation in %s", x, context)
	}

	// spec: "If a left-hand side is the blank identifier, any typed or
	// non-constant value except for the predeclared identifier nil may
	// be assigned to it."
	if T == nil {
	return
	}

	reason := ""
	if ok, code := x.assignableTo(check, T, &reason); !ok {
	if reason != "" {
	check.errorf(x, code, "cannot use %s as %s value in %s: %s", x, T, context, reason)
	} else {
	check.errorf(x, code, "cannot use %s as %s value in %s", x, T, context)
	}
	x.mode = invalid
	}
	}

	func (check Checker) initConst(lhs Const, x *operand) {
	if x.mode == invalid \|\| x.typ == Typ[Invalid] \|\| lhs.typ == Typ[Invalid] {
	if lhs.typ == nil {
	lhs.typ = Typ[Invalid]
	}
	return
	}

	// rhs must be a constant
	if x.mode != constant_ {
	check.errorf(x, _InvalidConstInit, "%s is not constant", x)
	if lhs.typ == nil {
	lhs.typ = Typ[Invalid]
	}
	return
	}
	assert(isConstType(x.typ))

	// If the lhs doesn't have a type yet, use the type of x.
	if lhs.typ == nil {
	lhs.typ = x.typ
	}

	check.assignment(x, lhs.typ, "constant declaration")
	if x.mode == invalid {
	return
	}

	lhs.val = x.val
	}

	func (check Checker) initVar(lhs Var, x *operand, context string) Type {
	if x.mode == invalid \|\| x.typ == Typ[Invalid] \|\| lhs.typ == Typ[Invalid] {
	if lhs.typ == nil {
	lhs.typ = Typ[Invalid]
	}
	return nil
	}

	// If the lhs doesn't have a type yet, use the type of x.
	if lhs.typ == nil {
	typ := x.typ
	if isUntyped(typ) {
	// convert untyped types to default types
	if typ == Typ[UntypedNil] {
	check.errorf(x, _UntypedNil, "use of untyped nil in %s", context)
	lhs.typ = Typ[Invalid]
	return nil
	}
	typ = Default(typ)
	}
	lhs.typ = typ
	}

	check.assignment(x, lhs.typ, context)
	if x.mode == invalid {
	return nil
	}

	return x.typ
	}

	func (check Checker) assignVar(lhs ast.Expr, x operand) Type {
	if x.mode == invalid \|\| x.typ == Typ[Invalid] {
	check.useLHS(lhs)
	return nil
	}

	// Determine if the lhs is a (possibly parenthesized) identifier.
	ident, _ := unparen(lhs).(*ast.Ident)

	// Don't evaluate lhs if it is the blank identifier.
	if ident != nil && ident.Name == "_" {
	check.recordDef(ident, nil)
	check.assignment(x, nil, "assignment to _ identifier")
	if x.mode == invalid {
	return nil
	}
	return x.typ
	}

	// If the lhs is an identifier denoting a variable v, this assignment
	// is not a 'use' of v. Remember current value of v.used and restore
	// after evaluating the lhs via check.expr.
	var v *Var
	var v_used bool
	if ident != nil {
	if obj := check.lookup(ident.Name); obj != nil {
	// It's ok to mark non-local variables, but ignore variables
	// from other packages to avoid potential race conditions with
	// dot-imported variables.
	if w, _ := obj.(*Var); w != nil && w.pkg == check.pkg {
	v = w
	v_used = v.used
	}
	}
	}

	var z operand
	check.expr(&z, lhs)
	if v != nil {
	v.used = v_used // restore v.used
	}

	if z.mode == invalid \|\| z.typ == Typ[Invalid] {
	return nil
	}

	// spec: "Each left-hand side operand must be addressable, a map index
	// expression, or the blank identifier. Operands may be parenthesized."
	switch z.mode {
	case invalid:
	return nil
	case variable, mapindex:
	// ok
	default:
	if sel, ok := z.expr.(*ast.SelectorExpr); ok {
	var op operand
	check.expr(&op, sel.X)
	if op.mode == mapindex {
	check.errorf(&z, _UnaddressableFieldAssign, "cannot assign to struct field %s in map", ExprString(z.expr))
	return nil
	}
	}
	check.errorf(&z, _UnassignableOperand, "cannot assign to %s", &z)
	return nil
	}

	check.assignment(x, z.typ, "assignment")
	if x.mode == invalid {
	return nil
	}

	return x.typ
	}

	// If returnPos is valid, initVars is called to type-check the assignment of
	// return expressions, and returnPos is the position of the return statement.
	func (check Checker) initVars(lhs []Var, origRHS []ast.Expr, returnPos token.Pos) {
	rhs, commaOk := check.exprList(origRHS, len(lhs) == 2 && !returnPos.IsValid())

	if len(lhs) != len(rhs) {
	// invalidate lhs
	for _, obj := range lhs {
	if obj.typ == nil {
	obj.typ = Typ[Invalid]
	}
	}
	// don't report an error if we already reported one
	for _, x := range rhs {
	if x.mode == invalid {
	return
	}
	}
	if returnPos.IsValid() {
	check.errorf(atPos(returnPos), _WrongResultCount, "wrong number of return values (want %d, got %d)", len(lhs), len(rhs))
	return
	}
	check.errorf(rhs[0], _WrongAssignCount, "cannot initialize %d variables with %d values", len(lhs), len(rhs))
	return
	}

	context := "assignment"
	if returnPos.IsValid() {
	context = "return statement"
	}

	if commaOk {
	var a [2]Type
	for i := range a {
	a[i] = check.initVar(lhs[i], rhs[i], context)
	}
	check.recordCommaOkTypes(origRHS[0], a)
	return
	}

	for i, lhs := range lhs {
	check.initVar(lhs, rhs[i], context)
	}
	}

	func (check *Checker) assignVars(lhs, origRHS []ast.Expr) {
	rhs, commaOk := check.exprList(origRHS, len(lhs) == 2)

	if len(lhs) != len(rhs) {
	check.useLHS(lhs...)
	// don't report an error if we already reported one
	for _, x := range rhs {
	if x.mode == invalid {
	return
	}
	}
	check.errorf(rhs[0], _WrongAssignCount, "cannot assign %d values to %d variables", len(rhs), len(lhs))
	return
	}

	if commaOk {
	var a [2]Type
	for i := range a {
	a[i] = check.assignVar(lhs[i], rhs[i])
	}
	check.recordCommaOkTypes(origRHS[0], a)
	return
	}

	for i, lhs := range lhs {
	check.assignVar(lhs, rhs[i])
	}
	}

	func (check *Checker) shortVarDecl(pos positioner, lhs, rhs []ast.Expr) {
	top := len(check.delayed)
	scope := check.scope

	// collect lhs variables
	seen := make(map[string]bool, len(lhs))
	lhsVars := make([]*Var, len(lhs))
	newVars := make([]*Var, 0, len(lhs))
	hasErr := false
	for i, lhs := range lhs {
	ident, _ := lhs.(*ast.Ident)
	if ident == nil {
	check.useLHS(lhs)
	// TODO(rFindley) this is redundant with a parser error. Consider omitting?
	check.errorf(lhs, _BadDecl, "non-name %s on left side of :=", lhs)
	hasErr = true
	continue
	}

	name := ident.Name
	if name != "_" {
	if seen[name] {
	check.errorf(lhs, _RepeatedDecl, "%s repeated on left side of :=", lhs)
	hasErr = true
	continue
	}
	seen[name] = true
	}

	// Use the correct obj if the ident is redeclared. The
	// variable's scope starts after the declaration; so we
	// must use Scope.Lookup here and call Scope.Insert
	// (via check.declare) later.
	if alt := scope.Lookup(name); alt != nil {
	check.recordUse(ident, alt)
	// redeclared object must be a variable
	if obj, _ := alt.(*Var); obj != nil {
	lhsVars[i] = obj
	} else {
	check.errorf(lhs, _UnassignableOperand, "cannot assign to %s", lhs)
	hasErr = true
	}
	continue
	}

	// declare new variable
	obj := NewVar(ident.Pos(), check.pkg, name, nil)
	lhsVars[i] = obj
	if name != "_" {
	newVars = append(newVars, obj)
	}
	check.recordDef(ident, obj)
	}

	// create dummy variables where the lhs is invalid
	for i, obj := range lhsVars {
	if obj == nil {
	lhsVars[i] = NewVar(lhs[i].Pos(), check.pkg, "_", nil)
	}
	}

	check.initVars(lhsVars, rhs, token.NoPos)

	// process function literals in rhs expressions before scope changes
	check.processDelayed(top)

	if len(newVars) == 0 && !hasErr {
	check.softErrorf(pos, _NoNewVar, "no new variables on left side of :=")
	return
	}

	// declare new variables
	// spec: "The scope of a constant or variable identifier declared inside
	// a function begins at the end of the ConstSpec or VarSpec (ShortVarDecl
	// for short variable declarations) and ends at the end of the innermost
	// containing block."
	scopePos := rhs[len(rhs)-1].End()
	for _, obj := range newVars {
	check.declare(scope, nil, obj, scopePos) // id = nil: recordDef already called
	}
	}