go.tools/go.types: more missing assigment checks implemented

Assignments to "comma, ok" expressions on the lhs of an
assignment are not permitted unless we have map index
"comma, ok" expression. Created new operand mode 'mapindex'
to distinguish this case. Renamed mode 'valueok' to the more
commonly used 'commaok' term, which also makes it easier to
distinguish from simply 'value'.

Added corresponding tests.

Fixes a TODO.

R=adonovan
CC=golang-dev
https://golang.org/cl/14526049

go/types/assignments.go

@@ -26,7 +26,7 @@ func (check *checker) assignment(x *operand, T Type) bool {
 	switch x.mode {
 	case invalid:
 		return true // error reported before
-	case constant, variable, value, valueok:
+	case constant, variable, mapindex, value, commaok:
 		// ok
 	default:
 		unreachable()
@@ -99,12 +99,7 @@ func (check *checker) initConst(lhs *Const, x *operand) {
 		return
 	}
 
-	// rhs type must be a valid constant type
-	if !isConstType(x.typ) {
-		check.errorf(x.pos(), "%s has invalid constant type", x)
-		return
-	}
-
+	assert(isConstType(x.typ))
 	lhs.val = x.val
 }
 
@@ -193,14 +188,18 @@ func (check *checker) assignVar(lhs ast.Expr, x *operand) Type {
 		return nil
 	}
 
-	if z.mode == constant || z.mode == value {
-		check.errorf(z.pos(), "cannot assign to non-variable %s", &z)
+	// 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:
+		check.errorf(z.pos(), "cannot assign to %s", &z)
 		return nil
 	}
 
-	// TODO(gri) z.mode can also be valueok which in some cases is ok (maps)
-	// but in others isn't (channels). Complete the checks here.
-
 	if !check.assignment(x, z.typ) {
 		if x.mode != invalid {
 			check.errorf(x.pos(), "cannot assign %s to %s", x, &z)
@@ -258,7 +257,7 @@ func (check *checker) initVars(lhs []*Var, rhs []ast.Expr, returnPos token.Pos)
 			}
 		}
 
-		if !returnPos.IsValid() && x.mode == valueok && l == 2 {
+		if !returnPos.IsValid() && (x.mode == mapindex || x.mode == commaok) && l == 2 {
 			// comma-ok expression (not permitted with return statements)
 			x.mode = value
 			t1 := check.initVar(lhs[0], &x)
@@ -328,7 +327,7 @@ func (check *checker) assignVars(lhs, rhs []ast.Expr) {
 			}
 		}
 
-		if x.mode == valueok && l == 2 {
+		if (x.mode == mapindex || x.mode == commaok) && l == 2 {
 			// comma-ok expression
 			x.mode = value
 			t1 := check.assignVar(lhs[0], &x)

go/types/call.go

@@ -130,7 +130,7 @@ func unpack(get getter, n int, allowCommaOk bool) (getter, int) {
 			}, t.Len()
 		}
 
-		if x0.mode == valueok {
+		if x0.mode == mapindex || x0.mode == commaok {
 			// comma-ok value
 			if allowCommaOk {
 				return func(x *operand, i int) {

go/types/eval.go

@@ -102,7 +102,7 @@ func EvalNode(fset *token.FileSet, node ast.Expr, pkg *Package, scope *Scope) (t
 	case constant:
 		val = x.val
 		fallthrough
-	case typexpr, variable, value, valueok:
+	case typexpr, variable, mapindex, value, commaok:
 		typ = x.typ
 	}
 

go/types/expr.go

@@ -107,7 +107,7 @@ func (check *checker) unary(x *operand, op token.Token) {
 			x.mode = invalid
 			return
 		}
-		x.mode = valueok
+		x.mode = commaok
 		x.typ = typ.elt
 		return
 	}
@@ -1168,7 +1168,7 @@ func (check *checker) expr0(x *operand, e ast.Expr, hint Type) exprKind {
 				}
 				goto Error
 			}
-			x.mode = valueok
+			x.mode = mapindex
 			x.typ = typ.elt
 			x.expr = e
 			return expression
@@ -1308,7 +1308,7 @@ func (check *checker) expr0(x *operand, e ast.Expr, hint Type) exprKind {
 			goto Error
 		}
 		check.typeAssertion(x.pos(), x, xtyp, T)
-		x.mode = valueok
+		x.mode = commaok
 		x.typ = T
 
 	case *ast.CallExpr:

go/types/operand.go

@@ -24,8 +24,9 @@ const (
 	typexpr                     // operand is a type
 	constant                    // operand is a constant; the operand's typ is a Basic type
 	variable                    // operand is an addressable variable
+	mapindex                    // operand is a map index expression (acts like a variable on lhs, commaok on rhs of an assignment)
 	value                       // operand is a computed value
-	valueok                     // like value, but operand may be used in a comma,ok expression
+	commaok                     // like value, but operand may be used in a comma,ok expression
 )
 
 var operandModeString = [...]string{
@@ -35,8 +36,9 @@ var operandModeString = [...]string{
 	typexpr:  "type",
 	constant: "constant",
 	variable: "variable",
+	mapindex: "map index expression",
 	value:    "value",
-	valueok:  "value, ok",
+	commaok:  "comma, ok expression",
 }
 
 // An operand represents an intermediate value during type checking.
@@ -83,11 +85,14 @@ func (x *operand) pos() token.Pos {
 // variable   <expr> (<untyped kind> <mode>                    )
 // variable   <expr> (               <mode>       of type <typ>)
 //
+// mapindex   <expr> (<untyped kind> <mode>                    )
+// mapindex   <expr> (               <mode>       of type <typ>)
+//
 // value      <expr> (<untyped kind> <mode>                    )
 // value      <expr> (               <mode>       of type <typ>)
 //
-// valueok    <expr> (<untyped kind> <mode>                    )
-// valueok    <expr> (               <mode>       of type <typ>)
+// commaok    <expr> (<untyped kind> <mode>                    )
+// commaok    <expr> (               <mode>       of type <typ>)
 //
 func (x *operand) String() string {
 	var buf bytes.Buffer

go/types/testdata/errors.src

@@ -34,7 +34,7 @@ func f(x int, m map[string]int) {
 
 	// value, ok's
 	const s = "foo"
-	m /* ERROR "m\[s\] \(value, ok of type int\) is not used" */ [s]
+	m /* ERROR "m\[s\] \(map index expression of type int\) is not used" */ [s]
 }
 
 // Valid ERROR comments can have a variety of forms.

go/types/testdata/stmt0.src

@@ -92,6 +92,34 @@ func assignments1() {
 	(_) = 0
 }
 
+func assignments2() {
+	var m map[string][]bool
+	var s []bool
+	var b bool
+	_ = s
+	_ = b
+
+	// assignments to map index expressions are ok
+	s, b = m["foo"]
+	m["foo"] = nil
+	m["foo"] = nil /* ERROR assignment count mismatch */ , false
+	_ = append(m["foo"])
+	_ = append(m["foo"], true)
+
+	var c chan int
+	_, b = <-c
+	<- /* ERROR cannot assign */ c = 0
+	<-c = 0 /* ERROR assignment count mismatch */ , false
+
+	var x interface{}
+	_, b = x.(int)
+	x /* ERROR cannot assign */ .(int) = 0
+	x.(int) = 0 /* ERROR assignment count mismatch */ , false
+
+	assignments2 /* ERROR used as value */ () = nil
+	int /* ERROR not an expression */ = 0
+}
+
 func issue6487() {
 	type S struct{x int}
 	_ = &S /* ERROR "cannot take address" */ {}.x