go.tools/go/types: x.f is addressable if x is addressable or x.f contains an indirection

Fixes golang/go#6487.

R=adonovan
CC=golang-dev
https://golang.org/cl/14047043

go/types/call.go

@@ -286,7 +286,11 @@ func (check *checker) selector(x *operand, e *ast.SelectorExpr) {
 		switch obj := obj.(type) {
 		case *Var:
 			check.recordSelection(e, FieldVal, x.typ, obj, index, indirect)
-			x.mode = variable
+			if x.mode == variable || indirect {
+				x.mode = variable
+			} else {
+				x.mode = value
+			}
 			x.typ = obj.typ
 
 		case *Func:

go/types/testdata/stmt0.src

@@ -92,6 +92,21 @@ func assignments1() {
 	(_) = 0
 }
 
+func issue6487() {
+	type S struct{x int}
+	_ = &S /* ERROR "cannot take address" */ {}.x
+	_ = &( /* ERROR "cannot take address" */ S{}.x)
+	_ = (&S{}).x
+	S /* ERROR "cannot assign" */ {}.x = 0
+	(&S{}).x = 0
+
+	type M map[string]S
+	var m M
+	m /* ERROR "cannot assign" */ ["foo"].x = 0
+	_ = &( /* ERROR "cannot take address" */ m["foo"].x)
+	_ = &m /* ERROR "cannot take address" */ ["foo"].x
+}
+
 func shortVarDecls() {
 	const c = 0
 	type d int