go/types: don't permit implicit assignments to unexported struct fields via literals

Change-Id: I74eec172ba5a319f91c95e33b047f489dfce0c95
Reviewed-on: https://go-review.googlesource.com/4090
Reviewed-by: Alan Donovan <adonovan@google.com>

go/types/expr.go

@@ -1057,7 +1057,12 @@ func (check *Checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
 						break // cannot continue
 					}
 					// i < len(fields)
-					etyp := fields[i].typ
+					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)

go/types/testdata/expr3.src

@@ -4,6 +4,8 @@
 
 package expr3
 
+import "time"
+
 func indexes() {
 	_ = 1 /* ERROR "cannot index" */ [0]
 	_ = indexes /* ERROR "cannot index" */ [0]
@@ -101,7 +103,6 @@ func indexes() {
 	_, ok = m["bar"]
 	_ = ok
 
-
 	var t string
 	_ = t[- /* ERROR "negative" */ 1]
 	_ = t[- /* ERROR "negative" */ 1 :]
@@ -201,6 +202,15 @@ func struct_literals() {
 		x int
 	}
 	_ = P /* ERROR "invalid composite literal type" */ {}
+
+	// unexported fields
+	_ = time.Time{}
+	_ = time.Time{sec /* ERROR "unknown field" */ : 0}
+	_ = time.Time{
+		0 /* ERROR implicit assignment to unexported field sec in time.Time literal */,
+		0 /* ERROR implicit assignment */ ,
+		nil /* ERROR implicit assignment */ ,
+	}
 }
 
 func array_literals() {