interp: fix redeclaration scope issue

Fixes #1378

_test/issue-1378.go

@@ -0,0 +1,21 @@
+package main
+
+import (
+	"fmt"
+	"time"
+)
+
+func main() {
+	t, err := time.Parse(time.RFC3339, "2006-01-02T15:04:05Z")
+	if err != nil {
+		panic(err)
+	}
+	fn := func() error {
+		_, err := t.GobEncode()
+		return err
+	}
+	fmt.Println(fn())
+}
+
+// Output:
+// <nil>

interp/cfg.go

@@ -2063,7 +2063,7 @@ func compDefineX(sc *scope, n *node) error {
 	for i, t := range types {
 		var index int
 		id := n.child[i].ident
-		if sym, _, ok := sc.lookup(id); ok && sym.kind == varSym && sym.typ.equals(t) {
+		if sym, level, ok := sc.lookup(id); ok && level == n.child[i].level && sym.kind == varSym && sym.typ.equals(t) {
 			// Reuse symbol in case of a variable redeclaration with the same type.
 			index = sym.index
 		} else {