interp: fix redeclaration of an interface variable

Fixes #1404.

_test/issue-1404.go

@@ -0,0 +1,22 @@
+package main
+
+type I interface {
+	inI()
+}
+
+type T struct {
+	name string
+}
+
+func (t *T) inI() {}
+
+func main() {
+	var i I = &T{name: "foo"}
+
+	if i, ok := i.(*T); ok {
+		println(i.name)
+	}
+}
+
+// Output:
+// foo

interp/cfg.go

@@ -2135,7 +2135,7 @@ func compDefineX(sc *scope, n *node) error {
 	for i, t := range types {
 		var index int
 		id := n.child[i].ident
-		if sym, level, ok := sc.lookup(id); ok && level == n.child[i].level && 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.id() == t.id() {
 			// Reuse symbol in case of a variable redeclaration with the same type.
 			index = sym.index
 		} else {