fix: correct type assertion for bin func types (#588)

_test/type21.go

@@ -0,0 +1,16 @@
+package main
+
+import (
+	"reflect"
+	"time"
+)
+
+func main() {
+	t := time.Date(2009, time.November, 10, 23, 4, 5, 0, time.UTC)
+	v := reflect.ValueOf(t.String)
+	f := v.Interface().(func() string)
+	println(f())
+}
+
+// Output:
+// 2009-11-10 23:04:05 +0000 UTC

interp/cfg.go

@@ -1503,7 +1503,12 @@ func (interp *Interpreter) cfg(root *node, pkgID string) ([]*node, error) {
 					}
 				}
 				if n.anc.action != aAssignX {
-					n.typ = n.child[1].typ
+					if n.child[0].typ.cat == valueT {
+						// Avoid special wrapping of interfaces and func types.
+						n.typ = &itype{cat: valueT, rtype: n.child[1].typ.TypeOf()}
+					} else {
+						n.typ = n.child[1].typ
+					}
 					n.findex = sc.add(n.typ)
 				}
 			} else {