interp: fix interface wrapper generation

Add early detection of cases where no wrapper is necessary because
the value type already implements the target interface.

It should both increase performances by avoiding the wrapper overhead,
and fix errors due to replacing valid values by incomplete wrappers,
caused by the presence of private methods in the interface definition,
as in #1191.

Fixes #1191.

_test/interface52.go

@@ -0,0 +1,17 @@
+package main
+
+import "testing"
+
+func main() {
+	t := testing.T{}
+	var tb testing.TB
+	tb = &t
+	if tb.TempDir() == "" {
+		println("FAIL")
+		return
+	}
+	println("PASS")
+}
+
+// Output:
+// PASS

interp/run.go

@@ -971,7 +971,8 @@ func genInterfaceWrapper(n *node, typ reflect.Type) func(*frame) reflect.Value {
 	if typ == nil || typ.Kind() != reflect.Interface || typ.NumMethod() == 0 || n.typ.cat == valueT {
 		return value
 	}
-	if nt := n.typ.TypeOf(); nt != nil && nt.Kind() == reflect.Interface {
+	nt := n.typ.frameType()
+	if nt != nil && nt.Implements(typ) {
 		return value
 	}
 	mn := typ.NumMethod()
@@ -990,6 +991,9 @@ func genInterfaceWrapper(n *node, typ reflect.Type) func(*frame) reflect.Value {
 
 	return func(f *frame) reflect.Value {
 		v := value(f)
+		if v.Type().Implements(typ) {
+			return v
+		}
 		vv := v
 		switch v.Kind() {
 		case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Ptr, reflect.Slice: