Support an internal plain text default filter.

Python/_warnings.c

@@ -180,17 +180,20 @@ check_matched(PyInterpreterState *interp, PyObject *obj, PyObject *arg, PyObject
     if (obj == Py_None)
         return 1;
 
-    if (arg != NULL) {
-        /* An internal plain text default filter must match exactly */
-        if (PyUnicode_CheckExact(obj)) {
-            int cmp_result = PyUnicode_Compare(obj, arg);
-            if (cmp_result == -1 && PyErr_Occurred()) {
-                return -1;
-            }
-            return !cmp_result;
+    /* An internal plain text default filter must match exactly */
+    if (PyUnicode_CheckExact(obj)) {
+        if (arg == NULL) {
+            return 0;
+        }
+        int cmp_result = PyUnicode_Compare(obj, arg);
+        if (cmp_result == -1 && PyErr_Occurred()) {
+            return -1;
         }
+        return !cmp_result;
+    }
 
-        /* Otherwise assume a regex filter and call its match() method */
+    /* Otherwise assume a regex filter and call its match() method */
+    if (arg != NULL) {
         result = PyObject_CallMethodOneArg(obj, &_Py_ID(match), arg);
     }
     else {