[BUGFIX] Fixing pybind11::error_already_set.matches to also work with exception subclasses (#1715)

* Fixing order of arguments in call to PyErr_GivenExceptionMatches in pybind11::error_already_set.matches

* Added tests on error_already_set::matches fix for exception base classes

include/pybind11/pytypes.h

@@ -344,7 +344,7 @@ public:
     /// Check if the currently trapped error type matches the given Python exception class (or a
     /// subclass thereof).  May also be passed a tuple to search for any exception class matches in
     /// the given tuple.
-    bool matches(handle ex) const { return PyErr_GivenExceptionMatches(ex.ptr(), m_type.ptr()); }
+    bool matches(handle exc) const { return PyErr_GivenExceptionMatches(m_type.ptr(), exc.ptr()); }
 
     const object& type() const { return m_type; }
     const object& value() const { return m_value; }

tests/test_exceptions.cpp

@@ -118,10 +118,38 @@ TEST_SUBMODULE(exceptions, m) {
     m.def("throws_logic_error", []() { throw std::logic_error("this error should fall through to the standard handler"); });
     m.def("exception_matches", []() {
         py::dict foo;
-        try { foo["bar"]; }
+        try {
+            // Assign to a py::object to force read access of nonexistent dict entry
+            py::object o = foo["bar"];
+        }
         catch (py::error_already_set& ex) {
             if (!ex.matches(PyExc_KeyError)) throw;
+            return true;
+        }
+        return false;
+    });
+    m.def("exception_matches_base", []() {
+        py::dict foo;
+        try {
+            // Assign to a py::object to force read access of nonexistent dict entry
+            py::object o = foo["bar"];
         }
+        catch (py::error_already_set &ex) {
+            if (!ex.matches(PyExc_Exception)) throw;
+            return true;
+        }
+        return false;
+    });
+    m.def("modulenotfound_exception_matches_base", []() {
+        try {
+            // On Python >= 3.6, this raises a ModuleNotFoundError, a subclass of ImportError
+            py::module::import("nonexistent");
+        }
+        catch (py::error_already_set &ex) {
+            if (!ex.matches(PyExc_ImportError)) throw;
+            return true;
+        }
+        return false;
     });
 
     m.def("throw_already_set", [](bool err) {

tests/test_exceptions.py

@@ -48,7 +48,9 @@ def test_python_call_in_catch():
 
 
 def test_exception_matches():
-    m.exception_matches()
+    assert m.exception_matches()
+    assert m.exception_matches_base()
+    assert m.modulenotfound_exception_matches_base()
 
 
 def test_custom(msg):