Avoid redundant type check after calling "ext_type.__new__()" (and in some other cases).

CHANGES.rst

@@ -51,6 +51,8 @@ Features added
 
 * Single argument function calls can avoid the argument tuple creation in some cases.
 
+* Some redundant extension type checks are avoided.
+
 * Formatting C enum values in f-strings is faster, as well as some other special cases.
 
 * Subscripting (item access) is faster in some cases.

Cython/Compiler/ExprNodes.py

@@ -12894,6 +12894,15 @@ class PyTypeTestNode(CoercionNode):
     def nonlocally_immutable(self):
         return self.arg.nonlocally_immutable()
 
+    def reanalyse(self):
+        if self.type != self.arg.type or not self.arg.is_temp:
+            return self
+        if not self.type.typeobj_is_available():
+            return self
+        if self.arg.may_be_none() and self.notnone:
+            return self.arg.as_none_safe_node("Cannot convert NoneType to %.200s" % self.type.name)
+        return self.arg
+
     def calculate_constant_result(self):
         # FIXME
         pass

Cython/Compiler/Optimize.py

@@ -2024,16 +2024,11 @@ class OptimizeBuiltinCalls(Visitor.NodeRefCleanupMixin,
     """
     ### cleanup to avoid redundant coercions to/from Python types
 
-    def _visit_PyTypeTestNode(self, node):
-        # disabled - appears to break assignments in some cases, and
-        # also drops a None check, which might still be required
+    def visit_PyTypeTestNode(self, node):
         """Flatten redundant type checks after tree changes.
         """
-        old_arg = node.arg
         self.visitchildren(node)
-        if old_arg is node.arg or node.arg.type != node.type:
-            return node
-        return node.arg
+        return node.reanalyse()
 
     def _visit_TypecastNode(self, node):
         # disabled - the user may have had a reason to put a type
@@ -2804,7 +2799,7 @@ class OptimizeBuiltinCalls(Visitor.NodeRefCleanupMixin,
                     PyTypeObjectPtr = PyrexTypes.CPtrType(
                         cython_scope.lookup('PyTypeObject').type)
                     pyx_tp_new_kwargs_func_type = PyrexTypes.CFuncType(
-                        PyrexTypes.py_object_type, [
+                        ext_type, [
                             PyrexTypes.CFuncTypeArg("type",   PyTypeObjectPtr, None),
                             PyrexTypes.CFuncTypeArg("args",   PyrexTypes.py_object_type, None),
                             PyrexTypes.CFuncTypeArg("kwargs", PyrexTypes.py_object_type, None),
@@ -2817,6 +2812,7 @@ class OptimizeBuiltinCalls(Visitor.NodeRefCleanupMixin,
                         node.pos, slot_func_cname,
                         pyx_tp_new_kwargs_func_type,
                         args=[type_arg, args_tuple, kwargs],
+                        may_return_none=False,
                         is_temp=True)
         else:
             # arbitrary variable, needs a None check for safety