Simplified inference on None returns

This patch simplifies how we infer that the function returns `None`
since the previous procedure caused problems related to type annotations
with generic standard collections (see PEP 585).

Related to issue #44.

icontract_lint/__init__.py

@@ -11,7 +11,7 @@
 import re
 import sys
 import traceback
-from typing import Set, List, Optional, TextIO, cast, Tuple
+from typing import Set, List, Optional, TextIO, cast, Tuple, Any
 
 if sys.version_info >= (3, 8):
     from typing import Final, TypedDict
@@ -410,17 +410,20 @@ def visit_FunctionDef(self, node: astroid.nodes.FunctionDef) -> None:  # pylint:
         # annotated with None.
         func_has_result = True
 
-        if node.returns is not None:
-            try:
-                inferred_returns = next(node.returns.infer())
-
-                if isinstance(inferred_returns, astroid.nodes.Const):
-                    if inferred_returns.value is None:
-                        func_has_result = False
-
-            except astroid.exceptions.NameInferenceError:
-                # Ignore uninferrable returns
-                pass
+        if node.returns is None:
+            # If no return annotation is specified, we assume that it has a result.
+            # We could use ``node.infer_call_result()`` to infer the result (see
+            # https://github.com/PyCQA/astroid/issues/1134), but that *could* lead to quite
+            # confusing false positives.
+            assert func_has_result, "Assume there is a result if there is no returns annotation."
+        else:
+            # Do not use ``node.returns.infer()`` as it causes problems with Python <3.10.
+            # See: https://github.com/Parquery/pyicontract-lint/issues/44 and
+            # https://github.com/PyCQA/astroid/issues/1134.
+            #
+            # We use ``node.returns.value`` instead, since it is the easiest way to check for
+            # ``... -> None`` annotation.
+            func_has_result = not (isinstance(node.returns, astroid.Const) and node.returns.value is None)
 
         # Infer the decorator instances
 

tests/test_postcondition.py

@@ -73,7 +73,48 @@ def some_func(x: int):
             with tests.common.sys_path_with(tmp_path):
                 self.assertListEqual([], icontract_lint.check_file(path=pth))
 
-    def test_result_none(self):
+    def test_that_it_accepts_result_annotated_with_pep_585(self):
+        text = textwrap.dedent("""\
+            from icontract import ensure
+
+            @ensure(lambda result: len(result) > 0)
+            def some_func() -> list[int]:
+                return [1, 2, 3]
+            """)
+
+        with tempfile.TemporaryDirectory() as tmp:
+            tmp_path = pathlib.Path(tmp)
+
+            pth = tmp_path / "some_module.py"
+            pth.write_text(text)
+
+            with tests.common.sys_path_with(tmp_path):
+                self.assertListEqual([], icontract_lint.check_file(path=pth))
+
+    def test_result_none_without_annotation_ignored_though_invalid(self):
+        # If the annotation is missing, we err on the side of the false negatives.
+        #
+        # Though we could use astroid to infer that the result is None, the error messages
+        # would be very confusing in the cases of false positives. Hence we ignore functions
+        # without the explicit annotation for the return value.
+        text = textwrap.dedent("""\
+            from icontract import ensure
+
+            @ensure(lambda result: result > 0)
+            def some_func(x: int):
+                pass
+            """)
+
+        with tempfile.TemporaryDirectory() as tmp:
+            tmp_path = pathlib.Path(tmp)
+
+            pth = tmp_path / "some_module.py"
+            pth.write_text(text)
+
+            with tests.common.sys_path_with(tmp_path):
+                self.assertListEqual([], icontract_lint.check_file(path=pth))
+
+    def test_result_none_with_explicit_annotation(self):
         text = textwrap.dedent("""\
             from icontract import ensure