typing: set warn_unreachable

This makes mypy raise an error whenever it detects code which is
statically unreachable, e.g.

    x: int
    if isinstance(x, str):
        ... # Statement is unreachable  [unreachable]

This is really neat and finds quite a few logic and typing bugs.

Sometimes the code is intentionally unreachable in terms of types, e.g.
raising TypeError when a function is given an argument with a wrong
type. In these cases a `type: ignore[unreachable]` is needed, but I
think it's a nice code hint.

testing/code/test_source.py

@@ -238,7 +238,7 @@ def test_getline_finally() -> None:
             c(1)  # type: ignore
         finally:
             if teardown:
-                teardown()
+                teardown()  # type: ignore[unreachable]
     source = excinfo.traceback[-1].statement
     assert str(source).strip() == "c(1)  # type: ignore"
 

testing/test_assertrewrite.py

@@ -381,7 +381,7 @@ class TestAssertionRewrite:
         )
 
         def f7() -> None:
-            assert False or x()
+            assert False or x()  # type: ignore[unreachable]
 
         assert (
             getmsg(f7, {"x": x})
@@ -416,7 +416,7 @@ class TestAssertionRewrite:
 
     def test_short_circuit_evaluation(self) -> None:
         def f1() -> None:
-            assert True or explode  # type: ignore[name-defined] # noqa: F821
+            assert True or explode  # type: ignore[name-defined,unreachable] # noqa: F821
 
         getmsg(f1, must_pass=True)
 
@@ -471,7 +471,7 @@ class TestAssertionRewrite:
         assert getmsg(f1) == "assert ((3 % 2) and False)"
 
         def f2() -> None:
-            assert False or 4 % 2
+            assert False or 4 % 2  # type: ignore[unreachable]
 
         assert getmsg(f2) == "assert (False or (4 % 2))"
 

testing/test_monkeypatch.py

@@ -360,7 +360,7 @@ def test_issue156_undo_staticmethod(Sample: "Type[Sample]") -> None:
     monkeypatch.setattr(Sample, "hello", None)
     assert Sample.hello is None
 
-    monkeypatch.undo()
+    monkeypatch.undo()  # type: ignore[unreachable]
     assert Sample.hello()
 
 

testing/test_pytester.py

@@ -23,7 +23,9 @@ def test_make_hook_recorder(testdir) -> None:
     recorder = testdir.make_hook_recorder(item.config.pluginmanager)
     assert not recorder.getfailures()
 
-    pytest.xfail("internal reportrecorder tests need refactoring")
+    # (The silly condition is to fool mypy that the code below this is reachable)
+    if 1 + 1 == 2:
+        pytest.xfail("internal reportrecorder tests need refactoring")
 
     class rep:
         excinfo = None