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.

1 files changed, 3 insertions, 3 deletions

diff --git a/testing/test_assertrewrite.py b/testing/test_assertrewrite.py
index 23f535173..63a6fdd12 100644
--- a/testing/test_assertrewrite.py
+++ b/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))"