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, 2 deletions

diff --git a/src/_pytest/compat.py b/src/_pytest/compat.py
index 93232f1bf..ff98492dc 100644
--- a/src/_pytest/compat.py
+++ b/src/_pytest/compat.py
@@ -176,8 +176,9 @@ def getfuncargnames(
         p.name
         for p in parameters.values()
         if (
-            p.kind is Parameter.POSITIONAL_OR_KEYWORD
-            or p.kind is Parameter.KEYWORD_ONLY
+            # TODO: Remove type ignore after https://github.com/python/typeshed/pull/4383
+            p.kind is Parameter.POSITIONAL_OR_KEYWORD  # type: ignore[unreachable]
+            or p.kind is Parameter.KEYWORD_ONLY  # type: ignore[unreachable]
         )
         and p.default is Parameter.empty
     )