diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index bf01667..f084159 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -46,11 +46,11 @@ jobs:
         run: uv run --dev ruff check .
         continue-on-error: true
 
-      - name: Type check
-        run: |
-          uv run --dev pyrefly check .
-          uv run --dev mypy .
-        continue-on-error: true
+      - name: Pyrefly type check
+        run: uv run --dev pyrefly check .
+
+      - name: Mypy type check
+        run: uv run --dev mypy .
 
       - name: Run tests
         run: |
@@ -61,7 +61,7 @@ jobs:
           uv run --dev pytest -vv --cov=alternative --cov-report=xml --junitxml=test-results.xml
 
       - name: Upload coverage to Codecov
-        if: always() && env.CODECOV_TOKEN != ''
+        if: (success() || hashFiles('coverage.xml') != '') && env.CODECOV_TOKEN != ''
         uses: codecov/codecov-action@v5
         with:
           token: ${{ env.CODECOV_TOKEN }}
@@ -69,7 +69,7 @@ jobs:
           fail_ci_if_error: true
 
       - name: Upload test results to Codecov
-        if: always() && env.CODECOV_TOKEN != ''
+        if: (success() || hashFiles('test-results.xml') != '') && env.CODECOV_TOKEN != ''
         uses: codecov/codecov-action@v5
         with:
           token: ${{ env.CODECOV_TOKEN }}
diff --git a/alternative.py b/alternative.py
index 1029269..0bfbd71 100644
--- a/alternative.py
+++ b/alternative.py
@@ -4,7 +4,7 @@
 import inspect
 import os
 from functools import wraps, lru_cache
-from typing import Callable
+from typing import Callable, Protocol
 from typing import cast, overload
 
 
@@ -31,11 +31,19 @@
 class _UNDEFINED: ...
 
 
+class _SupportsLessThan(Protocol):
+    def __lt__(self, other: object, /) -> bool: ...
+
+
 _UNDEFINED_VALUE = _UNDEFINED()
 
 type ImplementationSig[**P, R] = Callable[P, R] | Implementation[P, R]
-type AlternativesWrapper[**P, R] = Callable[[ImplementationSig], Alternatives[P, R]]
-type ImplementationWrapper[**P, R] = Callable[[ImplementationSig], Implementation[P, R]]
+type AlternativesWrapper[**P, R] = Callable[
+    [ImplementationSig[P, R]], Alternatives[P, R]
+]
+type ImplementationWrapper[**P, R] = Callable[
+    [ImplementationSig[P, R]], Implementation[P, R]
+]
 
 
 class AlternativeError(Exception):
@@ -104,7 +112,6 @@ def __init__(self, implementation: Callable[P, R], *, default: bool = False):
         self._debug_invoked_site: str | None = None
         # tracks the use of the set should be
         self._enumerated = False
-        self._debug_invoked_site: str | None = None
 
         self._callable: Callable[P, R] | None = None
         self._debug_callable_used: str | None = None
@@ -187,7 +194,7 @@ def callable(self) -> Callable[P, R]:
             else:
                 self._callable = self.reference
             self._debug_callable_used = _maybe_get_caller_path()
-            self.__call__ = self._callable
+            setattr(self, "__call__", self._callable)
             # access the list of implementations to freeze them
             assert self.implementations
         return self._callable
@@ -219,7 +226,15 @@ def measure[M](
         }
         try:
             # try to sort the dictionary by the measurements
-            return dict(sorted(result.items(), key=lambda x: x[1]))
+            return dict(
+                sorted(
+                    result.items(),
+                    key=cast(
+                        Callable[[tuple[Implementation[P, R], M]], _SupportsLessThan],
+                        lambda x: cast(_SupportsLessThan, x[1]),
+                    ),
+                )
+            )
         except TypeError:
             return result
 
@@ -252,10 +267,10 @@ def pytest_parametrize(
 
         if isinstance(test, _UNDEFINED):
 
-            def inner(f: Callable):
+            def decorator(f: Callable):
                 return self.pytest_parametrize(f, only_default=only_default)
 
-            return inner
+            return decorator
 
         implementations = self._select_parametrize_implementations(
             only_default=only_default
@@ -309,7 +324,7 @@ def pytest_parametrize_pairs(
 
         if isinstance(test, _UNDEFINED):
 
-            def inner(f: Callable):
+            def decorator(f: Callable):
                 return self.pytest_parametrize_pairs(
                     f,
                     n_cache=n_cache,
@@ -317,7 +332,7 @@ def inner(f: Callable):
                     only_default=only_default,
                 )
 
-            return inner
+            return decorator
 
         reference_implementation = lru_cache(maxsize=n_cache)(
             self.reference.implementation
@@ -390,7 +405,7 @@ def __repr__(self) -> str:
         return f"Implementation({implementation_name})"
 
     def __call__(self, *args: P.args, **kwargs: P.kwargs) -> R:
-        self.__call__ = self.implementation
+        setattr(self, "__call__", self.implementation)
         return self.__call__(*args, **kwargs)
 
     @overload
@@ -412,18 +427,20 @@ def add(
 @overload
 def reference[**P, R](
     implementation: _UNDEFINED = _UNDEFINED_VALUE, *, default: bool = False
-) -> AlternativesWrapper[P, R]: ...
+) -> Callable[[Callable[P, R]], Alternatives[P, R]]: ...
 
 
 @overload
-def reference[**P, R](
-    implementation: ImplementationSig[P, R], *, default: bool = False
+def reference[**P, R](  # pyrefly: ignore[inconsistent-overload]
+    implementation: Callable[P, R], *, default: bool = False
 ) -> Alternatives[P, R]: ...
 
 
 def reference[**P, R](
-    implementation=_UNDEFINED_VALUE, *, default=False
-) -> Alternatives[P, R] | AlternativesWrapper[P, R]:
+    implementation=_UNDEFINED_VALUE,
+    *,
+    default=False,
+) -> Alternatives[P, R] | Callable[[Callable[P, R]], Alternatives[P, R]]:
     if isinstance(implementation, _UNDEFINED):
 
         def inner(f: Callable[P, R]) -> Alternatives[P, R]:
diff --git a/pyproject.toml b/pyproject.toml
index c5dae07..fa64e2a 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -31,3 +31,11 @@ build-backend = "hatchling.build"
 [tool.pytest.ini_options]
 # do 5 rounds of 0.01 benchmarks, as the benchmarks are examples or very fast
 addopts = "--cov=alternative --cov-report=html --benchmark-max-time=0.01"
+
+
+[tool.mypy]
+python_version = "3.12"
+
+[[tool.mypy.overrides]]
+module = ["pytest"]
+ignore_missing_imports = true