Report func-returns-value for decorated methods

[jbbqqf]

Summary

Fixes #14179. Decorated functions — @staticmethod, @classmethod, and user-defined decorators that preserve a -> None return — were silently skipped by the [func-returns-value] check while their undecorated counterparts were not. The check is now consistent across all of them.

Why

ExpressionChecker.defn_returns_none handled FuncDef, OverloadedFuncDef, and Var, but not Decorator. A @staticmethod becomes a Decorator node whose synthetic var.is_inferred is True, so the existing Var branch deliberately skipped it. The function's -> None annotation is still propagated into decorator.var.type as a CallableType, so we can reuse it directly without losing precision for decorators that change the return type (e.g. one that turns () -> None into () -> int is correctly not flagged — verified in the test suite and ad-hoc).

What changed

• mypy/checkexpr.py: add a Decorator branch to defn_returns_none that inspects decorator.var.type (the type the call site actually sees).
• test-data/unit/check-errorcodes.test: new testErrorCodeFuncReturnsValueStaticAndClassMethod covering instance, static, class methods on both an instance and the class itself.

37 lines added, 0 removed across 2 files.

Reproduce BEFORE/AFTER yourself (copy-paste)

# Set up
git clone https://github.com/python/mypy.git /tmp/mypy-bisect && cd /tmp/mypy-bisect
git remote add fork https://github.com/jbbqqf/mypy.git && git fetch fork
python -m venv .venv && . .venv/bin/activate
pip install -e . >/dev/null

cat > /tmp/repro_14179.py <<'PY'
class Example:
    def instance_op(self) -> None: return None
    @staticmethod
    def static_op() -> None: return None
    @classmethod
    def class_op(cls) -> None: return None

ex = Example()
a = ex.instance_op()
b = ex.static_op()
c = ex.class_op()
PY

# BEFORE — only the instance method is flagged (the bug)
git checkout master >/dev/null 2>&1
pip install -e . --quiet
python -m mypy /tmp/repro_14179.py || true
# Expected: 1 error on instance_op only (static_op and class_op missed)

# AFTER — all three methods flagged consistently
git checkout fork/fix/14179-staticmethod-func-returns-value >/dev/null 2>&1
pip install -e . --quiet
python -m mypy /tmp/repro_14179.py || true
# Expected: 3 errors, one on each of instance_op / static_op / class_op

What I ran locally

$ pytest mypy/test/testcheck.py -x -k "testErrorCodeFuncReturnsValueStaticAndClassMethod"
12 workers [1 item]
.                                                                        [100%]
============================== 1 passed in 1.29s ===============================

$ pytest mypy/test/testcheck.py -x
================= 8035 passed, 26 skipped, 7 xfailed in 36.84s =================

The new regression test fails verbatim on origin/master:

Failed: Unexpected type checker output ... line 685
Expected:
  main:18: error: "instance_op" of "Example" does not return a value ...
  main:19: error: "static_op" of "Example" does not return a value ...  (diff)
  main:20: error: "class_op" of "Example" does not return a value ...   (diff)
  ...
Actual:
  main:18: error: "instance_op" of "Example" does not return a value ...

Edge cases considered

Case	Behavior	Verified?
@staticmethod def f() -> None	flagged	yes (new test)
@classmethod def f(cls) -> None	flagged	yes (new test)
Plain instance method -> None	flagged (unchanged)	yes (new test)
Custom decorator preserving () -> None via TypeVar bound to Callable	flagged	yes (ad-hoc)
Custom decorator returning Callable[..., int] wrapping a -> None function	NOT flagged (correct)	yes (ad-hoc)
@staticmethod def f() -> int	NOT flagged (correct)	yes (ad-hoc)
Existing @property, @overload, @abstractmethod tests	unchanged	yes (378 decorator-related tests still pass)

Notes

• The fix is intentionally tight: it routes the Decorator case through the same return-type check the other branches use, rather than e.g. peering at decorator.func.type (which would over-trigger for decorators that genuinely change the return type).
• Added a short code comment with the issue link so a future reviewer reading defn_returns_none cold doesn't have to rediscover why Decorator needs special handling.

---

🤖 Disclosure: I'm a data engineer; this PR was prepared with help from an AI assistant (Claude) — I reviewed every change, wrote the regression test first, and validated locally on the full testcheck.py suite before pushing. Happy to iterate on review feedback.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

spark (https://github.com/apache/spark)
+ python/pyspark/sql/profiler.py:80: error: "zero" of "PStatsParam" does not return a value (it only ever returns None)  [func-returns-value]
+ python/pyspark/sql/profiler.py:80: error: "zero" of "MemUsageParam" does not return a value (it only ever returns None)  [func-returns-value]

colour (https://github.com/colour-science/colour)
+ colour/colorimetry/tests/test_tristimulus_values.py:1716: error: "assert_allclose" does not return a value (it only ever returns None)  [func-returns-value]
+ colour/colorimetry/tests/test_tristimulus_values.py:1730: error: "assert_allclose" does not return a value (it only ever returns None)  [func-returns-value]
+ colour/colorimetry/tests/test_tristimulus_values.py:1765: error: "assert_allclose" does not return a value (it only ever returns None)  [func-returns-value]
+ colour/colorimetry/tests/test_tristimulus_values.py:1779: error: "assert_allclose" does not return a value (it only ever returns None)  [func-returns-value]

zulip (https://github.com/zulip/zulip)
+ zerver/tests/test_message_fetch.py:2416: error: "sort" of "list" does not return a value (it only ever returns None)  [func-returns-value]
+ zerver/tests/test_message_fetch.py:2436: error: "sort" of "list" does not return a value (it only ever returns None)  [func-returns-value]

egglog-python (https://github.com/egraphs-good/egglog-python)
+ python/egglog/exp/any_expr.py:665: error: "setitem" does not return a value (it only ever returns None)  [func-returns-value]
+ python/egglog/exp/any_expr.py:668: error: "delitem" does not return a value (it only ever returns None)  [func-returns-value]

pandera (https://github.com/pandera-dev/pandera)
+ tests/pandas/test_decorators.py:756: error: "optional_in" does not return a value (it only ever returns None)  [func-returns-value]

pandas (https://github.com/pandas-dev/pandas)
+ pandas/core/series.py:3978: error: "_update_inplace" of "NDFrame" does not return a value (it only ever returns None)  [func-returns-value]
+ pandas/core/series.py:6806: error: "_update_inplace" of "NDFrame" does not return a value (it only ever returns None)  [func-returns-value]
+ pandas/core/frame.py:8338: error: "_update_inplace" of "NDFrame" does not return a value (it only ever returns None)  [func-returns-value]
+ pandas/core/frame.py:8348: error: "_update_inplace" of "NDFrame" does not return a value (it only ever returns None)  [func-returns-value]
+ pandas/core/frame.py:8363: error: "_update_inplace" of "NDFrame" does not return a value (it only ever returns None)  [func-returns-value]

jax (https://github.com/google/jax)
+ jax/_src/environment_info.py:63: error: "print" does not return a value (it only ever returns None)  [func-returns-value]