[ty] Extend Liskov checks to also cover classmethods and staticmethods (#21598)
## Summary Building on https://github.com/astral-sh/ruff/pull/21436. There's nothing conceptually more complicated about this, it just requires its own set of tests and its own subdiagnostic hint. I also uncovered another inconsistency between mypy/pyright/pyrefly, which is fun. In this case, I suggest we go with pyright's behaviour. ## Test Plan mdtests/snapshots
This commit is contained in:
Alex Waygood
@@ -523,3 +523,63 @@ class Baz(NamedTuple):
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class Spam(Baz):
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def _asdict(self) -> tuple[int, ...]: ... # error: [invalid-method-override]
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```
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## Staticmethods and classmethods
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Methods decorated with `@staticmethod` or `@classmethod` are checked in much the same way as other
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methods.
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<!-- snapshot-diagnostics -->
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```pyi
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class Parent:
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def instance_method(self, x: int) -> int: ...
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@classmethod
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def class_method(cls, x: int) -> int: ...
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@staticmethod
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def static_method(x: int) -> int: ...
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class BadChild1(Parent):
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@staticmethod
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def instance_method(self, x: int) -> int: ... # error: [invalid-method-override]
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# TODO: we should emit `invalid-method-override` here.
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# Although the method has the same signature as `Parent.class_method`
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# when accessed on instances, it does not have the same signature as
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# `Parent.class_method` when accessed on the class object itself
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def class_method(cls, x: int) -> int: ...
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def static_method(x: int) -> int: ... # error: [invalid-method-override]
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class BadChild2(Parent):
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# TODO: we should emit `invalid-method-override` here.
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# Although the method has the same signature as `Parent.class_method`
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# when accessed on instances, it does not have the same signature as
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# `Parent.class_method` when accessed on the class object itself.
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#
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# Note that whereas `BadChild1.class_method` is reported as a Liskov violation by
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# mypy, pyright and pyrefly, pyright is the only one of those three to report a
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# Liskov violation on this method as of 2025-11-23.
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@classmethod
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def instance_method(self, x: int) -> int: ...
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@staticmethod
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def class_method(cls, x: int) -> int: ... # error: [invalid-method-override]
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@classmethod
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def static_method(x: int) -> int: ... # error: [invalid-method-override]
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class BadChild3(Parent):
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@classmethod
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def class_method(cls, x: bool) -> object: ... # error: [invalid-method-override]
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@staticmethod
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def static_method(x: bool) -> object: ... # error: [invalid-method-override]
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class GoodChild1(Parent):
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@classmethod
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def class_method(cls, x: int) -> int: ...
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@staticmethod
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def static_method(x: int) -> int: ...
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class GoodChild2(Parent):
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@classmethod
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def class_method(cls, x: object) -> bool: ...
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@staticmethod
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def static_method(x: object) -> bool: ...
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```
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@@ -0,0 +1,220 @@
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---
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source: crates/ty_test/src/lib.rs
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expression: snapshot
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---
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---
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mdtest name: liskov.md - The Liskov Substitution Principle - Staticmethods and classmethods
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mdtest path: crates/ty_python_semantic/resources/mdtest/liskov.md
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---
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# Python source files
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## mdtest_snippet.pyi
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```
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1 | class Parent:
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2 | def instance_method(self, x: int) -> int: ...
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3 | @classmethod
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4 | def class_method(cls, x: int) -> int: ...
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5 | @staticmethod
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6 | def static_method(x: int) -> int: ...
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7 |
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8 | class BadChild1(Parent):
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9 | @staticmethod
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10 | def instance_method(self, x: int) -> int: ... # error: [invalid-method-override]
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11 | # TODO: we should emit `invalid-method-override` here.
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12 | # Although the method has the same signature as `Parent.class_method`
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13 | # when accessed on instances, it does not have the same signature as
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14 | # `Parent.class_method` when accessed on the class object itself
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15 | def class_method(cls, x: int) -> int: ...
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16 | def static_method(x: int) -> int: ... # error: [invalid-method-override]
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17 |
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18 | class BadChild2(Parent):
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19 | # TODO: we should emit `invalid-method-override` here.
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20 | # Although the method has the same signature as `Parent.class_method`
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21 | # when accessed on instances, it does not have the same signature as
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22 | # `Parent.class_method` when accessed on the class object itself.
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23 | #
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24 | # Note that whereas `BadChild1.class_method` is reported as a Liskov violation by
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25 | # mypy, pyright and pyrefly, pyright is the only one of those three to report a
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26 | # Liskov violation on this method as of 2025-11-23.
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27 | @classmethod
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28 | def instance_method(self, x: int) -> int: ...
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29 | @staticmethod
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30 | def class_method(cls, x: int) -> int: ... # error: [invalid-method-override]
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31 | @classmethod
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32 | def static_method(x: int) -> int: ... # error: [invalid-method-override]
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33 |
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34 | class BadChild3(Parent):
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35 | @classmethod
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36 | def class_method(cls, x: bool) -> object: ... # error: [invalid-method-override]
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37 | @staticmethod
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38 | def static_method(x: bool) -> object: ... # error: [invalid-method-override]
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39 |
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40 | class GoodChild1(Parent):
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41 | @classmethod
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42 | def class_method(cls, x: int) -> int: ...
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43 | @staticmethod
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44 | def static_method(x: int) -> int: ...
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45 |
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46 | class GoodChild2(Parent):
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47 | @classmethod
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48 | def class_method(cls, x: object) -> bool: ...
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49 | @staticmethod
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50 | def static_method(x: object) -> bool: ...
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```
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# Diagnostics
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```
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error[invalid-method-override]: Invalid override of method `instance_method`
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--> src/mdtest_snippet.pyi:10:9
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8 | class BadChild1(Parent):
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9 | @staticmethod
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10 | def instance_method(self, x: int) -> int: ... # error: [invalid-method-override]
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Parent.instance_method`
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11 | # TODO: we should emit `invalid-method-override` here.
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12 | # Although the method has the same signature as `Parent.class_method`
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::: src/mdtest_snippet.pyi:2:9
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1 | class Parent:
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2 | def instance_method(self, x: int) -> int: ...
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| ------------------------------------ `Parent.instance_method` defined here
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3 | @classmethod
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4 | def class_method(cls, x: int) -> int: ...
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info: `BadChild1.instance_method` is a staticmethod but `Parent.instance_method` is an instance method
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info: This violates the Liskov Substitution Principle
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info: rule `invalid-method-override` is enabled by default
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```
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```
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error[invalid-method-override]: Invalid override of method `static_method`
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--> src/mdtest_snippet.pyi:16:9
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14 | # `Parent.class_method` when accessed on the class object itself
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15 | def class_method(cls, x: int) -> int: ...
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16 | def static_method(x: int) -> int: ... # error: [invalid-method-override]
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Parent.static_method`
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17 |
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18 | class BadChild2(Parent):
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::: src/mdtest_snippet.pyi:6:9
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4 | def class_method(cls, x: int) -> int: ...
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5 | @staticmethod
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6 | def static_method(x: int) -> int: ...
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| ---------------------------- `Parent.static_method` defined here
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7 |
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8 | class BadChild1(Parent):
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info: `BadChild1.static_method` is an instance method but `Parent.static_method` is a staticmethod
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info: This violates the Liskov Substitution Principle
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info: rule `invalid-method-override` is enabled by default
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```
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```
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error[invalid-method-override]: Invalid override of method `class_method`
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--> src/mdtest_snippet.pyi:30:9
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28 | def instance_method(self, x: int) -> int: ...
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29 | @staticmethod
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30 | def class_method(cls, x: int) -> int: ... # error: [invalid-method-override]
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Parent.class_method`
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31 | @classmethod
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32 | def static_method(x: int) -> int: ... # error: [invalid-method-override]
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::: src/mdtest_snippet.pyi:4:9
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2 | def instance_method(self, x: int) -> int: ...
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3 | @classmethod
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4 | def class_method(cls, x: int) -> int: ...
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| -------------------------------- `Parent.class_method` defined here
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5 | @staticmethod
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6 | def static_method(x: int) -> int: ...
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info: `BadChild2.class_method` is a staticmethod but `Parent.class_method` is a classmethod
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info: This violates the Liskov Substitution Principle
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info: rule `invalid-method-override` is enabled by default
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```
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```
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error[invalid-method-override]: Invalid override of method `static_method`
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--> src/mdtest_snippet.pyi:32:9
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30 | def class_method(cls, x: int) -> int: ... # error: [invalid-method-override]
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31 | @classmethod
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32 | def static_method(x: int) -> int: ... # error: [invalid-method-override]
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Parent.static_method`
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33 |
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34 | class BadChild3(Parent):
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::: src/mdtest_snippet.pyi:6:9
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4 | def class_method(cls, x: int) -> int: ...
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5 | @staticmethod
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6 | def static_method(x: int) -> int: ...
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| ---------------------------- `Parent.static_method` defined here
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7 |
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8 | class BadChild1(Parent):
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info: `BadChild2.static_method` is a classmethod but `Parent.static_method` is a staticmethod
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info: This violates the Liskov Substitution Principle
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info: rule `invalid-method-override` is enabled by default
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```
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```
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error[invalid-method-override]: Invalid override of method `class_method`
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--> src/mdtest_snippet.pyi:36:9
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34 | class BadChild3(Parent):
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35 | @classmethod
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36 | def class_method(cls, x: bool) -> object: ... # error: [invalid-method-override]
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Parent.class_method`
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37 | @staticmethod
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38 | def static_method(x: bool) -> object: ... # error: [invalid-method-override]
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::: src/mdtest_snippet.pyi:4:9
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2 | def instance_method(self, x: int) -> int: ...
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3 | @classmethod
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4 | def class_method(cls, x: int) -> int: ...
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| -------------------------------- `Parent.class_method` defined here
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5 | @staticmethod
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6 | def static_method(x: int) -> int: ...
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info: This violates the Liskov Substitution Principle
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info: rule `invalid-method-override` is enabled by default
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```
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```
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error[invalid-method-override]: Invalid override of method `static_method`
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--> src/mdtest_snippet.pyi:38:9
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36 | def class_method(cls, x: bool) -> object: ... # error: [invalid-method-override]
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37 | @staticmethod
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38 | def static_method(x: bool) -> object: ... # error: [invalid-method-override]
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| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Definition is incompatible with `Parent.static_method`
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39 |
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40 | class GoodChild1(Parent):
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::: src/mdtest_snippet.pyi:6:9
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|
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4 | def class_method(cls, x: int) -> int: ...
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5 | @staticmethod
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6 | def static_method(x: int) -> int: ...
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| ---------------------------- `Parent.static_method` defined here
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7 |
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8 | class BadChild1(Parent):
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info: This violates the Liskov Substitution Principle
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info: rule `invalid-method-override` is enabled by default
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```
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