[ty] Emit a diagnostic when frozen dataclass inherits a non-frozen dataclass and the other way around

crates/ty_python_semantic/resources/mdtest/dataclasses/dataclasses.md

@@ -521,6 +521,73 @@ frozen = MyFrozenChildClass()
 del frozen.x  # TODO this should emit an [invalid-assignment]
 ```
 
+### frozen/non-frozen inheritance
+
+If a non-frozen dataclass inherits from a frozen dataclass, an exception is raised at runtime. We
+catch this error:
+
+<!-- snapshot-diagnostics -->
+
+`a.py`:
+
+```py
+from dataclasses import dataclass
+
+@dataclass(frozen=True)
+class FrozenBase:
+    x: int
+
+@dataclass
+# error: [invalid-frozen-dataclass-subclass] "Non-frozen dataclass `Child` cannot inherit from frozen dataclass `FrozenBase`"
+class Child(FrozenBase):
+    y: int
+```
+
+Frozen dataclasses inheriting from non-frozen dataclasses are also illegal:
+
+`b.py`:
+
+```py
+from dataclasses import dataclass
+
+@dataclass
+class Base:
+    x: int
+
+@dataclass(frozen=True)
+# error: [invalid-frozen-dataclass-subclass] "Frozen dataclass `FrozenChild` cannot inherit from non-frozen dataclass `Base`"
+class FrozenChild(Base):
+    y: int
+```
+
+Example of diagnostics when there are multiple files involved:
+
+`module.py`:
+
+```py
+import dataclasses
+
+@dataclasses.dataclass(frozen=False)
+class NotFrozenBase:
+    x: int
+```
+
+`main.py`:
+
+```py
+from functools import total_ordering
+from typing import final
+from dataclasses import dataclass
+
+from module import NotFrozenBase
+
+@final
+@dataclass(frozen=True)
+@total_ordering
+class FrozenChild(NotFrozenBase):  # error: [invalid-frozen-dataclass-subclass]
+    y: str
+```
+
 ### `match_args`
 
 If `match_args` is set to `True` (the default), the `__match_args__` attribute is a tuple created

crates/ty_python_semantic/resources/mdtest/snapshots/dataclasses.md_-_Dataclasses_-_Other_dataclass_para…_-_frozen__non-frozen_in…_(9af2ab07b8e829e).snap

@@ -0,0 +1,154 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: dataclasses.md - Dataclasses - Other dataclass parameters - frozen/non-frozen inheritance
+mdtest path: crates/ty_python_semantic/resources/mdtest/dataclasses/dataclasses.md
+---
+
+# Python source files
+
+## a.py
+
+```
+ 1 | from dataclasses import dataclass
+ 2 | 
+ 3 | @dataclass(frozen=True)
+ 4 | class FrozenBase:
+ 5 |     x: int
+ 6 | 
+ 7 | @dataclass
+ 8 | # error: [invalid-frozen-dataclass-subclass] "Non-frozen dataclass `Child` cannot inherit from frozen dataclass `FrozenBase`"
+ 9 | class Child(FrozenBase):
+10 |     y: int
+```
+
+## b.py
+
+```
+ 1 | from dataclasses import dataclass
+ 2 | 
+ 3 | @dataclass
+ 4 | class Base:
+ 5 |     x: int
+ 6 | 
+ 7 | @dataclass(frozen=True)
+ 8 | # error: [invalid-frozen-dataclass-subclass] "Frozen dataclass `FrozenChild` cannot inherit from non-frozen dataclass `Base`"
+ 9 | class FrozenChild(Base):
+10 |     y: int
+```
+
+## module.py
+
+```
+1 | import dataclasses
+2 | 
+3 | @dataclasses.dataclass(frozen=False)
+4 | class NotFrozenBase:
+5 |     x: int
+```
+
+## main.py
+
+```
+ 1 | from functools import total_ordering
+ 2 | from typing import final
+ 3 | from dataclasses import dataclass
+ 4 | 
+ 5 | from module import NotFrozenBase
+ 6 | 
+ 7 | @final
+ 8 | @dataclass(frozen=True)
+ 9 | @total_ordering
+10 | class FrozenChild(NotFrozenBase):  # error: [invalid-frozen-dataclass-subclass]
+11 |     y: str
+```
+
+# Diagnostics
+
+```
+error[invalid-frozen-dataclass-subclass]: Non-frozen dataclass cannot inherit from frozen dataclass
+  --> src/a.py:7:1
+   |
+ 5 |     x: int
+ 6 |
+ 7 | @dataclass
+   | ---------- `Child` dataclass parameters
+ 8 | # error: [invalid-frozen-dataclass-subclass] "Non-frozen dataclass `Child` cannot inherit from frozen dataclass `FrozenBase`"
+ 9 | class Child(FrozenBase):
+   |       ^^^^^^----------^ Subclass `Child` is not frozen but base class `FrozenBase` is
+10 |     y: int
+   |
+info: This causes the class creation to fail
+info: Base class definition
+ --> src/a.py:3:1
+  |
+1 | from dataclasses import dataclass
+2 |
+3 | @dataclass(frozen=True)
+  | ----------------------- `FrozenBase` dataclass parameters
+4 | class FrozenBase:
+  |       ^^^^^^^^^^ `FrozenBase` definition
+5 |     x: int
+  |
+info: rule `invalid-frozen-dataclass-subclass` is enabled by default
+
+```
+
+```
+error[invalid-frozen-dataclass-subclass]: Frozen dataclass cannot inherit from non-frozen dataclass
+  --> src/b.py:7:1
+   |
+ 5 |     x: int
+ 6 |
+ 7 | @dataclass(frozen=True)
+   | ----------------------- `FrozenChild` dataclass parameters
+ 8 | # error: [invalid-frozen-dataclass-subclass] "Frozen dataclass `FrozenChild` cannot inherit from non-frozen dataclass `Base`"
+ 9 | class FrozenChild(Base):
+   |       ^^^^^^^^^^^^----^ Subclass `FrozenChild` is frozen but base class `Base` is not
+10 |     y: int
+   |
+info: This causes the class creation to fail
+info: Base class definition
+ --> src/b.py:3:1
+  |
+1 | from dataclasses import dataclass
+2 |
+3 | @dataclass
+  | ---------- `Base` dataclass parameters
+4 | class Base:
+  |       ^^^^ `Base` definition
+5 |     x: int
+  |
+info: rule `invalid-frozen-dataclass-subclass` is enabled by default
+
+```
+
+```
+error[invalid-frozen-dataclass-subclass]: Frozen dataclass cannot inherit from non-frozen dataclass
+  --> src/main.py:8:1
+   |
+ 7 | @final
+ 8 | @dataclass(frozen=True)
+   | ----------------------- `FrozenChild` dataclass parameters
+ 9 | @total_ordering
+10 | class FrozenChild(NotFrozenBase):  # error: [invalid-frozen-dataclass-subclass]
+   |       ^^^^^^^^^^^^-------------^ Subclass `FrozenChild` is frozen but base class `NotFrozenBase` is not
+11 |     y: str
+   |
+info: This causes the class creation to fail
+info: Base class definition
+ --> src/module.py:3:1
+  |
+1 | import dataclasses
+2 |
+3 | @dataclasses.dataclass(frozen=False)
+  | ------------------------------------ `NotFrozenBase` dataclass parameters
+4 | class NotFrozenBase:
+  |       ^^^^^^^^^^^^^ `NotFrozenBase` definition
+5 |     x: int
+  |
+info: rule `invalid-frozen-dataclass-subclass` is enabled by default
+
+```

crates/ty_python_semantic/src/types.rs

@@ -677,6 +677,12 @@ bitflags! {
     }
 }
 
+impl DataclassFlags {
+    pub(crate) const fn is_frozen(self) -> bool {
+        self.contains(Self::FROZEN)
+    }
+}
+
 pub(crate) const DATACLASS_FLAGS: &[(&str, DataclassFlags)] = &[
     ("init", DataclassFlags::INIT),
     ("repr", DataclassFlags::REPR),

crates/ty_python_semantic/src/types/class.rs

@@ -1856,6 +1856,28 @@ impl<'db> ClassLiteral<'db> {
             .filter_map(|decorator| decorator.known(db))
     }
 
+    /// Iterate through the decorators on this class, returning the position of the first one
+    /// that matches the given predicate.
+    pub(super) fn find_decorator_position(
+        self,
+        db: &'db dyn Db,
+        predicate: impl Fn(Type<'db>) -> bool,
+    ) -> Option<usize> {
+        self.decorators(db)
+            .iter()
+            .position(|decorator| predicate(*decorator))
+    }
+
+    /// Iterate through the decorators on this class, returning the index of the first one
+    /// that is either `@dataclass` or `@dataclass(...)`.
+    pub(super) fn find_dataclass_decorator_position(self, db: &'db dyn Db) -> Option<usize> {
+        self.find_decorator_position(db, |ty| match ty {
+            Type::FunctionLiteral(function) => function.is_known(db, KnownFunction::Dataclass),
+            Type::DataclassDecorator(_) => true,
+            _ => false,
+        })
+    }
+
     /// Is this class final?
     pub(super) fn is_final(self, db: &'db dyn Db) -> bool {
         self.known_function_decorators(db)