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[ty] Fix generic inference for non-dataclass inheriting from generic dataclass #21159

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merged 2 commits into from
Oct 31, 2025
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[ty] Fix generic inference for non-dataclass inheriting from generic dataclass #21159

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3dcafb9
[ty] Fix generic inference for non-dataclass inheriting from generic …
saada Oct 31, 2025
22c2c82
Fix test, remove one TODO
sharkdp Oct 31, 2025
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34 changes: 34 additions & 0 deletions crates/ty_python_semantic/resources/mdtest/dataclasses/dataclasses.md
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Original file line number Diff line number Diff line change
Expand Up @@ -838,6 +838,40 @@ class WrappedIntAndExtraData[T](Wrap[int]):
reveal_type(WrappedIntAndExtraData[bytes].__init__)
```

### Non-dataclass inheriting from generic dataclass

This is a regression test for <https://github.com/astral-sh/ty/issues/1427>.

When a non-dataclass inherits from a generic dataclass, the generic type parameters should still be
properly inferred when calling the inherited `__init__` method.

```py
from dataclasses import dataclass

@dataclass
class ParentDataclass[T]:
value: T

# Non-dataclass inheriting from generic dataclass
class ChildOfParentDataclass[T](ParentDataclass[T]): ...

def uses_dataclass[T](x: T) -> ChildOfParentDataclass[T]:
return ChildOfParentDataclass(x)

# TODO: ParentDataclass.__init__ should show generic types, not Unknown
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@sharkdp sharkdp Oct 31, 2025

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Thank you for highlighting this. I opened astral-sh/ty#1461 to track this.

# revealed: (self: ParentDataclass[Unknown], value: Unknown) -> None
reveal_type(ParentDataclass.__init__)

# revealed: (self: ParentDataclass[T@ChildOfParentDataclass], value: T@ChildOfParentDataclass) -> None
reveal_type(ChildOfParentDataclass.__init__)

result_int = uses_dataclass(42)
reveal_type(result_int) # revealed: ChildOfParentDataclass[Literal[42]]

result_str = uses_dataclass("hello")
reveal_type(result_str) # revealed: ChildOfParentDataclass[Literal["hello"]]
```

## Descriptor-typed fields

### Same type in `__get__` and `__set__`
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8 changes: 5 additions & 3 deletions crates/ty_python_semantic/src/types/class.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2176,7 +2176,8 @@ impl<'db> ClassLiteral<'db> {
});

if member.is_undefined() {
if let Some(synthesized_member) = self.own_synthesized_member(db, specialization, name)
if let Some(synthesized_member) =
self.own_synthesized_member(db, specialization, inherited_generic_context, name)
{
return Member::definitely_declared(synthesized_member);
}
Expand All @@ -2192,6 +2193,7 @@ impl<'db> ClassLiteral<'db> {
self,
db: &'db dyn Db,
specialization: Option<Specialization<'db>>,
inherited_generic_context: Option<GenericContext<'db>>,
name: &str,
) -> Option<Type<'db>> {
let dataclass_params = self.dataclass_params(db);
Expand Down Expand Up @@ -2320,7 +2322,7 @@ impl<'db> ClassLiteral<'db> {

let signature = match name {
"__new__" | "__init__" => Signature::new_generic(
self.inherited_generic_context(db),
inherited_generic_context.or_else(|| self.inherited_generic_context(db)),
Parameters::new(parameters),
return_ty,
),
Expand Down Expand Up @@ -2702,7 +2704,7 @@ impl<'db> ClassLiteral<'db> {
name: &str,
policy: MemberLookupPolicy,
) -> PlaceAndQualifiers<'db> {
if let Some(member) = self.own_synthesized_member(db, specialization, name) {
if let Some(member) = self.own_synthesized_member(db, specialization, None, name) {
Place::bound(member).into()
} else {
KnownClass::TypedDictFallback
Expand Down
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