[ty] correctly ignore field specifiers when not specified (#20002)

This commit corrects the type checker's behavior when handling
`dataclass_transform` decorators that don't explicitly specify
`field_specifiers`. According to [PEP 681 (Data Class
Transforms)](https://peps.python.org/pep-0681/#dataclass-transform-parameters),
when `field_specifiers` is not provided, it defaults to an empty tuple,
meaning no field specifiers are supported and
`dataclasses.field`/`dataclasses.Field` calls should be ignored.

Fixes astral-sh/ty#980

Author: leandrobbraga

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

@@ -84,3 +84,47 @@ reveal_type(field(default=1))  # revealed: dataclasses.Field[Literal[1]]
 reveal_type(field(default=None))  # revealed: dataclasses.Field[None]
 reveal_type(field(default_factory=get_default))  # revealed: dataclasses.Field[str]
 ```
+
+## dataclass_transform field_specifiers
+
+If `field_specifiers` is not specified, it defaults to an empty tuple, meaning no field specifiers
+are supported and `dataclasses.field` and `dataclasses.Field` should not be accepted by default.
+
+```py
+from typing_extensions import dataclass_transform
+from dataclasses import field, dataclass
+from typing import TypeVar
+
+T = TypeVar("T")
+
+@dataclass_transform()
+def create_model(*, init: bool = True):
+    def deco(cls: type[T]) -> type[T]:
+        return cls
+    return deco
+
+@create_model()
+class A:
+    name: str = field(init=False)
+
+# field(init=False) should be ignored for dataclass_transform without explicit field_specifiers
+reveal_type(A.__init__)  # revealed: (self: A, name: str = Unknown) -> None
+
+@dataclass
+class B:
+    name: str = field(init=False)
+
+# Regular @dataclass should respect field(init=False)
+reveal_type(B.__init__)  # revealed: (self: B) -> None
+```
+
+Test constructor calls:
+
+```py
+# This should NOT error because field(init=False) is ignored for A
+A(name="foo")
+
+# This should error because field(init=False) is respected for B
+# error: [unknown-argument]
+B(name="foo")
+```

crates/ty_python_semantic/src/types.rs

@@ -510,6 +510,10 @@ bitflags! {
         const KW_ONLY = 0b0000_1000_0000;
         const SLOTS = 0b0001_0000_0000;
         const WEAKREF_SLOT = 0b0010_0000_0000;
+        // This is not an actual argument from `dataclass(...)` but a flag signaling that no
+        // `field_specifiers` was specified for the `dataclass_transform`, see [1].
+        // [1]: https://typing.python.org/en/latest/spec/dataclasses.html#dataclass-transform-parameters
+        const NO_FIELD_SPECIFIERS = 0b0100_0000_0000;
     }
 }
 
@@ -542,6 +546,11 @@ impl From<DataclassTransformerParams> for DataclassParams {
             params.contains(DataclassTransformerParams::FROZEN_DEFAULT),
         );
 
+        result.set(
+            Self::NO_FIELD_SPECIFIERS,
+            !params.contains(DataclassTransformerParams::FIELD_SPECIFIERS),
+        );
+
         result
     }
 }

crates/ty_python_semantic/src/types/call/bind.rs

@@ -900,7 +900,7 @@ impl<'db> Bindings<'db> {
                                 order_default,
                                 kw_only_default,
                                 frozen_default,
-                                _field_specifiers,
+                                field_specifiers,
                                 _kwargs,
                             ] = overload.parameter_types()
                             {
@@ -919,6 +919,16 @@ impl<'db> Bindings<'db> {
                                     params |= DataclassTransformerParams::FROZEN_DEFAULT;
                                 }
 
+                                if let Some(field_specifiers_type) = field_specifiers {
+                                    // For now, we'll do a simple check: if field_specifiers is not
+                                    // None/empty, we assume it might contain dataclasses.field
+                                    // TODO: Implement proper parsing to check for
+                                    //   dataclasses.field/Field specifically
+                                    if !field_specifiers_type.is_none(db) {
+                                        params |= DataclassTransformerParams::FIELD_SPECIFIERS;
+                                    }
+                                }
+
                                 overload.set_return_type(Type::DataclassTransformer(params));
                             }
                         }

crates/ty_python_semantic/src/types/class.rs

@@ -2407,8 +2407,18 @@ impl<'db> ClassLiteral<'db> {
                 let mut kw_only = None;
                 if let Some(Type::KnownInstance(KnownInstanceType::Field(field))) = default_ty {
                     default_ty = Some(field.default_type(db));
-                    init = field.init(db);
-                    kw_only = field.kw_only(db);
+                    if self
+                        .dataclass_params(db)
+                        .map(|params| params.contains(DataclassParams::NO_FIELD_SPECIFIERS))
+                        .unwrap_or(false)
+                    {
+                        // This happens when constructing a `dataclass` with a `dataclass_transform`
+                        // without defining the `field_specifiers`, meaning it should ignore
+                        // `dataclasses.field` and `dataclasses.Field`.
+                    } else {
+                        init = field.init(db);
+                        kw_only = field.kw_only(db);
+                    }
                 }
 
                 let mut field = Field {

crates/ty_python_semantic/src/types/function.rs

@@ -164,6 +164,7 @@ bitflags! {
         const ORDER_DEFAULT = 1 << 1;
         const KW_ONLY_DEFAULT = 1 << 2;
         const FROZEN_DEFAULT = 1 << 3;
+        const FIELD_SPECIFIERS= 1 << 4;
     }
 }