[ty] Do not assume that fields have a default value (#20914)

## Summary

fixes https://github.com/astral-sh/ty/issues/1366

## Test Plan

Added regression test

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

@@ -497,6 +497,8 @@ class A:
     a: str = field(kw_only=False)
     b: int = 0
 
+reveal_type(A.__init__)  # revealed: (self: A, a: str, *, b: int = Literal[0]) -> None
+
 A("hi")
 ```
 

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

@@ -108,7 +108,7 @@ 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
+reveal_type(A.__init__)  # revealed: (self: A, name: str) -> None
 
 @dataclass
 class B:

crates/ty_python_semantic/src/types.rs

@@ -1636,14 +1636,16 @@ impl<'db> Type<'db> {
             (Type::KnownInstance(KnownInstanceType::Field(field)), right)
                 if relation.is_assignability() =>
             {
-                field.default_type(db).has_relation_to_impl(
-                    db,
-                    right,
-                    inferable,
-                    relation,
-                    relation_visitor,
-                    disjointness_visitor,
-                )
+                field.default_type(db).when_none_or(|default_type| {
+                    default_type.has_relation_to_impl(
+                        db,
+                        right,
+                        inferable,
+                        relation,
+                        relation_visitor,
+                        disjointness_visitor,
+                    )
+                })
             }
 
             // Dynamic is only a subtype of `object` and only a supertype of `Never`; both were
@@ -7499,7 +7501,9 @@ fn walk_known_instance_type<'db, V: visitor::TypeVisitor<'db> + ?Sized>(
             // Nothing to visit
         }
         KnownInstanceType::Field(field) => {
-            visitor.visit_type(db, field.default_type(db));
+            if let Some(default_ty) = field.default_type(db) {
+                visitor.visit_type(db, default_ty);
+            }
         }
     }
 }
@@ -7599,9 +7603,11 @@ impl<'db> KnownInstanceType<'db> {
                     KnownInstanceType::TypeVar(_) => f.write_str("typing.TypeVar"),
                     KnownInstanceType::Deprecated(_) => f.write_str("warnings.deprecated"),
                     KnownInstanceType::Field(field) => {
-                        f.write_str("dataclasses.Field[")?;
-                        field.default_type(self.db).display(self.db).fmt(f)?;
-                        f.write_str("]")
+                        f.write_str("dataclasses.Field")?;
+                        if let Some(default_ty) = field.default_type(self.db) {
+                            write!(f, "[{}]", default_ty.display(self.db))?;
+                        }
+                        Ok(())
                     }
                     KnownInstanceType::ConstraintSet(tracked_set) => {
                         let constraints = tracked_set.constraints(self.db);
@@ -7988,7 +7994,7 @@ impl get_size2::GetSize for DeprecatedInstance<'_> {}
 pub struct FieldInstance<'db> {
     /// The type of the default value for this field. This is derived from the `default` or
     /// `default_factory` arguments to `dataclasses.field()`.
-    pub default_type: Type<'db>,
+    pub default_type: Option<Type<'db>>,
 
     /// Whether this field is part of the `__init__` signature, or not.
     pub init: bool,
@@ -8004,7 +8010,8 @@ impl<'db> FieldInstance<'db> {
     pub(crate) fn normalized_impl(self, db: &'db dyn Db, visitor: &NormalizedVisitor<'db>) -> Self {
         FieldInstance::new(
             db,
-            self.default_type(db).normalized_impl(db, visitor),
+            self.default_type(db)
+                .map(|ty| ty.normalized_impl(db, visitor)),
             self.init(db),
             self.kw_only(db),
         )

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

@@ -978,11 +978,12 @@ impl<'db> Bindings<'db> {
                                 overload.parameter_type_by_name("kw_only").unwrap_or(None);
 
                             let default_ty = match (default, default_factory) {
-                                (Some(default_ty), _) => default_ty,
+                                (Some(default_ty), _) => Some(default_ty),
                                 (_, Some(default_factory_ty)) => default_factory_ty
                                     .try_call(db, &CallArguments::none())
-                                    .map_or(Type::unknown(), |binding| binding.return_type(db)),
-                                _ => Type::unknown(),
+                                    .ok()
+                                    .map(|binding| binding.return_type(db)),
+                                _ => None,
                             };
 
                             let init = init

crates/ty_python_semantic/src/types/class.rs

@@ -2898,7 +2898,7 @@ impl<'db> ClassLiteral<'db> {
                 let mut init = true;
                 let mut kw_only = None;
                 if let Some(Type::KnownInstance(KnownInstanceType::Field(field))) = default_ty {
-                    default_ty = Some(field.default_type(db));
+                    default_ty = field.default_type(db);
                     if self
                         .dataclass_params(db)
                         .map(|params| params.contains(DataclassParams::NO_FIELD_SPECIFIERS))