Fix schema.py

schema.py type-checks, but raises exception at runtime because Struct is
referenced before it is defined. This commit
- moves definition of Type to the top
- uses string names for forward type references
- replaces T with covariant TypeT
- fixes missing type parameter in Struct definition

projects/jdwp/defs/schema.py

@@ -4,13 +4,24 @@
 
 from __future__ import annotations
 from dataclasses import dataclass
-from typing import Optional, Generic, TypeVar, Type as TypeAlias
+from typing import Optional, Generic, TypeVar, Type as TypeAlias, Union
 from collections.abc import Mapping
 from enum import Enum
 from collections.abc import Set, Sequence
 from projects.jdwp.defs.constants import ErrorType
 
 
+Type = Union[
+    "PrimitiveType",
+    "Array",
+    "TaggedUnion",
+    "IntegralType",
+    "ArrayLength",
+    "Struct",
+    "UnionTag",
+]
+
+
 class PrimitiveType(Enum):
     """Primitive type class."""
 
@@ -72,27 +83,23 @@ class UnionTag(Generic[EnumT]):
     value: TypeAlias[EnumT]
 
 
-Type = (
-    PrimitiveType | Array | TaggedUnion | IntegralType | ArrayLength | Struct | UnionTag
-)
-
-T = TypeVar("T", bound=Type)
+TypeT = TypeVar("TypeT", bound=Type, covariant=True)
 
 
 @dataclass(frozen=True)
-class Field(Generic[T]):
+class Field(Generic[TypeT]):
     """Field class."""
 
     name: str
-    type: T
+    type: TypeT
     description: str
 
 
 @dataclass(frozen=True)
 class Struct:
     """Struct class."""
 
-    fields: Sequence[Field]
+    fields: Sequence[Field[Type]]
 
 
 @dataclass(frozen=True)

projects/jdwp/tests/BUCK

@@ -2,5 +2,8 @@
 
 python_test(
   name = "tests",
-  srcs = glob(["*.py"]),
+  deps = [
+    "//projects/jdwp:lib",
+  ],
+  srcs = glob(["**/*.py"]),
 )

projects/jdwp/tests/defs/schema.py

@@ -0,0 +1,8 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+
+import unittest
+
+
+class SchemaTests(unittest.TestCase):
+    def test_schema_can_be_imported(self):
+        from projects.jdwp.defs.schema import PrimitiveType