Carry Literal default values in Param D generic

For simple default types (None, int, str, bytes, bool, enum), store
D = Literal[value] instead of D = ann_type, allowing real values to be
extracted when converting back to inspect.Signature.

Author: msullivan

tests/test_type_dir.py

@@ -205,7 +205,7 @@ class Final:
             fin: typing.Final[int]
             x: tests.test_type_dir.Wrapper[int | None]
             ordinary: str
-            def foo(self: Self, a: int | None, *, b: int = ...) -> dict[str, int]: ...
+            def foo(self: Self, a: int | None, *, b: int = 0) -> dict[str, int]: ...
             def base[Z](self: Self, a: int | Z | None, b: ~K) -> dict[str, int | Z]: ...
             @classmethod
             def cbase(cls: type[typing.Self], a: int | None, b: ~K) -> dict[str, int]: ...
@@ -381,7 +381,7 @@ def test_type_members_func_1():
 typemap.typing.Param[typing.Literal['self'], tests.test_type_dir.Base[int], {PK}, typing.Never], \
 typemap.typing.Param[typing.Literal['a'], int | None, {PK}, typing.Never], \
 typemap.typing.Param[typing.Literal['b'], int, typing.Literal[\
-<ParamKind.KEYWORD_ONLY: 3>], int]], \
+<ParamKind.KEYWORD_ONLY: 3>], typing.Literal[0]]], \
 dict[str, int]]"
     )
 

typemap/type_eval/_eval_operators.py

@@ -1,6 +1,7 @@
 import collections
 import collections.abc
 import contextlib
+import enum
 import functools
 import inspect
 import itertools
@@ -409,7 +410,13 @@ def _callable_type_to_signature(callable_type: object) -> inspect.Signature:
         default_type = param_args[3] if len(param_args) > 3 else typing.Never
         default: typing.Any
         if default_type is not typing.Never:
-            default = _DUMMY_DEFAULT
+            if (
+                _typing_inspect.is_literal(default_type)
+                and len(default_type.__args__) == 1  # type: ignore[union-attr]
+            ):
+                default = default_type.__args__[0]  # type: ignore[union-attr]
+            else:
+                default = _DUMMY_DEFAULT
         else:
             default = inspect.Parameter.empty
 
@@ -565,12 +572,20 @@ def _ann(x):
             quals.append(ParamKind.POSITIONAL_OR_KEYWORD)
         ann_type = _ann(ann)
         has_default = p.default is not empty
+        if has_default:
+            v = p.default
+            if v is None or isinstance(v, (int, str, bytes, bool, enum.Enum)):
+                default_type = typing.Literal[(v,)]
+            else:
+                default_type = ann_type
+        else:
+            default_type = typing.Never
         params.append(
             Param[
                 typing.Literal[p.name],
                 ann_type,
                 typing.Literal[*quals],
-                ann_type if has_default else typing.Never,
+                default_type,
             ]
         )