Support generic annotations in overload.

Generic aliases pass as `callable`, even though they are not. A related issue found in #90: prior to Python 3.11, a parameter with a default of `None` was automatically coerced to `Optional` by `get_type_hints`.

README.md

@@ -166,6 +166,7 @@ dev
 * Python >=3.8 required
 * `Type[...]` dispatches on class arguments
 * `|` syntax for union types
+* `overload` supports generics
 
 1.9.1
 

multimethod/__init__.py

@@ -397,7 +397,7 @@ def __new__(cls, func):
 
     def __init__(self, func: Callable):
         for name, value in get_type_hints(func).items():
-            if not callable(value) or isinstance(value, type):
+            if not callable(value) or isinstance(value, type) or hasattr(value, '__origin__'):
                 func.__annotations__[name] = isa(value)
         self[inspect.signature(func)] = func
 

pyproject.toml

@@ -21,6 +21,7 @@ classifiers = [
     "Programming Language :: Python :: 3.9",
     "Programming Language :: Python :: 3.10",
     "Programming Language :: Python :: 3.11",
+    "Programming Language :: Python :: 3.12",
     "Topic :: Software Development :: Libraries :: Python Modules",
     "Typing :: Typed",
 ]

tests/test_overload.py

@@ -1,5 +1,5 @@
 import pytest
-from typing import List, Literal
+from typing import List, Literal, Optional
 from multimethod import DispatchError, isa, overload
 
 
@@ -45,3 +45,12 @@ def test_generic():
     assert not pred([0.0])
     assert pred(0.0)
     assert not pred(1.0)
+
+    @overload
+    def func(arg: Optional[str]):
+        return arg
+
+    assert func('') == ''
+    assert func(None) is None
+    with pytest.raises(DispatchError):
+        func(0)