fix: add support for UnionType annotation

tests/test_type_conversion.py

@@ -1,3 +1,4 @@
+import sys
 from enum import Enum
 from pathlib import Path
 from typing import Any, List, Optional, Tuple
@@ -128,3 +129,23 @@ def custom_click_type(
 
     result = runner.invoke(app, ["0x56"])
     assert result.exit_code == 0
+
+
+@pytest.mark.skipif(sys.version_info < (3, 10), reason="requires python3.10 or higher")
+def test_optional_via_uniontype():
+    app = typer.Typer()
+
+    @app.command()
+    def opt(user: str | None = None):
+        if user:
+            print(f"User: {user}")
+        else:
+            print("No user")
+
+    result = runner.invoke(app)
+    assert result.exit_code == 0
+    assert "No user" in result.output
+
+    result = runner.invoke(app, ["--user", "Camila"])
+    assert result.exit_code == 0
+    assert "User: Camila" in result.output

typer/main.py

@@ -13,6 +13,7 @@
 
 import click
 
+from ._typing import get_origin, is_union
 from .completion import get_completion_inspect_parameters
 from .core import MarkupMode, TyperArgument, TyperCommand, TyperGroup, TyperOption
 from .models import (
@@ -816,10 +817,10 @@ def get_click_param(
     is_tuple = False
     parameter_type: Any = None
     is_flag = None
-    origin = getattr(main_type, "__origin__", None)
+    origin = get_origin(main_type)
     if origin is not None:
-        # Handle Optional[SomeType]
-        if origin is Union:
+        # Handle `Optional[SomeType]` and `str | None`
+        if is_union(origin):
             types = []
             for type_ in main_type.__args__:
                 if type_ is NoneType: