Handle UnionType correctly in true_or_false (#3510)

Fixes #3508.

Author: ddfisher

mypy/types.py

@@ -1699,6 +1699,10 @@ def true_or_false(t: Type) -> Type:
     """
     Unrestricted version of t with both True-ish and False-ish values
     """
+    if isinstance(t, UnionType):
+        new_items = [true_or_false(item) for item in t.items]
+        return UnionType.make_simplified_union(new_items, line=t.line, column=t.column)
+
     new_t = copy_type(t)
     new_t.can_be_true = type(new_t).can_be_true
     new_t.can_be_false = type(new_t).can_be_false

test-data/unit/check-optional.test

@@ -594,3 +594,12 @@ x: Optional[Union[int, str]]
 reveal_type(x) # E: Revealed type is 'Union[builtins.int, builtins.str, builtins.None]'
 y: Optional[Union[int, None]]
 reveal_type(y) # E: Revealed type is 'Union[builtins.int, builtins.None]'
+
+[case testUnionTruthinessTracking]
+from typing import Optional, Any
+def test_or_shortcut(value: Optional[Any]) -> None:
+    if not value:
+        pass
+    if not value or value.get('foo') == 'hello':
+        pass
+[builtins fixtures/bool.pyi]