For Optional[T] or T, infer the type T instead of Optional[T].

Fixes #1817, fixes #1733.

Author: Guido van Rossum

mypy/checkexpr.py

@@ -1090,6 +1090,13 @@ def check_boolean_op(self, e: OpExpr, context: Context) -> Type:
                 mypy.checker.find_isinstance_check(e.left, self.chk.type_map,
                                                    self.chk.typing_mode_weak())
         elif e.op == 'or':
+            # Special case for `Optional[T] or T` -- this should infer T.
+            # If the type is a Union containing None, remove the None.
+            if isinstance(left_type, UnionType):
+                items = [i for i in left_type.items if not isinstance(i, NoneTyp)]
+                if len(items) < len(left_type.items):
+                    left_type = UnionType.make_simplified_union(items)
+
             _, right_map = \
                 mypy.checker.find_isinstance_check(e.left, self.chk.type_map,
                                                    self.chk.typing_mode_weak())

test-data/unit/check-optional.test

@@ -175,3 +175,33 @@ def f(x: None) -> str: pass
 def f(x: int) -> int: pass
 reveal_type(f(None))  # E: Revealed type is 'builtins.str'
 reveal_type(f(0))  # E: Revealed type is 'builtins.int'
+
+[case testOptionalTypeOrTypePlain]
+from typing import Optional
+def f(a: Optional[int]) -> int:
+    return a or 0
+[out]
+
+[case testOptionalTypeOrTypeTypeVar]
+from typing import Optional, TypeVar
+T = TypeVar('T')
+def f(a: Optional[T], b: T) -> T:
+    return a or b
+[out]
+
+[case testOptionalTypeOrTypeNoTriggerPlain]
+from typing import Optional
+def f(a: Optional[int], b: int) -> int:
+    return b or a
+[out]
+main: note: In function "f":
+main:3: error: Incompatible return value type (got "Optional[int]", expected "int")
+
+[case testOptionalTypeOrTypeNoTriggerTypeVar]
+from typing import Optional, TypeVar
+T = TypeVar('T')
+def f(a: Optional[T], b: T) -> T:
+    return b or a
+[out]
+main: note: In function "f":
+main:4: error: Incompatible return value type (got "Optional[T]", expected "T")