Union types: Support narrowing to Ellipsis (...) cases of Unions

After this change, narrowing to `Ellipsis` works similarly with
regards to narrowing as `None` in `Optional`s

It would be a good followup refactor to delegate some of the logic
from `is_singleton_type` to the actual mypy types so they could decide
for themselves if they are representing singleton objects

Fixes: python#13117
Co-authored-by: Zsolt Cserna <cserna.zsolt@gmail.com>

mypy/typeops.py

@@ -747,7 +747,9 @@ def is_singleton_type(typ: Type) -> bool:
             isinstance(typ, NoneType)
             or (isinstance(typ, LiteralType)
                 and (typ.is_enum_literal() or isinstance(typ.value, bool)))
-            or (isinstance(typ, Instance) and typ.type.is_enum and len(get_enum_values(typ)) == 1)
+            or (isinstance(typ, Instance) and (
+                typ.type.is_enum and len(get_enum_values(typ)) == 1
+                or typ.type.fullname == 'builtins.ellipsis'))
     )
 
 

test-data/unit/check-unions.test

@@ -137,6 +137,16 @@ def f() -> Union[int, None]: pass
 x = 1
 x = f()
 
+[case testUnionWithEllipsis]
+from typing import Union
+def f(x: Union[int, EllipsisType]) -> int:
+    if x is Ellipsis:
+        reveal_type(x) # N: Revealed type is "builtins.ellipsis"
+        x = 1
+    reveal_type(x) # N: Revealed type is "builtins.int"
+    return x
+[builtins fixtures/isinstancelist.pyi]
+
 [case testOptional]
 from typing import Optional
 def f(x: Optional[int]) -> None: pass

test-data/unit/fixtures/isinstancelist.pyi

@@ -10,9 +10,12 @@ class type:
     def __init__(self, x) -> None: pass
 
 class function: pass
-class ellipsis: pass
 class classmethod: pass
 
+class ellipsis: pass
+EllipsisType = ellipsis
+Ellipsis = ellipsis()
+
 def isinstance(x: object, t: Union[type, Tuple]) -> bool: pass
 def issubclass(x: object, t: Union[type, Tuple]) -> bool: pass