[suggest] Fix refine turning Optional[Any] into None (#8100)

Author: msullivan

mypy/suggestions.py

@@ -970,6 +970,12 @@ def refine_union(t: UnionType, s: ProperType) -> Type:
     one). If an element of the union is succesfully refined, we drop it
     from the union in favor of the refined versions.
     """
+    # Don't try to do any union refining if the types are already the
+    # same.  This prevents things like refining Optional[Any] against
+    # itself and producing None.
+    if t == s:
+        return t
+
     rhs_items = s.items if isinstance(s, UnionType) else [s]
 
     new_items = []

test-data/unit/fine-grained-suggest.test

@@ -1030,6 +1030,7 @@ foo.py:4: error: unexpected EOF while parsing
 # suggest: foo.optional3
 # suggest: foo.optional4
 # suggest: foo.optional5
+# suggest: foo.optional_any
 # suggest: foo.dict1
 # suggest: foo.tuple1
 [file foo.py]
@@ -1112,6 +1113,9 @@ def optional5(x: Optional[Any]):
 optional5(10)
 optional5(None)
 
+def optional_any(x: Optional[Any] = None):
+    pass
+
 def dict1(d: Dict[int, Any]):
     pass
 
@@ -1138,6 +1142,7 @@ tuple1(t)
 (Optional[foo.List[int]]) -> int
 (Union[foo.Set[int], foo.List[int]]) -> None
 (Optional[int]) -> None
+(Optional[Any]) -> None
 (foo.Dict[int, int]) -> None
 (Tuple[int, int]) -> None
 ==