Fix mypy primer bugs

Author: hatal175

mypy/checker.py

@@ -4702,14 +4702,20 @@ def conditional_len_map(self,
                 possible_types = union_items(current_type)
                 len_of_types = [len_of_type(typ) for typ in possible_types]
 
-                proposed_type = UnionType([
+                proposed_type = make_simplified_union([
                     self.narrow_type_by_length(typ, length)
                     for typ, l in zip(possible_types, len_of_types)
                     if l is None or l == length])
-                remaining_type = UnionType([
+                remaining_type = make_simplified_union([
                     typ for typ, l in zip(possible_types, len_of_types)
                     if l is None or l != length])
-                return {expr: proposed_type}, {expr: remaining_type}
+                if_map = (
+                    {} if is_same_type(proposed_type, current_type)
+                    else {expr: proposed_type})
+                else_map = (
+                    {} if is_same_type(remaining_type, current_type)
+                    else {expr: remaining_type})
+                return if_map, else_map
         else:
             return {}, {}
 

test-data/unit/check-narrowing.test

@@ -1078,8 +1078,8 @@ else:
     reveal_type(x) # N: Revealed type is "Tuple[builtins.int, builtins.int, builtins.int]"
 [builtins fixtures/len.pyi]
 
-[case testNarrowingLenListAndStrUnaffected]
-from typing import Tuple, Union, List
+[case testNarrowingLenTypeUnaffected]
+from typing import Tuple, Union, List, Any
 
 def make() -> Union[str, List[int]]:
     return ""
@@ -1090,6 +1090,13 @@ if len(x) == 3:
     reveal_type(x) # N: Revealed type is "Union[builtins.str, builtins.list[builtins.int]]"
 else:
     reveal_type(x) # N: Revealed type is "Union[builtins.str, builtins.list[builtins.int]]"
+
+def f(self, value: Any) -> Any:
+    if isinstance(value, list) and len(value) == 0:
+        reveal_type(value) # N: Revealed type is "builtins.list[Any]"
+        return value
+    reveal_type(value) # N: Revealed type is "Any"
+    return None
 [builtins fixtures/len.pyi]
 
 [case testNarrowingLenLiteral]