Fix TypeIs narrowing to NewType

mypy/checker.py

@@ -8859,13 +8859,13 @@ def conditional_types(
 
     proposed_type = make_simplified_union([type_range.item for type_range in proposed_type_ranges])
     items = proposed_type.items if isinstance(proposed_type, UnionType) else [proposed_type]
-    for i in range(len(items)):
-        item = get_proper_type(items[i])
-        # Avoid ever narrowing to a NewType. The principle is values of NewType should only be
-        # produce by explicit wrapping
-        while isinstance(item, Instance) and item.type.is_newtype:
-            item = item.type.bases[0]
-        items[i] = item
+    if consider_runtime_isinstance:
+        for i in range(len(items)):
+            item = get_proper_type(items[i])
+            # Runtime checks can't distinguish a NewType from its base type.
+            while isinstance(item, Instance) and item.type.is_newtype:
+                item = item.type.bases[0]
+            items[i] = item
     proposed_type = get_proper_type(UnionType.make_union(items))
 
     if isinstance(p_current_type, AnyType):

test-data/unit/check-typeis.test

@@ -9,6 +9,20 @@ def main(a: object) -> None:
         reveal_type(a)  # N: Revealed type is "builtins.object"
 [builtins fixtures/tuple.pyi]
 
+[case testTypeIsNewType]
+from typing import NewType
+from typing_extensions import TypeIs
+
+Admin = NewType("Admin", str)
+def is_admin(value: str) -> TypeIs[Admin]: ...
+
+def f(value: str) -> None:
+    if is_admin(value):
+        reveal_type(value)  # N: Revealed type is "__main__.Admin"
+    else:
+        reveal_type(value)  # N: Revealed type is "builtins.str"
+[builtins fixtures/tuple.pyi]
+
 [case testTypeIsElif]
 from typing_extensions import TypeIs
 from typing import Union