Improve narrowing logic for Enum int and str subclasses (#20609)

Fixes some of the primer regressions from
#20492

Fixes #19753

This should also happen to fix the case mentioned by cdce8p in
#20492 (comment)

Author: hauntsaninja

mypy/checker.py

@@ -6734,6 +6734,7 @@ def narrow_type_by_equality(
             is_target_for_value_narrowing = is_singleton_identity_type
             should_coerce_literals = True
             should_narrow_by_identity_equality = True
+            enum_comparison_is_ambiguous = False
 
         elif operator in {"==", "!="}:
             is_target_for_value_narrowing = is_singleton_equality_type
@@ -6746,9 +6747,8 @@ def narrow_type_by_equality(
                     break
 
             expr_types = [operand_types[i] for i in expr_indices]
-            should_narrow_by_identity_equality = not any(
-                map(has_custom_eq_checks, expr_types)
-            ) and not is_ambiguous_mix_of_enums(expr_types)
+            should_narrow_by_identity_equality = not any(map(has_custom_eq_checks, expr_types))
+            enum_comparison_is_ambiguous = True
         else:
             raise AssertionError
 
@@ -6781,11 +6781,18 @@ def narrow_type_by_equality(
             for i in expr_indices:
                 if i not in narrowable_indices:
                     continue
+                expr_type = coerce_to_literal(operand_types[i])
+                expr_type = try_expanding_sum_type_to_union(expr_type, None)
+                expr_enum_keys = ambiguous_enum_equality_keys(expr_type)
                 for j, target in value_targets:
                     if i == j:
                         continue
-                    expr_type = coerce_to_literal(operand_types[i])
-                    expr_type = try_expanding_sum_type_to_union(expr_type, None)
+                    if (
+                        # See comments in ambiguous_enum_equality_keys
+                        enum_comparison_is_ambiguous
+                        and len(expr_enum_keys | ambiguous_enum_equality_keys(target.item)) > 1
+                    ):
+                        continue
                     if_map, else_map = conditional_types_to_typemaps(
                         operands[i], *conditional_types(expr_type, [target])
                     )
@@ -6795,10 +6802,10 @@ def narrow_type_by_equality(
             for i in expr_indices:
                 if i not in narrowable_indices:
                     continue
+                expr_type = operand_types[i]
                 for j, target in type_targets:
                     if i == j:
                         continue
-                    expr_type = operand_types[i]
                     if_map, else_map = conditional_types_to_typemaps(
                         operands[i], *conditional_types(expr_type, [target])
                     )
@@ -9387,47 +9394,44 @@ def visit_starred_pattern(self, p: StarredPattern) -> None:
             self.lvalue = False
 
 
-def is_ambiguous_mix_of_enums(types: list[Type]) -> bool:
-    """Do types have IntEnum/StrEnum types that are potentially overlapping with other types?
+def ambiguous_enum_equality_keys(t: Type) -> set[str]:
+    """
+    Used when narrowing types based on equality.
 
-    If True, we shouldn't attempt type narrowing based on enum values, as it gets
-    too ambiguous.
+    Certain kinds of enums can compare equal to values of other types, so doing type math
+    the way `conditional_types` does will be misleading if you expect it to correspond to
+    conditions based on equality comparisons.
 
-    For example, return True if there's an 'int' type together with an IntEnum literal.
-    However, IntEnum together with a literal of the same IntEnum type is not ambiguous.
+    For example, StrEnum classes can compare equal to str values. So if we see
+    `val: StrEnum; if val == "foo": ...` we currently avoid narrowing.
+    Note that we do wish to continue narrowing for `if val == StrEnum.MEMBER: ...`
     """
     # We need these things for this to be ambiguous:
-    #  (1) an IntEnum or StrEnum type
+    #  (1) an IntEnum or StrEnum type or enum subclass of int or str
     #  (2) either a different IntEnum/StrEnum type or a non-enum type ("<other>")
-    #
-    # It would be slightly more correct to calculate this separately for IntEnum and
-    # StrEnum related types, as an IntEnum can't be confused with a StrEnum.
-    return len(_ambiguous_enum_variants(types)) > 1
-
-
-def _ambiguous_enum_variants(types: list[Type]) -> set[str]:
     result = set()
-    for t in types:
-        t = get_proper_type(t)
-        if isinstance(t, UnionType):
-            result.update(_ambiguous_enum_variants(t.items))
-        elif isinstance(t, Instance):
-            if t.last_known_value:
-                result.update(_ambiguous_enum_variants([t.last_known_value]))
-            elif t.type.is_enum and any(
-                base.fullname in ("enum.IntEnum", "enum.StrEnum") for base in t.type.mro
-            ):
-                result.add(t.type.fullname)
-            elif not t.type.is_enum:
-                # These might compare equal to IntEnum/StrEnum types (e.g. Decimal), so
-                # let's be conservative
-                result.add("<other>")
-        elif isinstance(t, LiteralType):
-            result.update(_ambiguous_enum_variants([t.fallback]))
-        elif isinstance(t, NoneType):
-            pass
-        else:
+    t = get_proper_type(t)
+    if isinstance(t, UnionType):
+        for item in t.items:
+            result.update(ambiguous_enum_equality_keys(item))
+    elif isinstance(t, Instance):
+        if t.last_known_value:
+            result.update(ambiguous_enum_equality_keys(t.last_known_value))
+        elif t.type.is_enum and any(
+            base.fullname in ("enum.IntEnum", "enum.StrEnum", "builtins.str", "builtins.int")
+            for base in t.type.mro
+        ):
+            result.add(t.type.fullname)
+        elif not t.type.is_enum:
+            # These might compare equal to IntEnum/StrEnum types (e.g. Decimal), so
+            # let's be conservative
             result.add("<other>")
+    elif isinstance(t, LiteralType):
+        result.update(ambiguous_enum_equality_keys(t.fallback))
+    elif isinstance(t, NoneType):
+        pass
+    else:
+        result.add("<other>")
     return result
 
 

test-data/unit/check-narrowing.test

@@ -2124,7 +2124,7 @@ else:
 [builtins fixtures/ops.pyi]
 
 [case testNarrowingWithIntEnum]
-# mypy: strict-equality
+# flags: --strict-equality --warn-unreachable
 from __future__ import annotations
 from typing import Any
 from enum import IntEnum
@@ -2179,7 +2179,7 @@ def f6(x: IE) -> None:
 [builtins fixtures/primitives.pyi]
 
 [case testNarrowingWithIntEnum2]
-# mypy: strict-equality
+# flags: --strict-equality --warn-unreachable
 from __future__ import annotations
 from typing import Any
 from enum import IntEnum, Enum
@@ -2284,6 +2284,27 @@ def f4(x: SE) -> None:
          reveal_type(x)  # N: Revealed type is "Literal[__main__.SE.B]"
 [builtins fixtures/primitives.pyi]
 
+[case testNarrowingWithEnumStrSubclass]
+# flags: --strict-equality --warn-unreachable
+from enum import Enum
+
+class ParameterLocation(str, Enum):
+    QUERY = "query"
+    HEADER = "header"
+    PATH = "path"
+
+def foo(location: ParameterLocation):
+    if location == "path":
+        reveal_type(location)  # N: Revealed type is "__main__.ParameterLocation"
+    else:
+        reveal_type(location)  # N: Revealed type is "__main__.ParameterLocation"
+
+    if location == ParameterLocation.PATH:
+        reveal_type(location)  # N: Revealed type is "Literal[__main__.ParameterLocation.PATH]"
+    else:
+        reveal_type(location)  # N: Revealed type is "Literal[__main__.ParameterLocation.QUERY] | Literal[__main__.ParameterLocation.HEADER]"
+[builtins fixtures/primitives.pyi]
+
 [case testConsistentNarrowingEqAndIn]
 # flags: --python-version 3.10