fix enum+literal getting coerced to Sequence[str]

Author: randolf-scholz

mypy/join.py

@@ -150,6 +150,12 @@ def join_instances(self, t: Instance, s: Instance) -> ProperType:
             t, s, subtype_context=SubtypeContext(ignore_type_params=True)
         ):
             result = self.join_instances_via_supertype(t, s)
+        elif s.type.bases and is_proper_subtype(
+            s, t, subtype_context=SubtypeContext(ignore_type_params=True)
+        ):
+            result = self.join_instances_via_supertype(s, t)
+        elif is_subtype(t, s, subtype_context=SubtypeContext(ignore_type_params=True)):
+            result = self.join_instances_via_supertype(t, s)
         else:
             # Now t is not a subtype of s, and t != s. Now s could be a subtype
             # of t; alternatively, we need to find a common supertype. This works

test-data/unit/check-literal.test

@@ -3006,3 +3006,20 @@ def check_co(obj: A_co[Literal[1]]) -> None:
     reveal_type(f(obj, ''))  # N: Revealed type is "builtins.object"
     reveal_type(g(1, obj))   # N: Revealed type is "builtins.int"
     reveal_type(g('', obj))  # N: Revealed type is "builtins.object"
+
+[case testJoinLiteralInstanceAndEnum]
+from typing import Final, TypeVar
+from enum import StrEnum
+
+T = TypeVar("T")
+def join(a: T, b: T) -> T: ...
+
+class Foo(StrEnum):
+    A = "a"
+
+CONST: Final = "const"
+
+reveal_type(CONST)  # N: Revealed type is "Literal['const']?"
+reveal_type(join(Foo.A, CONST))  # N: Revealed type is "builtins.str"
+reveal_type(join(CONST, Foo.A))  # N: Revealed type is "builtins.str"
+[builtins fixtures/tuple.pyi]