Make enums idempotent

mypy/checkexpr.py

@@ -627,7 +627,14 @@ def visit_call_expr_inner(self, e: CallExpr, allow_none_return: bool = False) ->
                 for p in self.chk.options.untyped_calls_exclude
             ):
                 self.msg.untyped_function_call(callee_type, e)
-
+        if isinstance(callee_type, Instance) and callee_type.type.has_base("enum.Enum"):
+            if e.args:
+                arg_type = get_proper_type(self.accept(e.args[0]))
+                if (
+                    isinstance(arg_type, LiteralType)
+                    and arg_type.fallback.type == callee_type.type
+                ):
+                    return arg_type
         ret_type = self.check_call_expr_with_callee_type(
             callee_type, e, fullname, object_type, member
         )

test-data/unit/check-expressions.test

@@ -2492,3 +2492,16 @@ x + T  # E: Unsupported left operand type for + ("int")
 T()  # E: "TypeVar" not callable
 [builtins fixtures/tuple.pyi]
 [typing fixtures/typing-full.pyi]
+
+[case testEnumIdempotency]
+[builtins fixtures/tuple.pyi]
+from enum import Enum
+
+class Color(Enum):
+    RED = 1
+    BLUE = 2
+
+def f() -> Color:
+    return Color(Color.RED)
+
+[out]