From 44e67b68d67365e40f921988ce4b2be6defc2a2f Mon Sep 17 00:00:00 2001
From: Ivan Levkivskyi <levkivskyi@gmail.com>
Date: Fri, 20 Feb 2026 23:13:08 +0000
Subject: [PATCH] Try allowing redefinitions for function arguments

---
 mypy/cache.py                       |  2 +-
 mypy/checker.py                     |  2 +-
 mypy/fastparse.py                   |  1 +
 mypy/nodes.py                       | 12 ++++--------
 test-data/unit/check-redefine2.test |  4 ++--
 5 files changed, 9 insertions(+), 12 deletions(-)

diff --git a/mypy/cache.py b/mypy/cache.py
index 9e3b39ffaaa40..80112e94c7f3b 100644
--- a/mypy/cache.py
+++ b/mypy/cache.py
@@ -69,7 +69,7 @@
 from mypy_extensions import u8
 
 # High-level cache layout format
-CACHE_VERSION: Final = 5
+CACHE_VERSION: Final = 6
 
 # Type used internally to represent errors:
 #   (path, line, column, end_line, end_column, severity, message, code)
diff --git a/mypy/checker.py b/mypy/checker.py
index ea3e9f072afc8..784ec3939ad77 100644
--- a/mypy/checker.py
+++ b/mypy/checker.py
@@ -4421,7 +4421,7 @@ def check_lvalue(
             if (
                 self.options.allow_redefinition_new
                 and isinstance(lvalue.node, Var)
-                and lvalue.node.is_inferred
+                and (lvalue.node.is_inferred or lvalue.node.is_argument)
             ):
                 inferred = lvalue.node
             self.store_type(lvalue, lvalue_type)
diff --git a/mypy/fastparse.py b/mypy/fastparse.py
index 701e449f8f338..c68d1dfc53b62 100644
--- a/mypy/fastparse.py
+++ b/mypy/fastparse.py
@@ -1145,6 +1145,7 @@ def make_argument(
 
         var = Var(arg.arg, arg_type)
         var.is_inferred = False
+        var.is_argument = True
         argument = Argument(var, arg_type, self.visit(default), kind, pos_only)
         argument.set_line(arg.lineno, arg.col_offset, arg.end_lineno, arg.end_col_offset)
         return argument
diff --git a/mypy/nodes.py b/mypy/nodes.py
index 4c238592f9927..0b31f20267053 100644
--- a/mypy/nodes.py
+++ b/mypy/nodes.py
@@ -1292,6 +1292,7 @@ class Var(SymbolNode):
         "has_explicit_value",
         "allow_incompatible_override",
         "invalid_partial_type",
+        "is_argument",
     )
 
     __match_args__ = ("name", "type", "final_value")
@@ -1351,6 +1352,8 @@ def __init__(self, name: str, type: mypy.types.Type | None = None) -> None:
         # If True, this means we didn't manage to infer full type and fall back to
         # something like list[Any]. We may decide to not use such types as context.
         self.invalid_partial_type = False
+        # Is it a variable symbol for a function argument?
+        self.is_argument = False
 
     @property
     def name(self) -> str:
@@ -1414,8 +1417,6 @@ def write(self, data: WriteBuffer) -> None:
         write_flags(
             data,
             [
-                self.is_self,
-                self.is_cls,
                 self.is_initialized_in_class,
                 self.is_staticmethod,
                 self.is_classmethod,
@@ -1453,8 +1454,6 @@ def read(cls, data: ReadBuffer) -> Var:
         v.setter_type = setter_type
         v._fullname = read_str(data)
         (
-            v.is_self,
-            v.is_cls,
             v.is_initialized_in_class,
             v.is_staticmethod,
             v.is_classmethod,
@@ -1474,7 +1473,7 @@ def read(cls, data: ReadBuffer) -> Var:
             v.from_module_getattr,
             v.has_explicit_value,
             v.allow_incompatible_override,
-        ) = read_flags(data, num_flags=21)
+        ) = read_flags(data, num_flags=19)
         tag = read_tag(data)
         if tag == LITERAL_COMPLEX:
             v.final_value = complex(read_float_bare(data), read_float_bare(data))
@@ -2368,9 +2367,6 @@ def __init__(
         analyzed: Expression | None = None,
     ) -> None:
         super().__init__()
-        if not arg_names:
-            arg_names = [None] * len(args)
-
         self.callee = callee
         self.args = args
         self.arg_kinds = arg_kinds  # ARG_ constants
diff --git a/test-data/unit/check-redefine2.test b/test-data/unit/check-redefine2.test
index 6e9441151c935..de6b216e471e5 100644
--- a/test-data/unit/check-redefine2.test
+++ b/test-data/unit/check-redefine2.test
@@ -562,8 +562,8 @@ def f2() -> None:
 def f3(x) -> None:
     if int():
         x = 0
-        reveal_type(x) # N: Revealed type is "Any"
-    reveal_type(x) # N: Revealed type is "Any"
+        reveal_type(x) # N: Revealed type is "builtins.int"
+    reveal_type(x) # N: Revealed type is "Any | builtins.int"
 
 [case tetNewRedefineDel]
 # flags: --allow-redefinition-new --local-partial-types