From 0fa3184fd7e87fb4cbf433eb90cd9986297ddb8d Mon Sep 17 00:00:00 2001
From: Yevhenii Bilozor <153777868+neopexrl@users.noreply.github.com>
Date: Sun, 26 Oct 2025 17:31:29 +0100
Subject: [PATCH 1/2] Enhance type resolution for instance variables

Added logic to resolve types for instance variables and handle partial types.
---
 mypy/checker.py | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/mypy/checker.py b/mypy/checker.py
index 2dd7f10e6f35..d0742cbc4512 100644
--- a/mypy/checker.py
+++ b/mypy/checker.py
@@ -3313,6 +3313,33 @@ def check_assignment(
                     rvalue_type, lvalue_type, infer_lvalue_type = self.check_member_assignment(
                         lvalue, instance_type, lvalue_type, rvalue, context=rvalue
                     )
+                    ## --PATCH-- (ISSUE №19996 --INTERNAL ERROR with allow_redefinition_new and undeclared instance variables)
+                    proper_rvalue_type = get_proper_type(rvalue_type)
+                    resolved_type = None
+
+                    def can_resolve_partial(lvalue_type, proper_rvalue_type):
+                        return lvalue_type.type is None and not isinstance(proper_rvalue_type, NoneType)
+
+                    def set_node_type(lvalue, typ):
+                        if getattr(lvalue, "node", None) and hasattr(lvalue.node, "type"):
+                            lvalue.node.type = typ
+
+                    if isinstance(lvalue_type, PartialType) and can_resolve_partial(lvalue_type, proper_rvalue_type):
+                        resolved_type = make_simplified_union([proper_rvalue_type, NoneType()])
+                    elif lvalue_type.type is not None and isinstance(proper_rvalue_type, Instance):
+                        if rvalue_type.type == lvalue_type.type:
+                            resolved_type = rvalue_type
+
+                    if resolved_type is not None:
+                        lvalue_type = resolved_type
+                        set_node_type(lvalue, resolved_type)
+                    else:
+                        self.fail(
+                            f"Cannot resolve type for instance variable '{lvalue.name}', please annotate",
+                            lvalue,
+                        )
+                    return
+                    
                 else:
                     # Hacky special case for assigning a literal None
                     # to a variable defined in a previous if

From d0328183c58e62570a6eb64ae0b28a36507546d5 Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Sun, 26 Oct 2025 16:35:40 +0000
Subject: [PATCH 2/2] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 mypy/checker.py | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/mypy/checker.py b/mypy/checker.py
index d0742cbc4512..b6ceb8e5849e 100644
--- a/mypy/checker.py
+++ b/mypy/checker.py
@@ -3318,13 +3318,17 @@ def check_assignment(
                     resolved_type = None
 
                     def can_resolve_partial(lvalue_type, proper_rvalue_type):
-                        return lvalue_type.type is None and not isinstance(proper_rvalue_type, NoneType)
+                        return lvalue_type.type is None and not isinstance(
+                            proper_rvalue_type, NoneType
+                        )
 
                     def set_node_type(lvalue, typ):
                         if getattr(lvalue, "node", None) and hasattr(lvalue.node, "type"):
                             lvalue.node.type = typ
 
-                    if isinstance(lvalue_type, PartialType) and can_resolve_partial(lvalue_type, proper_rvalue_type):
+                    if isinstance(lvalue_type, PartialType) and can_resolve_partial(
+                        lvalue_type, proper_rvalue_type
+                    ):
                         resolved_type = make_simplified_union([proper_rvalue_type, NoneType()])
                     elif lvalue_type.type is not None and isinstance(proper_rvalue_type, Instance):
                         if rvalue_type.type == lvalue_type.type:
@@ -3339,7 +3343,7 @@ def set_node_type(lvalue, typ):
                             lvalue,
                         )
                     return
-                    
+
                 else:
                     # Hacky special case for assigning a literal None
                     # to a variable defined in a previous if