From e113f0bde0953e9b77c646cf77f082568f2cfb2f Mon Sep 17 00:00:00 2001
From: Ivan Levkivskyi <levkivskyi@gmail.com>
Date: Tue, 3 Jun 2025 16:31:35 +0100
Subject: [PATCH] Fix crash on partial type used as context

---
 mypy/checker.py                     |  4 +++-
 test-data/unit/check-inference.test | 21 +++++++++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/mypy/checker.py b/mypy/checker.py
index e83473492f01..5201037242ac 100644
--- a/mypy/checker.py
+++ b/mypy/checker.py
@@ -3427,7 +3427,9 @@ def check_compatibility_all_supers(self, lvalue: RefExpr, rvalue: Expression) ->
             # store the rvalue type on the variable.
             actual_lvalue_type = None
             if lvalue_node.is_inferred and not lvalue_node.explicit_self_type:
-                rvalue_type = self.expr_checker.accept(rvalue, lvalue_node.type)
+                # Don't use partial types as context, similar to regular code path.
+                ctx = lvalue_node.type if not isinstance(lvalue_node.type, PartialType) else None
+                rvalue_type = self.expr_checker.accept(rvalue, ctx)
                 actual_lvalue_type = lvalue_node.type
                 lvalue_node.type = rvalue_type
             lvalue_type, _ = self.node_type_from_base(lvalue_node.name, lvalue_node.info, lvalue)
diff --git a/test-data/unit/check-inference.test b/test-data/unit/check-inference.test
index 4a3930533954..b563eef0f8aa 100644
--- a/test-data/unit/check-inference.test
+++ b/test-data/unit/check-inference.test
@@ -4128,3 +4128,24 @@ T = TypeVar("T")
 def f(x: Optional[T]) -> T: ...
 reveal_type(f(a))  # N: Revealed type is "Any"
 reveal_type(f(oa))  # N: Revealed type is "Any"
+
+[case testNoCrashOnPartialTypeAsContext]
+from typing import overload, TypeVar, Optional, Protocol
+
+T = TypeVar("T")
+class DbManager(Protocol):
+    @overload
+    def get(self, key: str) -> Optional[T]:
+        pass
+
+    @overload
+    def get(self, key: str, default: T) -> T:
+        pass
+
+class Foo:
+    def __init__(self, db: DbManager, bar: bool) -> None:
+        if bar:
+            self.qux = db.get("qux")
+        else:
+            self.qux = {}  # E: Need type annotation for "qux" (hint: "qux: dict[<type>, <type>] = ...")
+[builtins fixtures/dict.pyi]