Fix crash on partial type used as context (#19216)

Fixes #19213

Author: ilevkivskyi

mypy/checker.py

@@ -3426,7 +3426,9 @@ 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)

test-data/unit/check-inference.test

@@ -3979,3 +3979,24 @@ def check(mapping: Mapping[str, _T]) -> None:
     reveal_type(ok1)  # N: Revealed type is "Union[_T`-1, builtins.str]"
     ok2: Union[_T, str] = mapping.get("", "")
 [builtins fixtures/tuple.pyi]
+
+[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]