Improve error message

Author: JukkaL

mypy/binder.py

@@ -288,7 +288,11 @@ def update_from_options(self, frames: list[Frame]) -> bool:
                     # still equivalent to such type).
                     if isinstance(type, UnionType):
                         type = collapse_variadic_union(type)
-                    if old_semantics and isinstance(type, ProperType) and isinstance(type, UnionType):
+                    if (
+                        old_semantics
+                        and isinstance(type, ProperType)
+                        and isinstance(type, UnionType)
+                    ):
                         # Simplify away any extra Any's that were added to the declared
                         # type when popping a frame.
                         simplified = UnionType.make_union(

mypy/checker.py

@@ -4566,6 +4566,12 @@ def check_simple_assignment(
                 rvalue_type = self.expr_checker.accept(
                     rvalue, type_context=lvalue_type, always_allow_any=always_allow_any
                 )
+                if not is_valid_inferred_type(rvalue_type, self.options) and inferred is not None:
+                    self.msg.need_annotation_for_var(
+                        inferred, context, self.options.python_version
+                    )
+                    rvalue_type = rvalue_type.accept(SetNothingToAny())
+
             if (
                 isinstance(lvalue, NameExpr)
                 and inferred is not None

test-data/unit/check-redefine2.test

@@ -1081,8 +1081,10 @@ def func() -> None:
     l = None
 
     if int():
-        l = []
+        l = [] # E: Need type annotation for "l"
         l.append(1)
+    reveal_type(l) # N: Revealed type is "Union[None, builtins.list[Any]]"
+[builtins fixtures/list.pyi]
 
 [case testNewRedefineNarrowingSpecialCase]
 # flags: --allow-redefinition-new --local-partial-types --warn-unreachable