Fix line number if __iter__ is incorrectly reported as missing (#14893)

A type was used as error context, but types don't reliably have valid
line numbers during type checking. Pass context explicitly instead.

The error in the test case is actually a false positive, but I'm first
fixing the line number of the error, since it seems plausible that the
wrong line number could cause other problems.

Work on #14892.

mypy/checker.py

@@ -3299,14 +3299,14 @@ def check_assignment_to_multiple_lvalues(
                         rvalues.extend([TempNode(typ) for typ in typs.items])
                     elif self.type_is_iterable(typs) and isinstance(typs, Instance):
                         if iterable_type is not None and iterable_type != self.iterable_item_type(
-                            typs
+                            typs, rvalue
                         ):
                             self.fail(message_registry.CONTIGUOUS_ITERABLE_EXPECTED, context)
                         else:
                             if last_idx is None or last_idx + 1 == idx_rval:
                                 rvalues.append(rval)
                                 last_idx = idx_rval
-                                iterable_type = self.iterable_item_type(typs)
+                                iterable_type = self.iterable_item_type(typs, rvalue)
                             else:
                                 self.fail(message_registry.CONTIGUOUS_ITERABLE_EXPECTED, context)
                     else:
@@ -3635,7 +3635,7 @@ def check_multi_assignment_from_iterable(
         if self.type_is_iterable(rvalue_type) and isinstance(
             rvalue_type, (Instance, CallableType, TypeType, Overloaded)
         ):
-            item_type = self.iterable_item_type(rvalue_type)
+            item_type = self.iterable_item_type(rvalue_type, context)
             for lv in lvalues:
                 if isinstance(lv, StarExpr):
                     items_type = self.named_generic_type("builtins.list", [item_type])
@@ -6392,7 +6392,9 @@ def note(
             return
         self.msg.note(msg, context, offset=offset, code=code)
 
-    def iterable_item_type(self, it: Instance | CallableType | TypeType | Overloaded) -> Type:
+    def iterable_item_type(
+        self, it: Instance | CallableType | TypeType | Overloaded, context: Context
+    ) -> Type:
         if isinstance(it, Instance):
             iterable = map_instance_to_supertype(it, self.lookup_typeinfo("typing.Iterable"))
             item_type = iterable.args[0]
@@ -6401,7 +6403,7 @@ def iterable_item_type(self, it: Instance | CallableType | TypeType | Overloaded
                 # in case there is no explicit base class.
                 return item_type
         # Try also structural typing.
-        return self.analyze_iterable_item_type_without_expression(it, it)[1]
+        return self.analyze_iterable_item_type_without_expression(it, context)[1]
 
     def function_type(self, func: FuncBase) -> FunctionLike:
         return function_type(func, self.named_type("builtins.function"))

mypy/checkpattern.py

@@ -15,7 +15,7 @@
 from mypy.maptype import map_instance_to_supertype
 from mypy.meet import narrow_declared_type
 from mypy.messages import MessageBuilder
-from mypy.nodes import ARG_POS, Expression, NameExpr, TypeAlias, TypeInfo, Var
+from mypy.nodes import ARG_POS, Context, Expression, NameExpr, TypeAlias, TypeInfo, Var
 from mypy.patterns import (
     AsPattern,
     ClassPattern,
@@ -242,7 +242,7 @@ def visit_sequence_pattern(self, o: SequencePattern) -> PatternType:
             elif size_diff > 0 and star_position is None:
                 return self.early_non_match()
         else:
-            inner_type = self.get_sequence_type(current_type)
+            inner_type = self.get_sequence_type(current_type, o)
             if inner_type is None:
                 inner_type = self.chk.named_type("builtins.object")
             inner_types = [inner_type] * len(o.patterns)
@@ -309,12 +309,12 @@ def visit_sequence_pattern(self, o: SequencePattern) -> PatternType:
                 new_type = current_type
         return PatternType(new_type, rest_type, captures)
 
-    def get_sequence_type(self, t: Type) -> Type | None:
+    def get_sequence_type(self, t: Type, context: Context) -> Type | None:
         t = get_proper_type(t)
         if isinstance(t, AnyType):
             return AnyType(TypeOfAny.from_another_any, t)
         if isinstance(t, UnionType):
-            items = [self.get_sequence_type(item) for item in t.items]
+            items = [self.get_sequence_type(item, context) for item in t.items]
             not_none_items = [item for item in items if item is not None]
             if len(not_none_items) > 0:
                 return make_simplified_union(not_none_items)
@@ -324,7 +324,7 @@ def get_sequence_type(self, t: Type) -> Type | None:
         if self.chk.type_is_iterable(t) and isinstance(t, (Instance, TupleType)):
             if isinstance(t, TupleType):
                 t = tuple_fallback(t)
-            return self.chk.iterable_item_type(t)
+            return self.chk.iterable_item_type(t, context)
         else:
             return None
 

test-data/unit/check-classes.test

@@ -7797,3 +7797,19 @@ class Element(Generic[_T]):
 class Bar(Foo): ...
 e: Element[Bar]
 reveal_type(e.elements)  # N: Revealed type is "typing.Sequence[__main__.Element[__main__.Bar]]"
+
+[case testIterableUnpackingWithGetAttr]
+from typing import Union, Tuple
+
+class C:
+    def __getattr__(self, name):
+        pass
+
+class D:
+    def f(self) -> C:
+        return C()
+
+    def g(self) -> None:
+        # TODO: This is a false positive
+        a, b = self.f()  # E: "C" has no attribute "__iter__" (not iterable)
+[builtins fixtures/tuple.pyi]