Fix inference for attrs.fields

Fixes python#15393

Author: hauntsaninja

mypy/checker.py

@@ -4594,6 +4594,14 @@ def analyze_iterable_item_type(self, expr: Expression) -> tuple[Type, Type]:
 
         if isinstance(iterable, TupleType):
             joined: Type = UninhabitedType()
+            if iterable.partial_fallback.type.fullname != "builtins.tuple":
+                # If we're some fancier tuple variant, join with the fallback type
+                proper_iterator = get_proper_type(iterator)
+                if isinstance(proper_iterator, Instance) and not isinstance(
+                    get_proper_type(proper_iterator.args[0]), AnyType
+                ):
+                    joined = proper_iterator.args[0]
+
             for item in iterable.items:
                 joined = join_types(joined, item)
             return iterator, joined

test-data/unit/check-plugin-attrs.test

@@ -1570,6 +1570,9 @@ reveal_type(f(A)[0])  # N: Revealed type is "attr.Attribute[builtins.int]"
 reveal_type(f(A).b)  # N: Revealed type is "attr.Attribute[builtins.int]"
 f(A).x  # E: "____main___A_AttrsAttributes__" has no attribute "x"
 
+for ff in f(A):
+    reveal_type(ff)  # N: Revealed type is "attr.Attribute[Any]"
+
 [builtins fixtures/plugin_attrs.pyi]
 
 [case testAttrsGenericFields]

test-data/unit/fixtures/plugin_attrs.pyi

@@ -1,5 +1,5 @@
 # Builtins stub used to support attrs plugin tests.
-from typing import Union, overload
+from typing import Union, overload, Generic, Sequence, TypeVar, Type, Iterable, Iterator
 
 class object:
     def __init__(self) -> None: pass
@@ -24,6 +24,13 @@ class complex:
 
 class str: pass
 class ellipsis: pass
-class tuple: pass
 class list: pass
 class dict: pass
+
+T = TypeVar("T")
+Tco = TypeVar('Tco', covariant=True)
+class tuple(Sequence[Tco], Generic[Tco]):
+    def __new__(cls: Type[T], iterable: Iterable[Tco] = ...) -> T: ...
+    def __iter__(self) -> Iterator[Tco]: pass
+    def __contains__(self, item: object) -> bool: pass
+    def __getitem__(self, x: int) -> Tco: pass