attrs: remove fields type check (#15983)

Since python-attrs/attrs#890 (≥ 22.1.0)
`attrs.fields` is typed to accept a protocol.
Since python-attrs/attrs#997 (≥ 22.2.0)
`attrs.has` is a type-guard.

Support both by removing the explicit error reporting and letting it
fall through to the type stub.

Fixes #15980.

mypy/plugins/attrs.py

@@ -1111,9 +1111,4 @@ def fields_function_sig_callback(ctx: mypy.plugin.FunctionSigContext) -> Callabl
         assert ret_type is not None
         return ctx.default_signature.copy_modified(arg_types=arg_types, ret_type=ret_type)
 
-    ctx.api.fail(
-        f'Argument 1 to "fields" has incompatible type "{format_type_bare(proper_type, ctx.api.options)}"; expected an attrs class',
-        ctx.context,
-    )
-
     return ctx.default_signature

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

@@ -1596,16 +1596,18 @@ def f(t: TA) -> None:
 [builtins fixtures/plugin_attrs.pyi]
 
 [case testNonattrsFields]
-# flags: --no-strict-optional
 from typing import Any, cast, Type
-from attrs import fields
+from attrs import fields, has
 
 class A:
     b: int
     c: str
 
-fields(A)  # E: Argument 1 to "fields" has incompatible type "Type[A]"; expected an attrs class
-fields(None)  # E: Argument 1 to "fields" has incompatible type "None"; expected an attrs class
+if has(A):
+    fields(A)
+else:
+    fields(A)  # E: Argument 1 to "fields" has incompatible type "Type[A]"; expected "Type[AttrsInstance]"
+fields(None)  # E: Argument 1 to "fields" has incompatible type "None"; expected "Type[AttrsInstance]"
 fields(cast(Any, 42))
 fields(cast(Type[Any], 43))
 
@@ -2167,7 +2169,8 @@ TA = TypeVar('TA', bound=A)
 TB = TypeVar('TB', bound=B)
 
 def f(b_or_t: TA | TB | int) -> None:
-    a2 = attrs.evolve(b_or_t)   # E: Argument 1 to "evolve" has type "Union[TA, TB, int]" whose item "TB" is not bound to an attrs class  # E: Argument 1 to "evolve" has incompatible type "Union[TA, TB, int]" whose item "int" is not an attrs class
+    a2 = attrs.evolve(b_or_t)  # E: Argument 1 to "evolve" has type "Union[TA, TB, int]" whose item "TB" is not bound to an attrs class \
+                               # E: Argument 1 to "evolve" has incompatible type "Union[TA, TB, int]" whose item "int" is not an attrs class
 
 
 [builtins fixtures/plugin_attrs.pyi]
@@ -2216,7 +2219,8 @@ def h(t: TNone) -> None:
     _ = attrs.evolve(t, x=42)  # E: Argument 1 to "evolve" has a variable type "TNone" not bound to an attrs class
 
 def x(t: TUnion) -> None:
-    _ = attrs.evolve(t, x=42)  # E: Argument 1 to "evolve" has incompatible type "TUnion" whose item "str" is not an attrs class  # E: Argument 1 to "evolve" has incompatible type "TUnion" whose item "int" is not an attrs class
+    _ = attrs.evolve(t, x=42)  # E: Argument 1 to "evolve" has incompatible type "TUnion" whose item "str" is not an attrs class \
+                               # E: Argument 1 to "evolve" has incompatible type "TUnion" whose item "int" is not an attrs class
 
 [builtins fixtures/plugin_attrs.pyi]
 

test-data/unit/lib-stub/attrs/__init__.pyi

@@ -1,7 +1,14 @@
-from typing import TypeVar, overload, Callable, Any, Optional, Union, Sequence, Mapping, Generic
+from typing import TypeVar, overload, Callable, Any, Optional, Union, Sequence, Mapping, \
+    Protocol, ClassVar, Type
+from typing_extensions import TypeGuard
 
 from attr import Attribute as Attribute
 
+
+class AttrsInstance(Protocol):
+    __attrs_attrs__: ClassVar[Any]
+
+
 _T = TypeVar('_T')
 _C = TypeVar('_C', bound=type)
 
@@ -131,5 +138,5 @@ def field(
 
 def evolve(inst: _T, **changes: Any) -> _T: ...
 def assoc(inst: _T, **changes: Any) -> _T: ...
-
-def fields(cls: type) -> Any: ...
+def has(cls: type) -> TypeGuard[Type[AttrsInstance]]: ...
+def fields(cls: Type[AttrsInstance]) -> Any: ...