Widen type to Any when isinstance(..., Any) is True (#3751)

Fixes #3474

mypy/checker.py

@@ -3233,6 +3233,8 @@ def get_isinstance_type(expr: Expression,
         elif isinstance(typ, Instance) and typ.type.fullname() == 'builtins.type':
             object_type = Instance(typ.type.mro[-1], [])
             types.append(TypeRange(object_type, is_upper_bound=True))
+        elif isinstance(typ, AnyType):
+            types.append(TypeRange(typ, is_upper_bound=False))
         else:  # we didn't see an actual type, but rather a variable whose value is unknown to us
             return None
     if not types:

test-data/unit/check-isinstance.test

@@ -1975,3 +1975,33 @@ def f() -> None:
 [typing fixtures/typing-full.pyi]
 [builtins fixtures/dict.pyi]
 [out]
+
+[case testIsinstanceWidensWithAnyArg]
+from typing import Any
+class A: ...
+B: Any
+x: A
+x.foo()  # E: "A" has no attribute "foo"
+assert isinstance(x, B)
+x.foo()
+reveal_type(x)  # E: Revealed type is 'Any'
+[builtins fixtures/isinstance.pyi]
+
+[case testIsinstanceWidensUnionWithAnyArg]
+from typing import Any, Union
+class A: ...
+B: Any
+x: Union[A, B]
+reveal_type(x)  # E: Revealed type is 'Union[__main__.A, Any]'
+assert isinstance(x, B)
+reveal_type(x)  # E: Revealed type is 'Any'
+[builtins fixtures/isinstance.pyi]
+
+[case testIsinstanceIgnoredImport]
+from typing import Union
+from foo import A  # type: ignore
+def f(x: Union[A, str]) -> None:
+    x.method_only_in_a()  # E: Item "str" of "Union[Any, str]" has no attribute "method_only_in_a"
+    if isinstance(x, A):
+        x.method_only_in_a()
+[builtins fixtures/isinstance.pyi]