Whitelist some equality checks under --strict-equality (#7302)

Fixes #7275

As I mentioned in the issue, we can just whitelist some pairs of classes.

Author: ilevkivskyi

mypy/checkexpr.py

@@ -39,6 +39,7 @@
 from mypy import types
 from mypy.sametypes import is_same_type
 from mypy.erasetype import replace_meta_vars, erase_type
+from mypy.maptype import map_instance_to_supertype
 from mypy.messages import MessageBuilder
 from mypy import message_registry
 from mypy.infer import infer_type_arguments, infer_function_type_arguments
@@ -71,6 +72,12 @@
 MAX_UNIONS = 5  # type: Final
 
 
+# Types considered safe for comparisons with --strict-equality due to known behaviour of __eq__.
+# NOTE: All these types are subtypes of AbstractSet.
+OVERLAPPING_TYPES_WHITELIST = ['builtins.set', 'builtins.frozenset',
+                               'typing.KeysView', 'typing.ItemsView']  # type: Final
+
+
 class TooManyUnions(Exception):
     """Indicates that we need to stop splitting unions in an attempt
     to match an overload in order to save performance.
@@ -2051,6 +2058,14 @@ def dangerous_comparison(self, left: Type, right: Type,
             # We need to special case bytes, because both 97 in b'abc' and b'a' in b'abc'
             # return True (and we want to show the error only if the check can _never_ be True).
             return False
+        if isinstance(left, Instance) and isinstance(right, Instance):
+            # Special case some builtin implementations of AbstractSet.
+            if (left.type.fullname() in OVERLAPPING_TYPES_WHITELIST and
+                    right.type.fullname() in OVERLAPPING_TYPES_WHITELIST):
+                abstract_set = self.chk.lookup_typeinfo('typing.AbstractSet')
+                left = map_instance_to_supertype(left, abstract_set)
+                right = map_instance_to_supertype(right, abstract_set)
+                return not is_overlapping_types(left.args[0], right.args[0])
         return not is_overlapping_types(left, right, ignore_promotions=False)
 
     def get_operator_method(self, op: str) -> str:

test-data/unit/pythoneval.test

@@ -1385,3 +1385,23 @@ def thing(stuff: StuffDict) -> int: ...
 
 [out]
 _testNewAnalyzerTypedDictInStub_newsemanal.py:2: note: Revealed type is 'def (stuff: TypedDict('stub.StuffDict', {'foo': builtins.str, 'bar': builtins.int})) -> builtins.int'
+
+[case testStrictEqualityWhitelist]
+# mypy: strict-equality
+{1} == frozenset({1})
+frozenset({1}) == {1}
+
+frozenset({1}) == [1]  # Error
+
+{1: 2}.keys() == {1}
+{1: 2}.keys() == frozenset({1})
+{1: 2}.items() == {(1, 2)}
+
+{1: 2}.keys() == {'no'}  # Error
+{1: 2}.values() == {2}  # Error
+{1: 2}.keys() == [1]  # Error
+[out]
+_testStrictEqualityWhitelist.py:5: error: Non-overlapping equality check (left operand type: "FrozenSet[int]", right operand type: "List[int]")
+_testStrictEqualityWhitelist.py:11: error: Non-overlapping equality check (left operand type: "KeysView[int]", right operand type: "Set[str]")
+_testStrictEqualityWhitelist.py:12: error: Non-overlapping equality check (left operand type: "ValuesView[int]", right operand type: "Set[int]")
+_testStrictEqualityWhitelist.py:13: error: Non-overlapping equality check (left operand type: "KeysView[int]", right operand type: "List[int]")