Fix strict equality check if operand item type has custom __eq__ (#14513)

Don't complain about comparing lists, variable-length tuples or sets if
one of the operands has an item type with a custom `__eq__` method.

Fix #14511.

Author: JukkaL

mypy/checkexpr.py

@@ -2988,20 +2988,14 @@ def visit_comparison_expr(self, e: ComparisonExpr) -> Type:
                 # testCustomEqCheckStrictEquality for an example.
                 if not w.has_new_errors() and operator in ("==", "!="):
                     right_type = self.accept(right)
-                    # We suppress the error if there is a custom __eq__() method on either
-                    # side. User defined (or even standard library) classes can define this
-                    # to return True for comparisons between non-overlapping types.
-                    if not custom_special_method(
-                        left_type, "__eq__"
-                    ) and not custom_special_method(right_type, "__eq__"):
-                        # Also flag non-overlapping literals in situations like:
-                        #    x: Literal['a', 'b']
-                        #    if x == 'c':
-                        #        ...
-                        left_type = try_getting_literal(left_type)
-                        right_type = try_getting_literal(right_type)
-                        if self.dangerous_comparison(left_type, right_type):
-                            self.msg.dangerous_comparison(left_type, right_type, "equality", e)
+                    # Also flag non-overlapping literals in situations like:
+                    #    x: Literal['a', 'b']
+                    #    if x == 'c':
+                    #        ...
+                    left_type = try_getting_literal(left_type)
+                    right_type = try_getting_literal(right_type)
+                    if self.dangerous_comparison(left_type, right_type):
+                        self.msg.dangerous_comparison(left_type, right_type, "equality", e)
 
             elif operator == "is" or operator == "is not":
                 right_type = self.accept(right)  # validate the right operand
@@ -3064,6 +3058,12 @@ def dangerous_comparison(
 
         left, right = get_proper_types((left, right))
 
+        # We suppress the error if there is a custom __eq__() method on either
+        # side. User defined (or even standard library) classes can define this
+        # to return True for comparisons between non-overlapping types.
+        if custom_special_method(left, "__eq__") or custom_special_method(right, "__eq__"):
+            return False
+
         if self.chk.binder.is_unreachable_warning_suppressed():
             # We are inside a function that contains type variables with value restrictions in
             # its signature. In this case we just suppress all strict-equality checks to avoid
@@ -3094,14 +3094,18 @@ def dangerous_comparison(
             return False
         if isinstance(left, Instance) and isinstance(right, Instance):
             # Special case some builtin implementations of AbstractSet.
+            left_name = left.type.fullname
+            right_name = right.type.fullname
             if (
-                left.type.fullname in OVERLAPPING_TYPES_ALLOWLIST
-                and right.type.fullname in OVERLAPPING_TYPES_ALLOWLIST
+                left_name in OVERLAPPING_TYPES_ALLOWLIST
+                and right_name in OVERLAPPING_TYPES_ALLOWLIST
             ):
                 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 self.dangerous_comparison(left.args[0], right.args[0])
+            elif left_name in ("builtins.list", "builtins.tuple") and right_name == left_name:
+                return self.dangerous_comparison(left.args[0], right.args[0])
         if isinstance(left, LiteralType) and isinstance(right, LiteralType):
             if isinstance(left.value, bool) and isinstance(right.value, bool):
                 # Comparing different booleans is not dangerous.

test-data/unit/check-expressions.test

@@ -1985,6 +1985,24 @@ class B:
 A() == B()  # E: Unsupported operand types for == ("A" and "B")
 [builtins fixtures/bool.pyi]
 
+[case testStrictEqualitySequenceAndCustomEq]
+# flags: --strict-equality
+from typing import Tuple
+
+class C: pass
+class D:
+    def __eq__(self, other): return True
+
+a = [C()]
+b = [D()]
+a == b
+b == a
+t1: Tuple[C, ...]
+t2: Tuple[D, ...]
+t1 == t2
+t2 == t1
+[builtins fixtures/bool.pyi]
+
 [case testCustomEqCheckStrictEqualityOKInstance]
 # flags: --strict-equality
 class A:

test-data/unit/fixtures/bool.pyi

@@ -16,5 +16,5 @@ class float: pass
 class str: pass
 class unicode: pass
 class ellipsis: pass
-class list: pass
+class list(Generic[T]): pass
 class property: pass

test-data/unit/fixtures/set.pyi

@@ -6,6 +6,7 @@ T = TypeVar('T')
 
 class object:
     def __init__(self) -> None: pass
+    def __eq__(self, other): pass
 
 class type: pass
 class tuple(Generic[T]): pass