Fix crash on strict-equality with recursive types (#16483)

Fixes #16473

Potentially we can turn this helper function into a proper visitor, but
I don't think it is worth it as of right now.

---------

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

mypy/checkexpr.py

@@ -3617,8 +3617,9 @@ def dangerous_comparison(
         self,
         left: Type,
         right: Type,
-        original_container: Type | None = None,
         *,
+        original_container: Type | None = None,
+        seen_types: set[tuple[Type, Type]] | None = None,
         prefer_literal: bool = True,
     ) -> bool:
         """Check for dangerous non-overlapping comparisons like 42 == 'no'.
@@ -3639,6 +3640,12 @@ def dangerous_comparison(
         if not self.chk.options.strict_equality:
             return False
 
+        if seen_types is None:
+            seen_types = set()
+        if (left, right) in seen_types:
+            return False
+        seen_types.add((left, right))
+
         left, right = get_proper_types((left, right))
 
         # We suppress the error if there is a custom __eq__() method on either
@@ -3694,17 +3701,21 @@ def dangerous_comparison(
                 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 self.dangerous_comparison(left.args[0], right.args[0])
+                return self.dangerous_comparison(
+                    left.args[0], right.args[0], seen_types=seen_types
+                )
             elif left.type.has_base("typing.Mapping") and right.type.has_base("typing.Mapping"):
                 # Similar to above: Mapping ignores the classes, it just compares items.
                 abstract_map = self.chk.lookup_typeinfo("typing.Mapping")
                 left = map_instance_to_supertype(left, abstract_map)
                 right = map_instance_to_supertype(right, abstract_map)
                 return self.dangerous_comparison(
-                    left.args[0], right.args[0]
-                ) or self.dangerous_comparison(left.args[1], right.args[1])
+                    left.args[0], right.args[0], seen_types=seen_types
+                ) or self.dangerous_comparison(left.args[1], right.args[1], seen_types=seen_types)
             elif left_name in ("builtins.list", "builtins.tuple") and right_name == left_name:
-                return self.dangerous_comparison(left.args[0], right.args[0])
+                return self.dangerous_comparison(
+                    left.args[0], right.args[0], seen_types=seen_types
+                )
             elif left_name in OVERLAPPING_BYTES_ALLOWLIST and right_name in (
                 OVERLAPPING_BYTES_ALLOWLIST
             ):

mypy/meet.py

@@ -262,6 +262,7 @@ def is_overlapping_types(
     ignore_promotions: bool = False,
     prohibit_none_typevar_overlap: bool = False,
     ignore_uninhabited: bool = False,
+    seen_types: set[tuple[Type, Type]] | None = None,
 ) -> bool:
     """Can a value of type 'left' also be of type 'right' or vice-versa?
 
@@ -275,18 +276,27 @@ def is_overlapping_types(
         # A type guard forces the new type even if it doesn't overlap the old.
         return True
 
+    if seen_types is None:
+        seen_types = set()
+    if (left, right) in seen_types:
+        return True
+    if isinstance(left, TypeAliasType) and isinstance(right, TypeAliasType):
+        seen_types.add((left, right))
+
     left, right = get_proper_types((left, right))
 
     def _is_overlapping_types(left: Type, right: Type) -> bool:
         """Encode the kind of overlapping check to perform.
 
-        This function mostly exists so we don't have to repeat keyword arguments everywhere."""
+        This function mostly exists, so we don't have to repeat keyword arguments everywhere.
+        """
         return is_overlapping_types(
             left,
             right,
             ignore_promotions=ignore_promotions,
             prohibit_none_typevar_overlap=prohibit_none_typevar_overlap,
             ignore_uninhabited=ignore_uninhabited,
+            seen_types=seen_types.copy(),
         )
 
     # We should never encounter this type.

test-data/unit/check-expressions.test

@@ -2378,6 +2378,38 @@ assert a == b
 [builtins fixtures/dict.pyi]
 [typing fixtures/typing-full.pyi]
 
+[case testStrictEqualityWithRecursiveMapTypes]
+# flags: --strict-equality
+from typing import Dict
+
+R = Dict[str, R]
+
+a: R
+b: R
+assert a == b
+
+R2 = Dict[int, R2]
+c: R2
+assert a == c  # E: Non-overlapping equality check (left operand type: "Dict[str, R]", right operand type: "Dict[int, R2]")
+[builtins fixtures/dict.pyi]
+[typing fixtures/typing-full.pyi]
+
+[case testStrictEqualityWithRecursiveListTypes]
+# flags: --strict-equality
+from typing import List, Union
+
+R = List[Union[str, R]]
+
+a: R
+b: R
+assert a == b
+
+R2 = List[Union[int, R2]]
+c: R2
+assert a == c
+[builtins fixtures/list.pyi]
+[typing fixtures/typing-full.pyi]
+
 [case testUnimportedHintAny]
 def f(x: Any) -> None:  # E: Name "Any" is not defined \
                         # N: Did you forget to import it from "typing"? (Suggestion: "from typing import Any")

test-data/unit/fixtures/list.pyi

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