Fix bug around improperly upcasting during ==

Author: saulshanabrook

docs/changelog.md

@@ -5,6 +5,7 @@ _This project uses semantic versioning_
 ## UNRELEASED
 
 - Change builtins to not evaluate values in egraph and changes facts to compare structural equality instead of using an egraph when converting to a boolean, removing magic context (`EGraph.current` and `Schedule.current`) that was added in release 9.0.0.
+- Fix bug that improperly upcasted values for ==
 
 ## 9.0.1 (2025-03-20)
 

python/egglog/conversion.py

@@ -149,6 +149,12 @@ def min_convertable_tp(a: object, b: object, name: str) -> JustTypeRef:
     decls = _retrieve_conversion_decls()
     a_tp = _get_tp(a)
     b_tp = _get_tp(b)
+    # Make sure at least one of the types has the method, to avoid issue with == upcasting improperly
+    if not (
+        (isinstance(a_tp, JustTypeRef) and decls.has_method(a_tp.name, name))
+        or (isinstance(b_tp, JustTypeRef) and decls.has_method(b_tp.name, name))
+    ):
+        raise ConvertError(f"Neither {a_tp} nor {b_tp} has method {name}")
     a_converts_to = {
         to: c for ((from_, to), (c, _)) in CONVERSIONS.items() if from_ == a_tp and decls.has_method(to.name, name)
     }

python/tests/test_high_level.py

@@ -839,6 +839,25 @@ def test_eq_false(self):
         assert not (i64(3) == 4)  # noqa: SIM201
 
 
+def test_no_upcast_eq():
+    """
+    Verifies that if two items can be upcast to something, calling == on them won't use
+    equality
+    """
+
+    class A(Expr):
+        def __init__(self) -> None: ...
+
+    class B(Expr):
+        def __init__(self) -> None: ...
+        def __eq__(self, other: B) -> B: ...  # type: ignore[override]
+
+    converter(A, B, lambda a: B())
+
+    assert isinstance(A() == A(), Fact)
+    assert not isinstance(B() == B(), Fact)
+
+
 EXAMPLE_FILES = list((pathlib.Path(__file__).parent / "../egglog/examples").glob("*.py"))