Try swapping args to __eq__ on NotImplemented

src/value.c

@@ -201,7 +201,18 @@ int krk_valuesEqual(KrkValue a, KrkValue b) {
 		krk_push(b);
 		KrkValue result = krk_callSimple(OBJECT_VAL(type->_eq),2,0);
 		if (IS_BOOLEAN(result)) return AS_BOOLEAN(result);
-		return 0;
+		if (IS_NOTIMPL(result)) goto _next;
+		return !krk_isFalsey(result);
+	}
+
+_next:
+	type = krk_getType(b);
+	if (type && type->_eq) {
+		krk_push(b);
+		krk_push(a);
+		KrkValue result = krk_callSimple(OBJECT_VAL(type->_eq),2,0);
+		if (IS_BOOLEAN(result)) return AS_BOOLEAN(result);
+		return !krk_isFalsey(result);
 	}
 
 	return 0;

src/vm.c

@@ -1025,6 +1025,7 @@ int krk_isFalsey(KrkValue value) {
 		case KRK_VAL_NONE: return 1;
 		case KRK_VAL_BOOLEAN: return !AS_BOOLEAN(value);
 		case KRK_VAL_INTEGER: return !AS_INTEGER(value);
+		case KRK_VAL_NOTIMPL: return 1;
 		case KRK_VAL_OBJECT: {
 			switch (AS_OBJECT(value)->type) {
 				case KRK_OBJ_STRING: return !AS_STRING(value)->codesLength;

test/testEqInvert.krk

@@ -0,0 +1,9 @@
+class Foo():
+    def __init__(self, tag):
+        self.tag = tag
+    def __eq__(self, other):
+        print(self.tag,'==',other.tag)
+        return NotImplemented
+
+print(Foo('a') == Foo('b'))
+print(Foo('a') != Foo('b'))

test/testEqInvert.krk.expect

@@ -0,0 +1,6 @@
+a == b
+b == a
+False
+a == b
+b == a
+True