obj: Handle user instance hash based on Python adhoc rules.

User instances are hashable by default (using __hash__ inherited from
"object"). But if __eq__ is defined and __hash__ not defined in particular
class, instance is not hashable.

py/obj.c

@@ -166,20 +166,34 @@ mp_int_t mp_obj_hash(mp_obj_t o_in) {
         return mp_obj_tuple_hash(o_in);
     } else if (MP_OBJ_IS_TYPE(o_in, &mp_type_type)) {
         return (mp_int_t)o_in;
-    } else if (MP_OBJ_IS_OBJ(o_in)) {
+    } else if (mp_obj_is_instance_type(mp_obj_get_type(o_in))) {
         // if a valid __hash__ method exists, use it
-        mp_obj_t hash_method[2];
-        mp_load_method_maybe(o_in, MP_QSTR___hash__, hash_method);
-        if (hash_method[0] != MP_OBJ_NULL) {
-            mp_obj_t hash_val = mp_call_method_n_kw(0, 0, hash_method);
+        mp_obj_t method[2];
+        mp_load_method_maybe(o_in, MP_QSTR___hash__, method);
+        if (method[0] != MP_OBJ_NULL) {
+            mp_obj_t hash_val = mp_call_method_n_kw(0, 0, method);
             if (MP_OBJ_IS_INT(hash_val)) {
                 return mp_obj_int_get_truncated(hash_val);
             }
+            goto error;
         }
+
+        mp_load_method_maybe(o_in, MP_QSTR___eq__, method);
+        if (method[0] == MP_OBJ_NULL) {
+            // https://docs.python.org/3/reference/datamodel.html#object.__hash__
+            // "User-defined classes have __eq__() and __hash__() methods by default;
+            // with them, all objects compare unequal (except with themselves) and 
+            // x.__hash__() returns an appropriate value such that x == y implies
+            // both that x is y and hash(x) == hash(y)."
+            return (mp_int_t)o_in;
+        }
+        // "A class that overrides __eq__() and does not define __hash__() will have its __hash__() implicitly set to None.
+        // When the __hash__() method of a class is None, instances of the class will raise an appropriate TypeError"
     }
 
-    // TODO hash class and instances - in CPython by default user created classes' __hash__ resolves to their id
+    // TODO hash classes
 
+error:
     if (MICROPY_ERROR_REPORTING == MICROPY_ERROR_REPORTING_TERSE) {
         nlr_raise(mp_obj_new_exception_msg(&mp_type_TypeError, "unhashable type"));
     } else {

tests/basics/builtin_hash.py

@@ -19,3 +19,16 @@ class A:
 
 print(hash(A()))
 print({A():1})
+
+class B:
+    pass
+hash(B())
+
+
+class C:
+    def __eq__(self, another):
+        return True
+try:
+    hash(C())
+except TypeError:
+    print("TypeError")