feat(api): support passing Deferreds as predicates to joins

resolves ibis-project#10703

ibis/backends/tests/test_join.py

@@ -398,3 +398,20 @@ def test_positional_join(backend, con):
     result = con.execute(j)
     expected = pd.DataFrame({"x": [1, 2, 3], "x_right": [3, 2, 1]})
     backend.assert_frame_equal(result, expected)
+
+
+@pytest.mark.parametrize(
+    "preds",
+    [
+        pytest.param(ibis._.x.upper(), id="single"),
+        pytest.param([ibis._.x.upper()], id="single-list"),
+        pytest.param([ibis._.x.upper(), ibis._.x.lower()], id="multiple"),
+    ],
+)
+def test_join_deferred(backend: BackendTest, con, preds):
+    t1 = ibis.memtable({"x": ["a", "b", "c"]})
+    t2 = ibis.memtable({"x": ["A", "B", "Z"]})
+    j = t1.inner_join(t2, predicates=preds)
+    result = con.execute(j)
+    expected = pd.DataFrame({"x": ["a", "b"], "x_right": ["A", "B"]})
+    backend.assert_frame_equal(result, expected)

ibis/expr/types/joins.py

@@ -7,7 +7,6 @@
 
 import ibis.expr.operations as ops
 from ibis import util
-from ibis.common.deferred import Deferred
 from ibis.common.egraph import DisjointSet
 from ibis.common.exceptions import (
     ExpressionError,
@@ -168,7 +167,7 @@ def prepare_predicates(
 
     left, right = chain.to_expr(), right.to_expr()
     for pred in util.promote_list(predicates):
-        if isinstance(pred, (Value, Deferred, bool)):
+        if isinstance(pred, (Value, bool)):
             for bound in bind(left, pred):
                 yield deref_both.dereference(bound.op())
         else: