fix(optimizer): wrap merged results as predicates

optimizer/fold.go

@@ -296,10 +296,12 @@ func (fold *fold) Visit(node *Node) {
 					Name: "filter",
 					Arguments: []Node{
 						base.Arguments[0],
-						&BinaryNode{
-							Operator: "&&",
-							Left:     base.Arguments[1].(*PredicateNode).Node,
-							Right:    n.Arguments[1].(*PredicateNode).Node,
+						&PredicateNode{
+							Node: &BinaryNode{
+								Operator: "&&",
+								Left:     base.Arguments[1].(*PredicateNode).Node,
+								Right:    n.Arguments[1].(*PredicateNode).Node,
+							},
 						},
 					},
 				})

optimizer/fold_test.go

@@ -70,8 +70,10 @@ func TestOptimize_constant_folding_filter_filter(t *testing.T) {
 					Value: 2,
 				},
 			},
-			&ast.BoolNode{
-				Value: true,
+			&ast.PredicateNode{
+				Node: &ast.BoolNode{
+					Value: true,
+				},
 			},
 		},
 		Throws: false,

test/issues/857/issue_test.go

@@ -0,0 +1,39 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/expr-lang/expr"
+	"github.com/expr-lang/expr/internal/testify/require"
+)
+
+func TestIssue857(t *testing.T) {
+	foo := map[string]any{
+		"entry": map[string]any{
+			"alpha": "x",
+			"beta":  1,
+		},
+	}
+	bar := map[string]any{
+		"entry": map[string]any{
+			"alpha": "x",
+			"beta":  1,
+		},
+	}
+
+	env := map[string]any{
+		"foo": foo,
+		"bar": bar,
+	}
+
+	code := `
+		foo
+		| keys()
+		| filter(# in bar)
+		| filter(foo[#].alpha == bar[#].alpha)
+		| filter(foo[#].beta == bar[#].beta)
+	`
+
+	_, err := expr.Compile(code, expr.Env(env))
+	require.NoError(t, err)
+}