semantic check: identify boolean udfs (#4886)

Update the semantic analyzer isBool function to identify user-defined
functions that return a boolean value.

compiler/semantic/op.go

@@ -1014,7 +1014,7 @@ func (a *analyzer) semOpExpr(e ast.Expr, seq dag.Seq) (dag.Seq, error) {
 	if err != nil {
 		return nil, err
 	}
-	if isBool(out) {
+	if a.isBool(out) {
 		return append(seq, dag.NewFilter(out)), nil
 	}
 	return append(seq, &dag.Yield{
@@ -1023,12 +1023,12 @@ func (a *analyzer) semOpExpr(e ast.Expr, seq dag.Seq) (dag.Seq, error) {
 	}), nil
 }
 
-func isBool(e dag.Expr) bool {
+func (a *analyzer) isBool(e dag.Expr) bool {
 	switch e := e.(type) {
 	case *dag.Literal:
 		return e.Value == "true" || e.Value == "false"
 	case *dag.UnaryExpr:
-		return isBool(e.Operand)
+		return a.isBool(e.Operand)
 	case *dag.BinaryExpr:
 		switch e.Op {
 		case "and", "or", "in", "==", "!=", "<", "<=", ">", ">=":
@@ -1037,8 +1037,12 @@ func isBool(e dag.Expr) bool {
 			return false
 		}
 	case *dag.Conditional:
-		return isBool(e.Then) && isBool(e.Else)
+		return a.isBool(e.Then) && a.isBool(e.Else)
 	case *dag.Call:
+		// If udf recurse to inner expression.
+		if f, _ := a.scope.LookupExpr(e.Name); f != nil {
+			return a.isBool(f.(*dag.Func).Expr)
+		}
 		if e.Name == "cast" {
 			if len(e.Args) != 2 {
 				return false

compiler/ztests/udf-implied-where.yaml

@@ -0,0 +1,14 @@
+script: |
+  zc -C -s 'func h(e): ( has(e) ) h(foo)'
+
+outputs:
+  - name: stdout
+    data: |
+      reader
+      | (
+        func h(e): (
+          has(e)
+        )
+        
+        where h(foo)
+      )