fix: handle default comm clause in select

_test/select10.go

@@ -0,0 +1,14 @@
+package main
+
+func main() {
+	c := make(chan string)
+	select {
+	case <-c:
+		println("unexpected")
+	default:
+	}
+	println("bye")
+}
+
+// Output:
+// bye

_test/select11.go

@@ -0,0 +1,16 @@
+package main
+
+func main() {
+	c := make(chan string)
+	select {
+	case <-c:
+		println("unexpected")
+	default:
+		println("nothing received")
+	}
+	println("bye")
+}
+
+// Output:
+// nothing received
+// bye

interp/cfg.go

@@ -204,7 +204,7 @@ func (interp *Interpreter) cfg(root *node, pkgID string) ([]*node, error) {
 
 		case commClause:
 			sc = sc.pushBloc()
-			if n.child[0].action == aAssign {
+			if len(n.child) > 0 && n.child[0].action == aAssign {
 				ch := n.child[0].child[1].child[0]
 				var typ *itype
 				if typ, err = nodeType(interp, sc, ch); err != nil {
@@ -827,7 +827,7 @@ func (interp *Interpreter) cfg(root *node, pkgID string) ([]*node, error) {
 			case typeSwichAssign(n) && len(n.child) > 1:
 				n.start = n.child[1].start
 			case len(n.child) == 0:
-				// empty case body: jump to switch node (exit node)
+				// Empty case body: jump to switch node (exit node).
 				n.start = n.anc.anc.anc
 			default:
 				n.start = n.child[0].start
@@ -838,10 +838,14 @@ func (interp *Interpreter) cfg(root *node, pkgID string) ([]*node, error) {
 
 		case commClause:
 			wireChild(n)
-			if len(n.child) > 1 {
-				n.start = n.child[1].start // Skip chan operation, performed by select
-			} else {
+			switch len(n.child) {
+			case 0:
+				sc.pop()
+				return
+			case 1:
 				n.start = n.child[0].start // default clause
+			default:
+				n.start = n.child[1].start // Skip chan operation, performed by select
 			}
 			n.lastChild().tnext = n.anc.anc // exit node is selectStmt
 			sc = sc.pop()
@@ -1344,14 +1348,15 @@ func (interp *Interpreter) cfg(root *node, pkgID string) ([]*node, error) {
 			var cur *node
 			for _, c := range n.child[0].child {
 				var an *node // channel init action node
-				c0 := c.child[0]
-				switch {
-				case c0.kind == exprStmt && len(c0.child) == 1 && c0.child[0].action == aRecv:
-					an = c0.child[0].child[0]
-				case c0.action == aAssign:
-					an = c0.lastChild().child[0]
-				case c0.kind == sendStmt:
-					an = c0.child[0]
+				if len(c.child) > 0 {
+					switch c0 := c.child[0]; {
+					case c0.kind == exprStmt && len(c0.child) == 1 && c0.child[0].action == aRecv:
+						an = c0.child[0].child[0]
+					case c0.action == aAssign:
+						an = c0.lastChild().child[0]
+					case c0.kind == sendStmt:
+						an = c0.child[0]
+					}
 				}
 				if an != nil {
 					if cur == nil {

interp/run.go

@@ -2543,33 +2543,40 @@ func _select(n *node) {
 	assignedValues := make([]func(*frame) reflect.Value, nbClause)
 	okValues := make([]func(*frame) reflect.Value, nbClause)
 	cases := make([]reflect.SelectCase, nbClause+1)
+	next := getExec(n.tnext)
 
 	for i := 0; i < nbClause; i++ {
-		switch c0 := n.child[i].child[0]; {
-		case len(n.child[i].child) > 1:
-			// The comm clause contains a channel operation and a clause body.
-			clause[i] = getExec(n.child[i].child[1].start)
-			chans[i], assigned[i], ok[i], cases[i].Dir = clauseChanDir(n.child[i])
-			chanValues[i] = genValue(chans[i])
-			if assigned[i] != nil {
-				assignedValues[i] = genValue(assigned[i])
-			}
-			if ok[i] != nil {
-				okValues[i] = genValue(ok[i])
-			}
-		case c0.kind == exprStmt && len(c0.child) == 1 && c0.child[0].action == aRecv:
-			// The comm clause has an empty body clause after channel receive.
-			chanValues[i] = genValue(c0.child[0].child[0])
-			cases[i].Dir = reflect.SelectRecv
-		case c0.kind == sendStmt:
-			// The comm clause as an empty body clause after channel send.
-			chanValues[i] = genValue(c0.child[0])
-			cases[i].Dir = reflect.SelectSend
-			assignedValues[i] = genValue(c0.child[1])
-		default:
-			// The comm clause has a default clause.
-			clause[i] = getExec(c0.start)
+		if len(n.child[i].child) == 0 {
+			// The comm clause is an empty default, exit select.
 			cases[i].Dir = reflect.SelectDefault
+			clause[i] = func(*frame) bltn { return next }
+		} else {
+			switch c0 := n.child[i].child[0]; {
+			case len(n.child[i].child) > 1:
+				// The comm clause contains a channel operation and a clause body.
+				clause[i] = getExec(n.child[i].child[1].start)
+				chans[i], assigned[i], ok[i], cases[i].Dir = clauseChanDir(n.child[i])
+				chanValues[i] = genValue(chans[i])
+				if assigned[i] != nil {
+					assignedValues[i] = genValue(assigned[i])
+				}
+				if ok[i] != nil {
+					okValues[i] = genValue(ok[i])
+				}
+			case c0.kind == exprStmt && len(c0.child) == 1 && c0.child[0].action == aRecv:
+				// The comm clause has an empty body clause after channel receive.
+				chanValues[i] = genValue(c0.child[0].child[0])
+				cases[i].Dir = reflect.SelectRecv
+			case c0.kind == sendStmt:
+				// The comm clause as an empty body clause after channel send.
+				chanValues[i] = genValue(c0.child[0])
+				cases[i].Dir = reflect.SelectSend
+				assignedValues[i] = genValue(c0.child[1])
+			default:
+				// The comm clause has a default clause.
+				clause[i] = getExec(c0.start)
+				cases[i].Dir = reflect.SelectDefault
+			}
 		}
 	}