interp: fix default comm clause in select

Do not attempt to init a non-existent channel setting when in
default communication clause in select.

Fixes #1442.

_test/issue-1442.go

@@ -0,0 +1,41 @@
+package main
+
+import (
+	"context"
+)
+
+func main() {
+	ctx, _ := context.WithCancel(context.Background())
+	ch := make(chan string, 20)
+	defer close(ch)
+
+	go func(ctx context.Context, ch <-chan string) {
+		for {
+			select {
+			case <-ctx.Done():
+				return
+			case tmp := <-ch:
+				_ = tmp
+			}
+		}
+	}(ctx, ch)
+
+	for _, i := range "abcdef" {
+		for _, j := range "0123456789" {
+			// i, j := "a", "0"
+			for _, k := range "ABCDEF" {
+				select {
+				case <-ctx.Done():
+					return
+				default:
+					tmp := string(i) + string(j) + string(k)
+					ch <- tmp
+				}
+			}
+		}
+	}
+	return
+}
+
+// Output:
+//

_test/select1.go

@@ -10,11 +10,11 @@ func main() {
 	c2 := make(chan string)
 
 	go func() {
-		time.Sleep(1e7)
+		time.Sleep(1e8)
 		c1 <- "one"
 	}()
 	go func() {
-		time.Sleep(2e7)
+		time.Sleep(2e8)
 		c2 <- "two"
 	}()
 

_test/select14.go

@@ -6,8 +6,8 @@ import (
 )
 
 const (
-	period    = 100 * time.Millisecond
-	precision = 7 * time.Millisecond
+	period    = 300 * time.Millisecond
+	precision = 30 * time.Millisecond
 )
 
 func main() {

interp/cfg.go

@@ -1920,9 +1920,13 @@ func (interp *Interpreter) cfg(root *node, sc *scope, importPath, pkgName string
 			wireChild(n)
 			// Move action to block statement, so select node can be an exit point.
 			n.child[0].gen = _select
-			// Chain channel init actions in commClauses prior to invoke select.
+			// Chain channel init actions in commClauses prior to invoking select.
 			var cur *node
 			for _, c := range n.child[0].child {
+				if c.kind == commClauseDefault {
+					// No channel init in this case.
+					continue
+				}
 				var an, pn *node // channel init action nodes
 				if len(c.child) > 0 {
 					switch c0 := c.child[0]; {