Test that `next' is only used inside a decorator.

internal/vm/checker/checker.go

@@ -230,11 +230,18 @@ func (c *checker) VisitAfter(node ast.Node) ast.Node {
 		return n
 
 	case *ast.NextStmt:
-		// The last element in this list will be the empty stack created by the DecoDecl on the way in.
+		// The last element in this list will be the empty stack created by the
+		// DecoDecl on the way in.  If there's no last element, then we can't
+		// have entered a DecoDecl yet.
 		last := len(c.decoScopes) - 1
+		if last < 0 {
+			c.errors.Add(n.Pos(), fmt.Sprintf("Can't use `next' outside of a decorator."))
+			return n
+		}
 		decoScope := c.decoScopes[last]
 		if len(decoScope.Symbols) > 0 {
 			c.errors.Add(n.Pos(), fmt.Sprintf("Can't use `next' statement twice in a decorator."))
+			return n
 		}
 		// Merge the current scope into it.
 		decoScope.CopyFrom(c.scope)

internal/vm/checker/checker_test.go

@@ -167,6 +167,16 @@ const ID /bar/
 foo = $1
 }`,
 		[]string{"counter with buckets:1:9-11: Can't specify buckets for non-histogram metric `foo'."}},
+
+	{"next outside of decorator",
+		`def x{
+next
+}
+@x {
+next
+}
+`,
+		[]string{"next outside of decorator:5:1-4: Can't use `next' outside of a decorator."}},
 }
 
 func TestCheckInvalidPrograms(t *testing.T) {