Fix type check for fast call path

checker/checker.go

@@ -331,7 +331,8 @@ func (v *visitor) FunctionNode(node *ast.FunctionNode) reflect.Type {
 			if !isInterface(fn) &&
 				fn.IsVariadic() &&
 				fn.NumIn() == inputParamsCount &&
-				fn.NumOut() == 1 {
+				fn.NumOut() == 1 &&
+				fn.Out(0).Kind() == reflect.Interface {
 				rest := fn.In(fn.NumIn() - 1) // function has only one param for functions and two for methods
 				if rest.Kind() == reflect.Slice && rest.Elem().Kind() == reflect.Interface {
 					node.Fast = true

expr_test.go

@@ -855,6 +855,10 @@ func TestExpr(t *testing.T) {
 			`map(filter(Tweets, {len(.Text) > 10}), {Format(.Date)})`,
 			[]interface{}{"23 Oct 17 18:30 UTC", "23 Oct 17 18:30 UTC"},
 		},
+		{
+			`Concat("a", 1, [])`,
+			`a1[]`,
+		},
 	}
 
 	for _, tt := range tests {
@@ -1140,6 +1144,14 @@ func (*mockEnv) Variadic(x string, xs ...int) []int {
 	return xs
 }
 
+func (*mockEnv) Concat(list ...interface{}) string {
+	out := ""
+	for _, e := range list {
+		out += fmt.Sprintf("%v", e)
+	}
+	return out
+}
+
 func (*mockEnv) Float(i interface{}) float64 {
 	switch t := i.(type) {
 	case int: