Add extra check of correct eval usage

Author: antonmedv

expr.go

@@ -1,6 +1,7 @@
 package expr
 
 import (
+	"fmt"
 	"reflect"
 
 	"github.com/antonmedv/expr/checker"
@@ -13,6 +14,10 @@ import (
 
 // Eval parses, compiles and runs given input.
 func Eval(input string, env interface{}) (interface{}, error) {
+	if _, ok := env.(conf.Option); ok {
+		return nil, fmt.Errorf("misused expr.Eval: second argument (env) should be passed without expr.Env")
+	}
+
 	tree, err := parser.Parse(input)
 	if err != nil {
 		return nil, err

expr_test.go

@@ -903,3 +903,9 @@ type segment struct {
 	Destination string
 	Date        time.Time
 }
+
+func TestExpr_eval_with_env(t *testing.T) {
+	_, err := expr.Eval("true", expr.Env(map[string]interface{}{}))
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "misused")
+}