Do not test for built-in's when operands are not built-in.

src/fn_call.rs

@@ -289,13 +289,18 @@ impl Engine {
         }
 
         // See if it is built in.
-        if args.len() == 2 {
+        if args.len() == 2 && !args[0].is_variant() && !args[1].is_variant() {
             match run_builtin_binary_op(fn_name, args[0], args[1])? {
                 Some(v) => return Ok((v, false)),
                 None => (),
             }
 
-            if is_ref && fn_name.ends_with('=') {
+            // Op-assignment?
+            if is_ref {
+                match fn_name {
+                    _ if fn_name.len() <= 1 => (),
+                    "==" | "!=" | ">=" | "<=" => (),
+                    _ if fn_name.ends_with('=') => {
                         let (first, second) = args.split_first_mut().unwrap();
 
                         match run_builtin_op_assignment(fn_name, first, second[0])? {
@@ -303,6 +308,9 @@ impl Engine {
                             None => (),
                         }
                     }
+                    _ => (),
+                }
+            }
         }
 
         // Return default value (if any)

src/token.rs

@@ -1659,25 +1659,25 @@ pub fn is_valid_identifier(name: impl Iterator<Item = char>) -> bool {
 
 #[cfg(feature = "unicode-xid-ident")]
 #[inline(always)]
-fn is_id_first_alphabetic(x: char) -> bool {
+pub fn is_id_first_alphabetic(x: char) -> bool {
     unicode_xid::UnicodeXID::is_xid_start(x)
 }
 
 #[cfg(feature = "unicode-xid-ident")]
 #[inline(always)]
-fn is_id_continue(x: char) -> bool {
+pub fn is_id_continue(x: char) -> bool {
     unicode_xid::UnicodeXID::is_xid_continue(x)
 }
 
 #[cfg(not(feature = "unicode-xid-ident"))]
 #[inline(always)]
-fn is_id_first_alphabetic(x: char) -> bool {
+pub fn is_id_first_alphabetic(x: char) -> bool {
     x.is_ascii_alphabetic()
 }
 
 #[cfg(not(feature = "unicode-xid-ident"))]
 #[inline(always)]
-fn is_id_continue(x: char) -> bool {
+pub fn is_id_continue(x: char) -> bool {
     x.is_ascii_alphanumeric() || x == '_'
 }