Fix builtin $var expansion

A special case added for #1252 needed adjustment.

Fixes #5639

src/parse_util.cpp

@@ -1195,9 +1195,10 @@ static bool detect_errors_in_plain_statement(const wcstring &buff_src,
 
         // Check that we don't do an invalid builtin (issue #1252).
         if (!errored && decoration == parse_statement_decoration_builtin &&
-            !builtin_exists(command)) {
+            expand_one(*unexp_command, 0, null_environment_t{}, parse_errors) &&
+            !builtin_exists(*unexp_command)) {
             errored = append_syntax_error(parse_errors, source_start, UNKNOWN_BUILTIN_ERR_MSG,
-                                          command.c_str());
+                                          unexp_command->c_str());
         }
     }
     return errored;

tests/vars_as_commands.in

@@ -9,6 +9,9 @@ $CMD1
 set CMD2 echo '(' not expanded again
 $CMD2
 
+# Test using variables with the builtin decorator
+builtin $CMD1
+
 # Test implicit cd
 set CMD3 /usr/bin
 $CMD3 && echo $PWD

tests/vars_as_commands.out

@@ -1,3 +1,4 @@
 basic command as variable
 ( not expanded again
+basic command as variable
 /usr/bin