Disallow parsing 'and' and 'or' as commands

Except for and --help and or --help

Fixes #6089

src/parse_productions.cpp

@@ -314,6 +314,14 @@ RESOLVE(block_header) {
 }
 
 RESOLVE(decorated_statement) {
+    // and/or are typically parsed in job_conjunction at the beginning of a job
+    // However they may be reached here through e.g. true && and false.
+    // Refuse to parse them as a command except for --help. See #6089.
+    if ((token1.keyword == parse_keyword_and || token1.keyword == parse_keyword_or) &&
+        !token2.is_help_argument) {
+        return NO_PRODUCTION;
+    }
+
     // If this is e.g. 'command --help' then the command is 'command' and not a decoration. If the
     // second token is not a string, then this is a naked 'command' and we should execute it as
     // undecorated.

src/parse_tree.cpp

@@ -156,6 +156,7 @@ static wcstring token_type_user_presentable_description(
 
     switch (type) {
         // Hackish. We only support the following types.
+        case symbol_decorated_statement:
         case symbol_statement:
             return L"a command";
         case symbol_argument: