fwd: Make some errors more friendly

Author: lhmouse

asteria/compiler/statement_sequence.cpp

@@ -62,6 +62,11 @@ do_accept_identifier_opt(Token_Stream& tstrm, bool user_decl)
     if(!qtok)
       return nullopt;
 
+    if(user_decl && qtok->is_keyword())
+      throw Compiler_Error(xtc_format,
+                compiler_status_keyword_not_declarable, qtok->sloc(),
+                "Keyword `$1` not user-declarable", qtok->as_keyword_c_str());
+
     // See whether it is an identifier.
     if(!qtok->is_identifier())
       return nullopt;

asteria/fwd.cpp

@@ -376,6 +376,9 @@ describe_compiler_status(Compiler_Status status) noexcept
       case compiler_status_interval_closure_expected:
         return "`)` or `]` expected";
 
+      case compiler_status_keyword_not_declarable:
+        return "keyword not declarable";
+
       default:
         return "[unknown compiler status]";
     }

asteria/fwd.hpp

@@ -742,6 +742,7 @@ enum Compiler_Status : uint32_t
     compiler_status_multiple_default                           = 3004,
     compiler_status_duplicate_name_in_structured_binding       = 3005,
     compiler_status_duplicate_name_in_parameter_list           = 3006,
+    compiler_status_keyword_not_declarable                     = 3007,
   };
 
 ROCKET_CONST