From 4881754402d11dd8b7a4fccc359991e33fb2498b Mon Sep 17 00:00:00 2001
From: Charles Baker <charles.baker@sweetsoftware.co.nz>
Date: Sun, 16 Jul 2023 08:10:39 +1200
Subject: [PATCH] Avoid initializing Parser with invalid ParserStateMachine

When a grammar fails to generate a parser the ParserStateMachine has a
null start state.  When this is passed to construct a Parser quietly
avoid creating any parser specific state, esp. ParserNode which will
assert if passed a null state.
---
 src/lalr/Parser.ipp | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/src/lalr/Parser.ipp b/src/lalr/Parser.ipp
index dad237c..87bf9be 100644
--- a/src/lalr/Parser.ipp
+++ b/src/lalr/Parser.ipp
@@ -82,10 +82,13 @@ Parser<Iterator, UserData, Char, Traits, Allocator>::Parser( const ParserStateMa
         ++action;
     }
 
-    nodes_.reserve( 64 );       
-    user_data_.reserve( 64 );       
-    nodes_.push_back( ParserNode(state_machine_->start_state, nullptr, 0, 1) );
-    user_data_.push_back( UserData() );
+    if ( state_machine_->start_state )
+    {
+        nodes_.reserve( 64 );
+        user_data_.reserve( 64 );
+        nodes_.push_back( ParserNode(state_machine_->start_state, nullptr, 0, 1) );
+        user_data_.push_back( UserData() );
+    }
 }
 
 /**