Added etl::monostate as a separate header monostate.h so that it can be used without have to include variant.h

Author: John Wellbelove

include/etl/monostate.h

@@ -0,0 +1,46 @@
+///\file
+
+/******************************************************************************
+The MIT License(MIT)
+
+Embedded Template Library.
+https://github.com/ETLCPP/etl
+https://www.etlcpp.com
+
+Copyright(c) 2025 jwellbelove
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files(the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and / or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions :
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+******************************************************************************/
+
+#ifndef ETL_MONOSTATE_INCLUDED
+#define ETL_MONOSTATE_INCLUDED
+
+#include "platform.h"
+
+namespace etl
+{
+  //***************************************************************************
+  /// A 'no-value' placeholder.
+  //***************************************************************************
+  struct monostate
+  {
+  };
+}
+
+#endif

include/etl/private/variant_legacy.h

@@ -40,6 +40,7 @@ SOFTWARE.
 #include "../error_handler.h"
 #include "../null_type.h"
 #include "../placement_new.h"
+#include "../monostate.h"
 
 #include <stdint.h>
 
@@ -75,9 +76,7 @@ namespace etl
     /// Monostate for variants.
     ///\ingroup variant
     //***************************************************************************
-    struct monostate
-    {
-    };
+    typedef etl::monostate monostate;
 
     //***************************************************************************
     /// Base exception for the variant class.

include/etl/private/variant_variadic.h

@@ -44,6 +44,7 @@ SOFTWARE.
 #include "../memory.h"
 #include "../compare.h"
 #include "../initializer_list.h"
+#include "../monostate.h"
 
 #include <stdint.h>
 
@@ -304,14 +305,6 @@ namespace etl
   #include "variant_select_do_operator.h"
 #endif
 
-  //***************************************************************************
-  /// Monostate for variants.
-  ///\ingroup variant
-  //***************************************************************************
-  struct monostate
-  {
-  };
-
   constexpr bool operator >(etl::monostate, etl::monostate) noexcept { return false; }
 	constexpr bool operator <(etl::monostate, etl::monostate) noexcept { return false; }
 	constexpr bool operator !=(etl::monostate, etl::monostate) noexcept { return false; }

test/vs2022/etl.vcxproj

@@ -3173,6 +3173,7 @@
     <ClInclude Include="..\..\include\etl\lcm.h" />
     <ClInclude Include="..\..\include\etl\math.h" />
     <ClInclude Include="..\..\include\etl\message_broker.h" />
+    <ClInclude Include="..\..\include\etl\monostate.h" />
     <ClInclude Include="..\..\include\etl\poly_span.h" />
     <ClInclude Include="..\..\include\etl\private\bitset_legacy.h" />
     <ClInclude Include="..\..\include\etl\private\bitset_new.h" />

test/vs2022/etl.vcxproj.filters

@@ -1440,6 +1440,9 @@
     <ClInclude Include="..\..\include\etl\private\variant_select_do_operator.h">
       <Filter>ETL\Private</Filter>
     </ClInclude>
+    <ClInclude Include="..\..\include\etl\monostate.h">
+      <Filter>ETL\Utilities</Filter>
+    </ClInclude>
   </ItemGroup>
   <ItemGroup>
     <ClCompile Include="..\test_string_char.cpp">