For NS_IMPLEMENT_REFLECTION2, 'helper' is not used, fix is adding '[[maybe_unused]]':

NS_COLD_FUNC void classType::StaticFillClassType([[maybe_unused]] Noesis::TypeClassCreator& helper)

Same with NS_IMPLEMENT_INLINE_REFLECTION3:

NS_COLD_FUNC static void StaticFillClassType([[maybe_unused]] Noesis::TypeClassCreator& helper)

Thanks!

Also getting an issue in non-debug Clang build

15>C:\_Development\Repositories\rtg\libs\NoesisUtils\Private\App.Providers.cpp(13,1): error : unexpected argument 's' to '#pragma optimize'; expected "" [-Werror,-Wignored-pragmas]
15>C:\_Development\Repositories\rtg\third_party\Noesis\Include\NsCore\CompilerSettings.h(64,47): note: expanded from macro 'NS_BEGIN_COLD_REGION'
15>C:\_Development\Repositories\rtg\libs\NoesisUtils\Private\Behavior.cpp(38,1): error : unexpected argument 's' to '#pragma optimize'; expected "" [-Werror,-Wignored-pragmas]
15>C:\_Development\Repositories\rtg\third_party\Noesis\Include\NsCore\CompilerSettings.h(64,47): note: expanded from macro 'NS_BEGIN_COLD_REGION'

Temp fix:
Another:

2>C:\_Development\Repositories\rtg\libs\NoesisUtils\Private\LocExtension.cpp(125,22): error : inequality comparison result unused [-Werror,-Wunused-comparison]
2>C:\_Development\Repositories\rtg\third_party\Noesis\Include\NsCore\Error.h(65,41): note: expanded from macro 'NS_CHECK'
2>C:\_Development\Repositories\rtg\third_party\Noesis\Include\NsCore\CompilerSettings.h(58,60): note: expanded from macro 'NS_UNUSED'
2>C:\_Development\Repositories\rtg\libs\NoesisUtils\Private\LocExtension.cpp(125,22): note: use '|=' to turn this inequality comparison into an or-assignment

Okay, I think I know what's going on here. I'm using Visual Studio and Clang together, i.e. I generate my CMake solution for Visual Studio using the following:
This means that when compiling, the following two statements are true
So it might be worth trying that build path, as it ships with VS and uses the Microsoft-provided clang-cl (if you tick the right installer option), thus some of the code in e.g. CompilerSettings.h has issues.

For NS_IMPLEMENT_REFLECTION2, 'helper' is not used, fix is adding '[[maybe_unused]]':
Same with NS_IMPLEMENT_INLINE_REFLECTION3:

For cases where the reflection block is empty, we provide NS_IMPLEMENT_REFLECTION_ and NS_IMPLEMENT_INLINE_REFLECTION_. These macros are provided in a separate header (ReflectionImplementEmpty.h) to optimize compiling in this specific scenarios.
We still don't support C++17, but definitely [maybe_unused] could be used here. Could you please create a ticket for this?

So it might be worth trying that build path, as it ships with VS and uses the Microsoft-provided clang-cl (if you tick the right installer option), thus some of the code in e.g. CompilerSettings.h has issues.

Could you please also report this? So far we haven't tried Visual Studio with clang.