[SPIR-V] 'vk::SpirvOpaqueType<NNNN>' is an object and cannot be used as a type parameter` a.k.a. SpirvType as `T` of SpirvType

Description

Follow up to https://github.com/microsoft/DirectXShaderCompiler/issues/5092 as requested by @s-perron

Steps to Reproduce

Declare one SpirvOpaqueType (or even SpirvType) and shove it in another opaque type, examples:

• image type, inside combined sampler image

template<typename SampledType, spv::Dim dim, uint32_t Depth, uint32_t Arrayed, uint32_t MS, uint32_t Sampled, spv::ImageFormat Format=spv::ImageFormatUnknown, spv::AccessQualifier AccessQualifier=spv::AccessQualifierMax>
using image = vk::SpirvOpaqueType<
    /*spv::OpTypeImage*/ 25,
    SampledType,
    vk::Literal<vk::integral_constant<uint,dim> >,
    vk::Literal<vk::integral_constant<uint,Depth> >,
    vk::Literal<vk::integral_constant<uint,Arrayed> >,
    vk::Literal<vk::integral_constant<uint,MS> >,
    vk::Literal<vk::integral_constant<uint,Sampled> >,
    vk::Literal<vk::integral_constant<uint,Format> >
// TODO: use `nbl::hlsl::conditional_t<AccessQualifier==Max` to choose between two SpirvOpaqueTypes
>;

template<typename Image>
using sampledImage = vk::SpirvOpaqueType</*spv::OpTypeSampledImage*/ 27,Image>;

// %type_2d_image = OpTypeImage %float 2D 2 0 0 1 Unknown
// %type_sampled_image = OpTypeSampledImage %type_2d_image
// %_ptr_UniformConstant_type_2d_image_0 = OpTypePointer UniformConstant %type_2d_image_0
using sampler2D = spirv::pointer<spv::StorageClassUniformConstant,spirv::sampledImage<spirv::image<float,spv::Dim2D,2,0,0,1> > >;

• Acceleration Structure Type inside pointer type

using pointer_t = vk::SpirvOpaqueType<spv::OpTypePointer,vk::Literal<vk::integral_constant<uint32_t,StorageClass> >,T>;

using RayQueryKHR = vk::SpirvOpaqueType<spv::OpTypeRayQueryKHR>;

using RayQueryEXT = pointer_t<spv::StorageClassFunction,RayQueryKHR>;

• obtaining a Spir-V pointer of a HLSL object / casting to native Spir-V type

using pointer_t = vk::SpirvOpaqueType<spv::OpTypePointer,vk::Literal<vk::integral_constant<uint32_t,StorageClass> >,T>;

template<uint32_t StorageClass, typename T, typename U>
[[vk::ext_instruction(spv::OpCopyObject)]]
pointer_t<StorageClass,T> copyObject([[vk::ext_reference]] U v);

Actual Behavior

Always some variation of

vk::SpirvOpaqueType<NNNN>' is an object and cannot be used as a type parameter

Environment

• DXC version: commit ac36a797d3470e8ee906b98457a59270d01db30d
• Host Operating System: Windows

Start by reproducing the diagnostic with the nested image, acceleration-structure pointer, and OpCopyObject examples in the issue using DXC commit ac36a797d3470e8ee906b98457a59270d01db30d. Trace how SpirvOpaqueType arguments are classified during compilation; done means nested SpirvOpaqueType and SpirvType arguments are accepted without the object/type-parameter diagnostic.