[SPIR-V] Cannot cast initializer type 'int' into variable type 'RaytracingAccelerationStructure'

Description
My usage of loading RaytracingAccelerationStructure from ResourceDescriptorHeap doesn't compile in Spirv mode, but it does in regular dx12 mode when using templated casting in a very specific syntax.

Steps to Reproduce
This fails to compile with the error message: error: Cannot cast initializer type 'int' into variable type 'RaytracingAccelerationStructure' T operator[](uint index) { return T(ResourceDescriptorHeap[index]); }

template<typename T>
struct BindlessResource
{
	T operator[](uint index) { return T(ResourceDescriptorHeap[index]); }
};
static const BindlessResource<RaytracingAccelerationStructure> bindless_accelerationstructures;

The workaround is to have different casting syntax:

template<typename T>
struct BindlessResource
{
	T operator[](uint index) { return (T)ResourceDescriptorHeap[index]; }
};
static const BindlessResource<RaytracingAccelerationStructure> bindless_accelerationstructures;

Environment

• DXC version version: 1.9 (2025 last release)
• Host Operating System: Windows 11

Start by reproducing the templated BindlessResource case in DXC's SPIR-V mode, using ResourceDescriptorHeap and RaytracingAccelerationStructure as shown in the issue. Trace the handling of constructor-style casts versus C-style casts, then confirm that the constructor form compiles without breaking the regular DX12 behavior.