[SPIR-V] Variable cannot be replaced: invalid instruction - functor struct with no members variable not declared as static

[devshgraphicsprogramming]

Description
I've made a "functor" for accessing my scratch groupshared memory in templated algorithms:

groupshared uint32_t scratch[ScratchSz];

template<uint32_t offset>
struct ScratchProxy
{
	uint32_t get(const uint32_t ix)
	{
		return scratch[ix+offset];
	}

	void set(const uint32_t ix, const uint32_t value)
	{
		scratch[ix+offset] = value;
	}
};

ScratchProxy<0> arithmeticAccessor;

I can "fix" this in two ways:

• declare the artihmeticAccessor as static
• add a dummy member to ScratchProxy struct

I think this has something to do with the fact that all non-static globals map to a hidden UBO/ConstantBuffer, and this struct's sizeof() is 0

Would be nice for this case to either:

• produce a more intelligible and helpful error/diagnostic (e.g. "cannot use zero-sized structs in SSBO/UBO"
• compile successfully like DXIL output by noticing we don't actually have to store/load anything from $Globals

Steps to Reproduce
Compile this repro: https://godbolt.org/z/987efPv1b
with: -HV 202x -T cs_6_7 -fspv-target-env=vulkan1.3 -enable-16bit-types -fvk-use-scalar-layout -Wno-c++11-extensions -O0 -spirv

The -fvk-use-scalar-layout doesn't matter, fails either way.

Obviously it passes without -spirv and emits DXIL.

Actual Behavior

Fails compilation with

fatal error: failed to legalize SPIR-V: Variable cannot be replaced: invalid instruction
  OpEntryPoint GLCompute %main "main" %gl_GlobalInvocationID %output

Environment

• DXC version: latest & godbolt DXC trunk
• Host Operating System: Windows and Linux