DispatchMesh fails when given an emtpy struct

Description

Call DispatchMesh in an amplification shader with an empty struct, and observe that there's disagreement in the compiler as to the size of the struct (0 vs 4).

Steps to Reproduce

dxc a.hlsl -T as_6_6 -E taskMain

struct S{};
[shader("amplification")]
[numthreads(1,1,1)]
void taskMain(uint tig_0 : SV_GROUPINDEX)
{
    S s;
    DispatchMesh(1U, 1U, 1U, s);
    return;
}

Actual Behavior

error: validation errors
a.hlsl:14:5: error: For amplification shader with entry 'taskMain', payload size 4 is greater than declared size of 0 bytes.
note: at 'call void @dx.op.dispatchMesh.struct.S(i32 173, i32 1, i32 1, i32 1, %struct.S* nonnull %1)' in block '#0' of function 'taskMain'.
Validation failed.

Environment

• DXC version

  $ dxc --version
  libdxcompiler.so: 1.7(dev;0-00000000)
  

• Host Operating System: NixOS

Start by running the shown dxc command with the minimal a.hlsl amplification shader and trace handling of DispatchMesh with the empty struct S. Identify where the compiler and validator derive the payload size; done means they agree and the reproduced validation error no longer occurs.