Undefined value allowed for buffer load index

This shader compiles without warning or error even though the load address is undef.

// dxc /Tps_6_0 t.hlsl
ByteAddressBuffer b;

[RootSignature("UAV(u0), SRV(t0)")]
float main(uint a : A) : SV_Target
{
    uint X;
    return b.Load(X);
}

The generated dxil has undef for the index

call %dx.types.ResRet.i32 @dx.op.bufferLoad.i32(i32 68, %dx.types.Handle %1, i32 undef, i32 undef)

FXC will produce an error for this shader

t.hlsl(7,12-20): warning X4000: use of potentially uninitialized variable (X)
t.hlsl(7,12-20): error X4575: reading uninitialized value

Start by reproducing the shader with the shown dxc command and compare its diagnostics with FXC's X4000 and X4575 messages. Trace the compiler's handling of the uninitialized buffer-load index; done means dxc rejects this shader instead of emitting DXIL with an undef index.