Expr -> Symbol MethodError when combining mapslices and reshapes

I'm trying to write an absolute minimal amount of code to implement a multi-headed self-attention layer. I want to try to do this with TensorCast.jl, both to learn the syntax better, and perhaps as a nice demo of ML in Julia.

Right now I am trying to compute the query matrix in one go. This works:

batch = 4
length = 100
m = 32

# Data:
x = randn(Float32, length, m, batch)

heads = 10

# Layer to compute Q for all heads:
Q = Dense(m, m*heads)

# Computation:
@cast q1[ℓ,ch,n] := Q(x[ℓ,:,n])[ch]
@cast q2[ℓ,c,h,n] := q1[ℓ,(c,h),n] (h in 1:heads)

However, if I try to combine them in one go:

@cast q2[ℓ,c,h,n] := Q(x[ℓ,:,n])[(c,h)] (h in 1:heads)

it gives me the error:

ERROR: LoadError: MethodError: Cannot `convert` an object of type Expr to an object of type Symbol

Closest candidates are:
  convert(::Type{T}, ::T) where T
   @ Base Base.jl:64
  Symbol(::Any...)
   @ Base strings/basic.jl:229

Stacktrace:
  [1] setindex!(h::Dict{Symbol, Nothing}, v0::Nothing, key0::Expr)
    @ Base ./dict.jl:361
  [2] push!(s::Set{Symbol}, x::Expr)
    @ Base ./set.jl:103
  [3] checknorepeats(flat::Vector{Any}, call::TensorCast.CallInfo, msg::String)
    @ TensorCast ~/.julia/packages/TensorCast/mQB8h/src/macro.jl:1484
  [4] standardglue(ex::Any, target::Vector{Any}, store::NamedTuple{(:dict, :assert, :mustassert, :seen, :need, :top, :main), Tuple{Dict{Any, Any}, Vararg{Vector{Any}, 6}}}, call::TensorCast.CallInfo)
    @ TensorCast ~/.julia/packages/TensorCast/mQB8h/src/macro.jl:420
  [5] (::TensorCast.var"#3#5"{TensorCast.CallInfo, NamedTuple{(:dict, :assert, :mustassert, :seen, :need, :top, :main), Tuple{Dict{Any, Any}, Vararg{Vector{Any}, 6}}}})(x::Expr)
    @ TensorCast ~/.julia/packages/TensorCast/mQB8h/src/macro.jl:189
  [6] walk(x::Expr, inner::Function, outer::TensorCast.var"#3#5"{TensorCast.CallInfo, NamedTuple{(:dict, :assert, :mustassert, :seen, :need, :top, :main), Tuple{Dict{Any, Any}, Vararg{Vector{Any}, 6}}}})
    @ MacroTools ~/.julia/packages/MacroTools/qijNY/src/utils.jl:112
  [7] postwalk(f::Function, x::Expr)
    @ MacroTools ~/.julia/packages/MacroTools/qijNY/src/utils.jl:122
  [8] _macro(exone::Expr, extwo::Expr, exthree::Nothing; call::TensorCast.CallInfo, dict::Dict{Any, Any})
    @ TensorCast ~/.julia/packages/TensorCast/mQB8h/src/macro.jl:189
  [9] _macro
    @ ~/.julia/packages/TensorCast/mQB8h/src/macro.jl:154 [inlined]
 [10] var"@cast"(__source__::LineNumberNode, __module__::Module, exs::Vararg{Any})
    @ TensorCast ~/.julia/packages/TensorCast/mQB8h/src/macro.jl:74

Is this sort of thing possible? Or maybe it's too tricky to stack notation like this?

Thanks,
Miles

Start with the minimal Julia reproducer in the issue and trace the @cast expansion into macro.jl, especially checknorepeats at line 1484. Compare the combined expression with the working two-step form and identify why an Expr reaches the Symbol set. Done means the combined mapslices/reshape-style notation no longer raises the conversion error and produces the same q2 result.