Plate-local alias in symbolic UDF return size emits an out-of-scope declaration #60

Problem

A plate cell may bind an input to a local name before calling a helper whose declared vector return length depends on that input. The local alias causes the emitted outer storage declaration to refer to a name outside its scope. The same helper call with the input passed directly emits ragged per-cell storage and compiles.

Verified at StanBlocks commit 75d9feb41f6f9148799c4c1f3236aacd5bdfdf7c with Julia 1.10.12.

Minimal reproducer

using StanBlocks

StanBlocks.@deffun begin
    alias_probe_cell(reads::int[n])::vector[max(reads)] = rep_vector(1.0, max(reads))
end

data = (; reads=[[1, 2], [1, 2, 3]], y=[1.0, 1.0])

alias_model = StanBlocks.@slic data begin
    out ~ plate(reads, y; outer=(2,)) do reads, y
        local_reads = reads
        conc = alias_probe_cell(local_reads)
        y ~ normal(conc[1], 1.0)
        conc[2]
    end
end

direct_model = StanBlocks.@slic data begin
    out ~ plate(reads, y; outer=(2,)) do reads, y
        conc = alias_probe_cell(reads)
        y ~ normal(conc[1], 1.0)
        conc[2]
    end
end

for (label, model) in (("alias", alias_model), ("direct", direct_model))
    result = StanBlocks.stanc_check(StanBlocks.stan_code(model); warn_pedantic=false)
    println(label, ": ", result.ok)
end

Observed output: alias: false, direct: true. The alias case emits matrix[max(out_local_reads), 2] out_conc; in transformed data, then stanc reports Identifier "out_local_reads" not in scope. The direct case emits per-cell lengths and a ragged out_conc__pl_mem_1 carrier and passes stanc.

Expected: the local alias retains the same per-cell return length as its input, with valid ragged storage. The two calls have the same helper arguments and model meaning.

This occurs in a pure StanBlocks model; no application-specific code or data are required.