fix: split mixed positional/keyword arguments in the comma form f(a, b = c)

[bvdmitri]

Problem

In Julia f(a, b = c) and f(a; b = c) are the same call. Inside a @model body they were not — only the semicolon form worked, and the comma form failed during macro expansion with

syntax: invalid named tuple element "GraphPPL.proxylabel(:anonymous, 1.0, nothing, GraphPPL.False())"

which points at generated code rather than the user's line.

Root cause

convert_to_kwargs_expression was all-or-nothing: it rewrote a call into the keyword form only when is_kwargs_expression(args) held, and that requires every argument to be :kw/:parameters. Anything mixed fell through untouched, so the :kw node stayed among the positional arguments and combine_args emitted a tuple literal containing it:

input	args	kwargs	combine_args
Normal(m; var = v)	[:m]	[:(var = v)]	MixedArguments((m,), (var = v,)) ✅
Normal(m, var = v)	[:m, :(var = v)]	nothing	(m, $(Expr(:kw, :var, :v))) ❌

The trap the issue warns about is real: Julia puts an explicit ; group first, in a :parameters node, so a naive "split off the trailing keywords" breaks Normal(0, 1; a = 1, b = 2). split_positional_and_keyword_args handles both shapes, and rebuilding an already-correct semicolon call reproduces it exactly.

The second half

convert_anonymous_variables runs after convert_to_kwargs_expression in the pipeline (src/backends/default.jl:8-20) and generates fresh tilde expressions for nested calls, splicing the captured arguments back verbatim — which reintroduced the comma form. That is the path the original report hit, since its repro is a nested call. Fixing only convert_to_kwargs_expression left the reported case still broken; reconstruct_call closes it.

What this does and does not buy

• A nested deterministic call over constants is evaluated directly, so mixed arguments genuinely work there. This is the shape reported in the issue, and it now builds.
• A node that has to be materialized still cannot take both, by design. It now reaches that stated limitation instead of failing as invalid syntax.

Error message

The materialized-node error is also rewritten. It read MixedArguments not supported for rhs_interfaces when node has to be materialized — rhs_interfaces means nothing to a model author, and this change makes the error easier to reach. It now names the function, reports what it got, and says what to do.

Known adjacent gap, not addressed here

A nested deterministic call with mixed arguments and a stochastic argument fails with MethodError: no method matching filter(::typeof(is_nodelabel), ::MixedArguments) — materialize_anonymous_variable! has no MixedArguments method. Verified this is pre-existing on 4.9.0 and reachable through the semicolon form identically, so it is a separate bug rather than a regression from this change. This PR's contract is that the two spellings behave the same, which now holds in that case too.

Tests

Asserted directly as "the comma and semicolon forms are indistinguishable", across ~, .~ and := — including the Normal(0, 1; a = 1, b = 2) regression guard, both spellings at once (f(a, b = c; d = e)), and a unit testitem for the splitter.

Verified red before the src/ change (5 failed, 6 errored) and green after. Full local suite: 265/265 test items, 76866 pass, 1 broken (pre-existing), Aqua included.

Closes #328

[codecov]

Codecov Report

❌ Patch coverage is 89.28571% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.04%. Comparing base (8882c5b) to head (94d696f).

Files with missing lines	Patch %	Lines
src/model_macro.jl	89.28%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            4.9.0     #330      +/-   ##
==========================================
+ Coverage   90.96%   91.04%   +0.08%     
==========================================
  Files          16       16              
  Lines        2279     2301      +22     
==========================================
+ Hits         2073     2095      +22     
  Misses        206      206              

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