From 9a24e99d35a4bdce56a0460e4dea7094dd34070c Mon Sep 17 00:00:00 2001
From: Bagaev Dmitry <bvdmitri@gmail.com>
Date: Tue, 15 Sep 2026 14:45:14 +0200
Subject: [PATCH] fix!: reject keyword arguments in `@model` signatures instead
 of warning

Keyword arguments in a `@model` function signature were parsed and then silently
dropped, with only a `@warn`. The body never saw them, so a user writing
`@model function m(x; a = 1)` got no `a` and a later `UndefVarError` as the only
real signal, and the warning is easy to miss in a noisy REPL.

Raise an error at macro expansion instead, consistent with unsupported
positional arguments at the call site, and name the offending keyword arguments
plus the supported form.

BREAKING: `@model` definitions carrying keyword arguments now fail to expand
rather than silently ignoring them. Nothing in this repository relied on the old
behaviour: no test or doc asserted the warning, and no `@model` signature with
keyword arguments exists in `src/`, `test/`, `docs/` or `README.md`. Downstream
packages that defined such signatures will need to drop them, but their keyword
arguments were never in effect to begin with.

Closes #307.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
---
 src/model_macro.jl        |  8 +++++++-
 test/model_macro_tests.jl | 25 +++++++++++++++++++++++++
 2 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/src/model_macro.jl b/src/model_macro.jl
index 5a1388f8..3a30e3ab 100644
--- a/src/model_macro.jl
+++ b/src/model_macro.jl
@@ -957,7 +957,13 @@ function model_macro_interior(backend_type, model_specification)
 
     num_interfaces = Base.length(ms_args)
     if !isnothing(ms_kwargs) && length(ms_kwargs) > 0
-        @warn("Model specification language does not support keyword arguments. Ignoring $(length(ms_kwargs)) keyword arguments.")
+        # Keyword arguments in a model signature were previously parsed and then silently dropped, with only
+        # a warning. The body never saw them, so the only real signal was a later `UndefVarError`. Fail here
+        # instead, consistent with how unsupported positional arguments are rejected at the call site.
+        kwargs_names = map(kwarg -> (kwarg isa Expr && kwarg.head === :kw) ? kwarg.args[1] : kwarg, ms_kwargs)
+        error(
+            "The `$(ms_name)` model macro does not support keyword arguments in the model signature, but got $(length(ms_kwargs)): $(join(kwargs_names, ", ")). Declare all model interfaces as positional arguments, `$(ms_name)($(join(ms_args, ", ")))`, they are passed by name at the call site."
+        )
     end
 
     boilerplate_functions = GraphPPL.get_boilerplate_functions(backend_type, ms_name, ms_args, num_interfaces)
diff --git a/test/model_macro_tests.jl b/test/model_macro_tests.jl
index 30d53bd3..d629fd37 100644
--- a/test/model_macro_tests.jl
+++ b/test/model_macro_tests.jl
@@ -2070,3 +2070,28 @@ end
         somemodel()
     )
 end
+
+@testitem "`@model` should reject keyword arguments in the model signature" begin
+    import GraphPPL: model_macro_interior
+
+    include("testutils.jl")
+
+    # Keyword arguments used to be parsed and then silently dropped with only a warning, so the body
+    # never saw them and the user's only signal was a later `UndefVarError`
+    kwargs_spec = :(function model_with_kwargs(x; a = 1, b = 2)
+        y ~ Normal(x, a)
+    end)
+
+    @test_throws "does not support keyword arguments in the model signature" model_macro_interior(
+        TestUtils.TestGraphPPLBackend, kwargs_spec
+    )
+    # the offending keyword arguments are named, and the supported form is shown
+    @test_throws "but got 2: a, b" model_macro_interior(TestUtils.TestGraphPPLBackend, kwargs_spec)
+    @test_throws "model_with_kwargs(x)" model_macro_interior(TestUtils.TestGraphPPLBackend, kwargs_spec)
+
+    # positional-only signatures are unaffected
+    positional_spec = :(function model_without_kwargs(x, a)
+        y ~ Normal(x, a)
+    end)
+    @test model_macro_interior(TestUtils.TestGraphPPLBackend, positional_spec) isa Expr
+end