fix: clear error when a multi-output submodel call has the wrong number of LHS outputs

src/graph_engine.jl

@@ -2061,6 +2061,26 @@ make_node!(materialize::True, node_type::Composite, behaviour::Stochastic, model
     Composite(), model, ctx, options, fform, lhs_interface, rhs_interfaces, static(length(rhs_interfaces) + length(lhs_interface))
 )
 
+# A multi-output submodel call must provide exactly as many outputs on the left-hand side as there are
+# interfaces left unspecified on the right-hand side. When it does not, the total arity does not match the
+# `StaticInt{N}` of any generated `make_node!` method and dispatch fails with a `MethodError` that says
+# nothing about the real problem, so this less specific fallback reports it instead.
+function make_node!(
+    ::Composite,
+    model::Model,
+    ctx::Context,
+    options::NodeCreationOptions,
+    fform::F,
+    lhs_interface::Union{Tuple, NamedTuple},
+    rhs_interfaces::NamedTuple,
+    ::StaticInt{N}
+) where {F, N}
+    n = "\n"
+    error(
+        lazy"Node '$(fform)' cannot be called with $(length(lhs_interface)) output(s) on the left-hand side and $(length(rhs_interfaces)) interface(s) on the right-hand side, $(N) in total.$(n)$(n)The number of outputs on the left-hand side must be equal to the number of interfaces of '$(fform)' that are left unspecified on the right-hand side. Currently specified interfaces are: $(keys(rhs_interfaces)), but check the documentation to see the specification options."
+    )
+end
+
 """
     make_node!
 

test/multi_and_zero_output_tests.jl

@@ -73,7 +73,33 @@ end
         (p, q) ~ two_iface_sub(x = x, b = b)
     end
 
-    @test_throws "no method matching make_node!" create_model(main_mismatch()) do model, ctx
+    # Too many outputs on the LHS: only `a` is missing, but two are provided
+    @test_throws "cannot be called with 2 output(s) on the left-hand side" create_model(main_mismatch()) do model, ctx
+        x = datalabel(model, ctx, NodeCreationOptions(kind = :data), :x, 1.0)
+        return (x = x,)
+    end
+    @test_throws "must be equal to the number of interfaces" create_model(main_mismatch()) do model, ctx
+        x = datalabel(model, ctx, NodeCreationOptions(kind = :data), :x, 1.0)
+        return (x = x,)
+    end
+
+    # Too few outputs on the LHS: both `a` and `b` are missing, but only one is provided
+    @model function main_too_few(x)
+        (p,) ~ two_iface_sub(x = x)
+    end
+
+    @test_throws "cannot be called with 1 output(s) on the left-hand side" create_model(main_too_few()) do model, ctx
+        x = datalabel(model, ctx, NodeCreationOptions(kind = :data), :x, 1.0)
+        return (x = x,)
+    end
+
+    # The same applies to the named-output (NamedTuple LHS) form
+    @model function main_named_mismatch(x)
+        b ~ Normal(0, 1)
+        (a = p, b = q) ~ two_iface_sub(x = x, b = b)
+    end
+
+    @test_throws "cannot be called with 2 output(s) on the left-hand side" create_model(main_named_mismatch()) do model, ctx
         x = datalabel(model, ctx, NodeCreationOptions(kind = :data), :x, 1.0)
         return (x = x,)
     end