don't error on implicit parameter equation

src/systems/abstractsystem.jl

@@ -2808,18 +2808,16 @@ function process_parameter_equations(sys::AbstractSystem)
         vars!(varsbuf, eq; op = Union{Differential, Initial, Pre})
         # singular equations
         isempty(varsbuf) && continue
+
+        # only consider explicit parameter dependencies
+        isparameter(eq.lhs) || continue
+
         if all(varsbuf) do sym
             is_parameter(sys, sym) ||
                 symbolic_type(sym) == ArraySymbolic() &&
-                    is_sized_array_symbolic(sym) &&
-                    all(Base.Fix1(is_parameter, sys), collect(sym))
-        end
-            if !isparameter(eq.lhs)
-                throw(ArgumentError("""
-                LHS of parameter dependency equation must be a single parameter. Found \
-                $(eq.lhs).
-                """))
-            end
+                is_sized_array_symbolic(sym) &&
+                all(Base.Fix1(is_parameter, sys), collect(sym))
+        end
             push!(pareq_idxs, i)
         end
     end

test/input_output_handling.jl

@@ -465,3 +465,17 @@ end
     x = [1.0]
     @test_nowarn f[1](x, u, p, 0.0)
 end
+
+@testset "implicit focing of inputs" begin
+    @mtkmodel ImplicitForcing begin
+        @variables begin
+            u(t), [description = "Input Variable", input=true]
+            y(t), [description = "fully implicit output", output=true]
+        end
+        @equations begin
+            0 ~ sqrt(u) # implicitly forces output y because u=f(y) in  closed loop
+        end
+    end
+    @named implicit = ImplicitForcing()
+    mtkcompile(implicit; inputs = unbound_inputs(implicit)) # test for no error
+end