Eigen solvers in ARTIFICIAL_CELLS can't use voltage. (#1516)

src/codegen/codegen_neuron_cpp_visitor.cpp

@@ -908,7 +908,9 @@ void CodegenNeuronCppVisitor::print_sdlists_init([[maybe_unused]] bool print_ini
 
 CodegenCppVisitor::ParamVector CodegenNeuronCppVisitor::functor_params() {
     auto params = internal_method_parameters();
-    params.push_back({"", "double", "", "v"});
+    if (!info.artificial_cell) {
+        params.push_back({"", "double", "", "v"});
+    }
 
     return params;
 }

test/usecases/nonlinear/art_nonlin.mod

@@ -0,0 +1,21 @@
+NEURON {
+    ARTIFICIAL_CELL art_nonlin
+}
+
+STATE { x }
+
+FUNCTION solve() {
+    : Iterative solvers need a starting guess.
+    x = 1.0
+    SOLVE eq
+
+    solve = x
+}
+
+NONLINEAR eq {
+    ~ x*x = 4.0
+}
+
+FUNCTION residual(x) {
+    residual = x - 2.0
+}

test/usecases/nonlinear/test_nonlin.py

@@ -1,15 +1,25 @@
 from neuron import h, gui
 
 
-def test_nonlin():
+def check_solve(make_inst):
     s = h.Section()
     s.insert("nonlin")
-    inst = s(0.5).nonlin
+
+    inst = make_inst(s)
 
     x = inst.solve()
     error = inst.residual(x)
     assert abs(error) < 1e-9, f"{x = }, {error = }"
 
 
+def test_nonlin():
+    check_solve(lambda s: s(0.5).nonlin)
+
+
+def test_art_nonlin():
+    check_solve(lambda s: h.art_nonlin())
+
+
 if __name__ == "__main__":
     test_nonlin()
+    test_art_nonlin()