Merge pull request #1425 from SciML/myb/check

Fix #1413 and #1389; check the validity of the equations before building the function

Author: YingboMa

src/systems/abstractsystem.jl

@@ -911,7 +911,7 @@ function structural_simplify(sys::AbstractSystem; simplify=false)
     sys = initialize_system_structure(alias_elimination(sys))
     check_consistency(sys)
     if sys isa ODESystem
-        sys = initialize_system_structure(dae_index_lowering(sys))
+        sys = initialize_system_structure(dae_index_lowering(ode_order_lowering(sys)))
     end
     sys = tearing(sys, simplify=simplify)
     fullstates = [map(eq->eq.lhs, observed(sys)); states(sys)]

src/systems/diffeqs/abstractodesystem.jl

@@ -90,6 +90,7 @@ function generate_function(
 
     eqs = [eq for eq in equations(sys) if !isdifferenceeq(eq)]
     foreach(check_derivative_variables, eqs)
+    check_lhs(eqs, Differential, Set(dvs))
     # substitute x(t) by just x
     rhss = implicit_dae ? [_iszero(eq.lhs) ? eq.rhs : eq.rhs - eq.lhs for eq in eqs] :
                           [eq.rhs for eq in eqs]

src/utils.jl

@@ -130,6 +130,16 @@ function check_variables(dvs, iv)
     end
 end
 
+function check_lhs(eq::Equation, op, dvs::Set)
+    v = unwrap(eq.lhs)
+    _iszero(v) && return
+    (operation(v) isa op && only(arguments(v)) in dvs) && return
+    error("$v is not a valid LHS. Please run structural_simplify before simulation.")
+end
+check_lhs(eqs, op, dvs::Set) = for eq in eqs
+    check_lhs(eq, op, dvs)
+end
+
 """
     collect_ivs(eqs, op = Differential)
 

test/odesystem.jl

@@ -517,3 +517,21 @@ RHS2 = RHS
 @unpack RHS = fol
 @test isequal(RHS, RHS2)
 
+#1413 and 1389
+@parameters t α β
+@variables x(t) y(t) z(t)
+D = Differential(t)
+
+eqs = [
+       D(x) ~ 0.1x + 0.9y,
+       D(y) ~ 0.5x + 0.5y,
+       z ~ α*x - β*y
+       ]
+
+@named sys = ODESystem(eqs, t, [x,y,z],[α,β])
+@test_throws Any ODEFunction(sys)
+
+eqs = copy(eqs)
+eqs[end] = D(D(z)) ~ α*x - β*y
+@named sys = ODESystem(eqs, t, [x,y,z],[α,β])
+@test_throws Any ODEFunction(sys)