Special case Sym in variable collection

Fix #701

Author: YingboMa

src/systems/diffeqs/odesystem.jl

@@ -137,16 +137,25 @@ function ODESystem(eqs, iv=nothing; kwargs...)
 end
 
 function collect_vars!(states, parameters, expr, iv)
-    for var in vars(expr)
-        if isparameter(var) || isparameter(var.op)
-            isequal(var, iv) || push!(parameters, var)
-        else
-            push!(states, var)
+    if expr isa Sym
+        collect_var!(states, parameters, expr, iv)
+    else
+        for var in vars(expr)
+            collect_var!(states, parameters, var, iv)
         end
     end
     return nothing
 end
 
+function collect_var!(states, parameters, var, iv)
+    if isparameter(var) || isparameter(var.op)
+        isequal(var, iv) || push!(parameters, var)
+    else
+        push!(states, var)
+    end
+    return nothing
+end
+
 Base.:(==)(sys1::ODESystem, sys2::ODESystem) =
     _eq_unordered(sys1.eqs, sys2.eqs) && isequal(sys1.iv, sys2.iv) &&
     _eq_unordered(sys1.states, sys2.states) && _eq_unordered(sys1.ps, sys2.ps)

test/odesystem.jl

@@ -211,3 +211,11 @@ sys = ODESystem(eqs)
 @test all(isequal.(states(sys), [x, y, z]))
 @test all(isequal.(parameters(sys), [σ, β]))
 @test equations(sys) == eqs
+
+# issue 701
+using ModelingToolkit
+@parameters t a
+@variables x(t)
+@derivatives D'~t
+sys = ODESystem([D(x) ~ a])
+@test sys.eqs[1].rhs isa Sym