Just error if an equation only contains parameters (and no variables)

Author: hersle

src/utils.jl

@@ -201,6 +201,12 @@ function check_equations(eqs, iv)
         isequal(single_iv, iv) ||
             throw(ArgumentError("Differential w.r.t. variable ($single_iv) other than the independent variable ($iv) are not allowed."))
     end
+
+    for eq in eqs
+        vars, pars = collect_vars(eq, iv)
+        isempty(vars) && !isempty(pars) && 
+            throw(ArgumentError("Equation $eq contains only parameters, but relationships between parameters should be specified with defaults or parameter_dependencies."))
+    end
 end
 """
 Get all the independent variables with respect to which differentials are taken.

test/odesystem.jl

@@ -427,7 +427,6 @@ eqs = [
 @parameters p q
 @test_throws ArgumentError ODESystem([p ~ q], t; name = :foo)
 @test_throws ArgumentError ODESystem([p ~ 1], t; name = :foo)
-@test_throws ArgumentError ODESystem([1 ~ 2], t; name = :foo)
 
 @variables x(t)
 @parameters M b k