Check that equations contain variables

src/utils.jl

@@ -171,7 +171,7 @@ end
 """
     check_equations(eqs, iv)
 
-Assert that equations are well-formed when building ODE, i.e., only containing a single independent variable.
+Assert that ODE equations are well-formed.
 """
 function check_equations(eqs, iv)
     ivs = collect_ivs(eqs)
@@ -183,6 +183,17 @@ 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)
+        if isempty(vars)
+            if isempty(pars)
+                throw(ArgumentError("Equation $eq contains no variables or parameters."))
+            else
+                throw(ArgumentError("Equation $eq contains only parameters, but relationships between parameters should be specified with defaults or parameter_dependencies."))
+            end
+        end
+    end
 end
 """
 Get all the independent variables with respect to which differentials are taken.
@@ -439,6 +450,12 @@ function find_derivatives!(vars, expr, f)
     return vars
 end
 
+function collect_vars(args...; kwargs...)
+    unknowns, parameters = [], []
+    collect_vars!(unknowns, parameters, args...; kwargs...)
+    return unknowns, parameters
+end
+
 function collect_vars!(unknowns, parameters, expr, iv; op = Differential)
     if issym(expr)
         collect_var!(unknowns, parameters, expr, iv)

test/odesystem.jl

@@ -420,7 +420,14 @@ der = Differential(w)
 eqs = [
     der(u1) ~ t
 ]
-@test_throws ArgumentError ModelingToolkit.ODESystem(eqs, t, vars, pars, name = :foo)
+@test_throws ArgumentError ODESystem(eqs, t, vars, pars, name = :foo)
+
+ # equations without variables are forbidden
+ # https://github.com/SciML/ModelingToolkit.jl/issues/2727
+@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