Revert "Check that equations contain variables" (commited to wrong br…

…anch and pushed it...)

This reverts commit 47cdd3b.

src/utils.jl

@@ -171,7 +171,7 @@ end
 """
     check_equations(eqs, iv)
 
-Assert that ODE equations are well-formed.
+Assert that equations are well-formed when building ODE, i.e., only containing a single independent variable.
 """
 function check_equations(eqs, iv)
     ivs = collect_ivs(eqs)
@@ -183,17 +183,6 @@ 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.
@@ -450,12 +439,6 @@ 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

@@ -420,14 +420,7 @@ der = Differential(w)
 eqs = [
     der(u1) ~ t
 ]
-@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)
+@test_throws ArgumentError ModelingToolkit.ODESystem(eqs, t, vars, pars, name = :foo)
 
 @variables x(t)
 @parameters M b k