Make get_differential_vars type stable

lib/OrdinaryDiffEqCore/src/dense/generic_dense.jl

@@ -990,16 +990,6 @@ function interpolation_differential_vars(differential_vars, y₀, idxs)
         else
             return Trues(size(idxs))
         end
-    elseif differential_vars isa DifferentialVarsUndefined #for non diagonal mass matrices, use linear interpolation.
-        if y₀ isa Number
-            return false
-        elseif idxs === nothing
-            return Falses(size(y₀))
-        elseif idxs isa Number
-            return false
-        else
-            return Falses(size(idxs))
-        end
     elseif idxs isa Number
         return return differential_vars[idxs]
     elseif idxs === nothing

lib/OrdinaryDiffEqCore/src/misc_utils.jl

@@ -103,29 +103,29 @@ macro fold(arg)
     end
 end
 
-struct DifferentialVarsUndefined end
-
 """
     get_differential_vars(f, idxs, timeseries::uType)
 
-Returns an array of booleans for which values are the differential variables
-vs algebraic variables. Returns `nothing` for the cases where all variables
-are differential variables. Returns `DifferentialVarsUndefined` if it cannot
-be determined (i.e. the mass matrix is not diagonal).
+Returns an array of booleans, for which `true` refers to a differential variable and `false`
+to an algebraic variable. If the function has no mass matrix, returns `nothing` (implying 
+all variables are differential). 
 """
 function get_differential_vars(f, u)
-    differential_vars = nothing
     if hasproperty(f, :mass_matrix)
         mm = f.mass_matrix
         mm = mm isa MatrixOperator ? mm.A : mm
 
-        if mm isa UniformScaling || all(!iszero, mm)
+        if mm isa UniformScaling 
             return nothing
+        elseif all(!iszero, mm)
+            return trues(size(mm, 1))
         elseif !(mm isa SciMLOperators.AbstractSciMLOperator) && isdiag(mm)
-            differential_vars = reshape(diag(mm) .!= 0, size(u))
+            return reshape(diag(mm) .!= 0, size(u))
         else
-            return DifferentialVarsUndefined()
+            return reshape(.~all(iszero, mm, dims=1) .!= 0, size(u))
         end
+    else
+        return nothing
     end
 end