Revert "Remove branch for DifferentialVarsUndefined"

This reverts commit 5141b52.

Author: Ickaser

lib/OrdinaryDiffEqCore/src/dense/generic_dense.jl

@@ -990,6 +990,16 @@ 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

@@ -103,12 +103,15 @@ macro fold(arg)
     end
 end
 
+struct DifferentialVarsUndefined end
+
 """
     get_differential_vars(f, idxs, timeseries::uType)
 
-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). 
+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).
 """
 function get_differential_vars(f, u)
     if hasproperty(f, :mass_matrix)
@@ -122,7 +125,7 @@ function get_differential_vars(f, u)
         elseif !(mm isa SciMLOperators.AbstractSciMLOperator) && isdiag(mm)
             return reshape(diag(mm) .!= 0, size(u))
         else
-            return reshape(.~all(iszero, mm, dims=1) .!= 0, size(u))
+            return DifferentialVarsUndefined()
         end
     else
         return nothing