feat: allow NLLS for ImplicitDiscreteProblem

Author: vyudu

lib/ImplicitDiscreteSolve/src/solve.jl

@@ -22,10 +22,10 @@ function initialize!(integrator, cache::IDSolveCache)
     else
         (u_next, p) -> f(u_next, p.u, p.p, p.t_next)
     end
-    nlls = length(f.resid_prototype) == length(integrator.u)
+    nlls = !isnothing(f.resid_prototype) && (length(f.resid_prototype) == length(integrator.u))
 
     prob = if nlls
-        NonlinearLeastSquaresProblem{isinplace(f)}(_f, cache.state.u, cache.state)
+        NonlinearLeastSquaresProblem{isinplace(f)}(NonlinearFunction(_f; resid_prototype = f.resid_prototype), cache.state.u, cache.state)
     else
         NonlinearProblem{isinplace(f)}(_f, cache.state.u, cache.state)
     end
@@ -48,9 +48,10 @@ function _initialize_dae!(integrator, prob::ImplicitDiscreteProblem,
         else
             (u_next, p) -> f(u_next, p.u, p.p, p.t_next)
         end
-        nlls = length(f.resid_prototype) == length(integrator.u)
+
+        nlls = !isnothing(f.resid_prototype) && (length(f.resid_prototype) == length(integrator.u))
         prob = if nlls
-            NonlinearLeastSquaresProblem{isinplace(f)}(_f, cache.state.u, cache.state)
+            NonlinearLeastSquaresProblem{isinplace(f)}(NonlinearFunction(_f; resid_prototype = f.resid_prototype), cache.state.u, cache.state)
         else
             NonlinearProblem{isinplace(f)}(_f, cache.state.u, cache.state)
         end