Merge pull request #2560 from SciML/linear_interp

Refactor & Generalize: System for opting alg into linear interpolation

Author: ChrisRackauckas

lib/OrdinaryDiffEqCore/src/alg_utils.jl

@@ -444,3 +444,6 @@ function Base.show(io::IO, ::MIME"text/plain", alg::OrdinaryDiffEqAlgorithm)
     end
     print(io, ")")
 end
+
+# Defaults in the current system: currently opt out DAEAlgorithms until complete
+default_linear_interpolation(alg, prob) = alg isa DAEAlgorithm || prob isa DiscreteProblem

lib/OrdinaryDiffEqCore/src/solve.jl

@@ -26,9 +26,7 @@ function DiffEqBase.__init(
                          saveat isa Number || prob.tspan[1] in saveat,
         save_end = nothing,
         callback = nothing,
-        dense = save_everystep &&
-                    !(alg isa DAEAlgorithm) && !(prob isa DiscreteProblem) &&
-                    isempty(saveat),
+        dense = save_everystep && isempty(saveat) && !default_linear_interpolation(prob, alg),
         calck = (callback !== nothing && callback !== CallbackSet()) ||
                     (dense) || !isempty(saveat), # and no dense output
         dt = isdiscretealg(alg) && isempty(tstops) ?