Fix get_du/get_du! reading the wrong derivative under CompositeAlgorithm

lib/OrdinaryDiffEqCore/Project.toml

@@ -1,7 +1,7 @@
 name = "OrdinaryDiffEqCore"
 uuid = "bbf590c4-e513-4bbe-9b18-05decba2e5d8"
 authors = ["ParamThakkar123 <paramthakkar864@gmail.com>"]
-version = "4.12.0"
+version = "4.12.1"
 
 [deps]
 ADTypes = "47edcb42-4c32-4615-8424-f2b9edc5f35b"

lib/OrdinaryDiffEqCore/src/alg_utils.jl

@@ -126,6 +126,15 @@ Return whether `alg` provides a special interpolant for its stiff branch
 """
 has_stiff_interpolation(alg) = false
 
+"""
+    get_current_has_stiff_interpolation(alg, cache) -> Bool
+
+`has_stiff_interpolation` for the algorithm that is currently active, which for a
+`CompositeAlgorithm` is the sub-algorithm selected by `cache.current` rather than
+the wrapper itself.
+"""
+get_current_has_stiff_interpolation(alg, cache) = has_stiff_interpolation(alg)
+
 """
     OrdinaryDiffEqCore.has_stage_limiter(alg)
 
@@ -160,6 +169,10 @@ function get_current_isfsal(alg::CompositeAlgorithm, cache)
     return _eval_index(isfsal, alg.algs, cache.current)::Bool
 end
 
+function get_current_has_stiff_interpolation(alg::CompositeAlgorithm, cache)
+    return _eval_index(has_stiff_interpolation, alg.algs, cache.current)::Bool
+end
+
 all_fsal(alg, cache) = isfsal(alg)
 all_fsal(alg::CompositeAlgorithm, cache) = _all_fsal(alg.algs)
 

lib/OrdinaryDiffEqCore/src/integrators/integrator_interface.jl

@@ -81,8 +81,8 @@ end
 @inline function SciMLBase.get_du(integrator::ODEIntegrator)
     isdiscretecache(integrator.cache) &&
         error("Derivatives are not defined for this stepper.")
-    return if isfsal(integrator.alg) &&
-            !has_stiff_interpolation(integrator.alg)
+    return if get_current_isfsal(integrator.alg, integrator.cache) &&
+            !get_current_has_stiff_interpolation(integrator.alg, integrator.cache)
         # Special stiff interpolations do not store the
         # right value in fsallast
         integrator.fsallast
@@ -116,8 +116,8 @@ end
     if isdiscretecache(integrator.cache)
         out .= integrator.cache.tmp
     else
-        return if isfsal(integrator.alg) &&
-                !has_stiff_interpolation(integrator.alg)
+        return if get_current_isfsal(integrator.alg, integrator.cache) &&
+                !get_current_has_stiff_interpolation(integrator.alg, integrator.cache)
             # Special stiff interpolations do not store the
             # right value in fsallast
             out .= integrator.fsallast

test/InterfaceII/get_du.jl

@@ -23,6 +23,10 @@ res = copy(cache)
 for alg in [
         Vern6(), Vern7(), Vern8(), Vern9(), Rodas4(), Rodas4P(),
         Rodas5(), Rodas5P(), TRBDF2(), KenCarp4(), FBDF(), QNDF(),
+        # A CompositeAlgorithm must report the derivative of whichever
+        # sub-algorithm is currently active, not of the wrapper.
+        AutoTsit5(Rosenbrock23()), AutoTsit5(Rodas5P()),
+        AutoVern7(Rodas4()), AutoVern9(Rodas4()), AutoVern9(Rodas5P()),
     ]
     sol = solve(
         prob, alg, tstops = [0.2], callback = dusave, abstol = 1.0e-12, reltol = 1.0e-12