From 883d752bd07754d259d0e447a0f9ed52c30cf4c3 Mon Sep 17 00:00:00 2001
From: Avik Pal <avikpal@mit.edu>
Date: Sun, 24 Dec 2023 02:44:53 -0500
Subject: [PATCH] Properly support scalars

---
 Project.toml                       |  2 +-
 src/common_interface/algorithms.jl |  2 +-
 src/common_interface/solve.jl      |  6 +++++-
 test/kinsol_nonlinear_solve.jl     | 17 +++++++++++++++++
 4 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/Project.toml b/Project.toml
index fa63f85..480d39b 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,7 +1,7 @@
 name = "Sundials"
 uuid = "c3572dad-4567-51f8-b174-8c6c989267f4"
 authors = ["Chris Rackauckas <accounts@chrisrackauckas.com>"]
-version = "4.23.0"
+version = "4.23.1"
 
 [deps]
 CEnum = "fa961155-64e5-5f13-b03f-caf6b980ea82"
diff --git a/src/common_interface/algorithms.jl b/src/common_interface/algorithms.jl
index e50d9b2..d1f1e60 100644
--- a/src/common_interface/algorithms.jl
+++ b/src/common_interface/algorithms.jl
@@ -3,7 +3,7 @@
 # Abstract Types
 abstract type SundialsODEAlgorithm{Method, LinearSolver} <: DiffEqBase.AbstractODEAlgorithm end
 abstract type SundialsDAEAlgorithm{LinearSolver} <: DiffEqBase.AbstractDAEAlgorithm end
-abstract type SundialsNonlinearSolveAlgorithm{LinearSolver} end
+abstract type SundialsNonlinearSolveAlgorithm{LinearSolver} <: SciMLBase.AbstractNonlinearAlgorithm end
 
 SciMLBase.alg_order(alg::Union{SundialsODEAlgorithm, SundialsDAEAlgorithm}) = alg.max_order
 
diff --git a/src/common_interface/solve.jl b/src/common_interface/solve.jl
index 6f7b086..f5ea33f 100644
--- a/src/common_interface/solve.jl
+++ b/src/common_interface/solve.jl
@@ -80,7 +80,11 @@ function DiffEqBase.__solve(prob::Union{
 
     f!(resid, u)
     retcode = interpret_sundials_retcode(flag)
-    DiffEqBase.build_solution(prob, alg, u, resid; retcode = retcode)
+    if prob.u0 isa Number
+        DiffEqBase.build_solution(prob, alg, u[1], resid[1]; retcode = retcode)
+    else
+        DiffEqBase.build_solution(prob, alg, u, resid; retcode = retcode)
+    end
 end
 
 function DiffEqBase.__init(prob::DiffEqBase.AbstractODEProblem{uType, tupType, isinplace},
diff --git a/test/kinsol_nonlinear_solve.jl b/test/kinsol_nonlinear_solve.jl
index 35cd788..addd5ed 100644
--- a/test/kinsol_nonlinear_solve.jl
+++ b/test/kinsol_nonlinear_solve.jl
@@ -37,3 +37,20 @@ prob_oop = NonlinearProblem{false}(f_oop, u0)
     f_oop(sol.u, nothing)
     @test maximum(abs, du) < 1e-6
 end
+
+# Scalar
+f_scalar(u, p) = 2 - 2u
+u0 = 0.0
+prob_scalar = NonlinearProblem{false}(f_scalar, u0)
+
+@testset "linear_solver = $(linear_solver) | globalization_strategy = $(globalization_strategy)" for linear_solver in (:Dense,
+        :LapackDense, :GMRES, :FGMRES, :PCG, :TFQMR), globalization_strategy in (:LineSearch, :None)
+    local sol
+    alg = KINSOL(; linear_solver, globalization_strategy)
+    sol = solve(prob_scalar, alg; abstol)
+    @test SciMLBase.successful_retcode(sol.retcode)
+    @test sol.u isa Number
+
+    resid = f_scalar(sol.u, nothing)
+    @test abs(resid) < 1e-6
+end