Fix SCCNonlinearProblem not having u0 field (issue #758) (#759)

* Fix SCCNonlinearProblem not having u0 field (issue #758)

SCCNonlinearProblem does not have a u0 field - each subproblem has its own u0.
The solve function was incorrectly trying to access prob.u0 when u0 was not
provided, causing a FieldError.

Changes:
- Remove the u0 = prob.u0 fallback since SCCNonlinearProblem has no u0 field
- Add regression test to prevent this from happening again

Closes #758

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Co-Authored-By: Claude Opus 4.5 <[email protected]>

* Bump SCCNonlinearSolve version to 1.8.0

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Co-Authored-By: Claude Opus 4.5 <[email protected]>

* Fix test to properly import SciMLBase

The test item needs to explicitly use SciMLBase to access NonlinearProblem
and other types from the module.

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Co-Authored-By: Claude Opus 4.5 <[email protected]>

* Fix ExplicitImports check in ChainRulesCore extension

Use `import SCCNonlinearSolve: ...` instead of `using SCCNonlinearSolve` to
avoid implicit import of the module name, which causes ExplicitImports tests
to fail on macOS CI.

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Co-Authored-By: Claude Opus 4.5 <[email protected]>

* Fix code formatting in test file

Apply SciML formatting style (spaces around operators and assignments)
to the test file to pass CI code-style checks.

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Co-Authored-By: Claude Opus 4.5 <[email protected]>

* Fix indentation in ChainRulesCore extension

Align continuation indent for multi-line import statement.

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Co-Authored-By: Claude Opus 4.5 <[email protected]>

* Fix formatting: add spaces around ≈ operator

Restore spaces around ≈ operator that were accidentally removed.

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Co-Authored-By: Claude Opus 4.5 <[email protected]>

---------

Co-authored-by: ChrisRackauckas <[email protected]>
Co-authored-by: Claude Opus 4.5 <[email protected]>

Author: 3 people

lib/SCCNonlinearSolve/Project.toml

@@ -1,7 +1,7 @@
 name = "SCCNonlinearSolve"
 uuid = "9dfe8606-65a1-4bb3-9748-cb89d1561431"
+version = "1.8.0"
 authors = ["Avik Pal <[email protected]> and contributors"]
-version = "1.7.0"
 
 [deps]
 CommonSolve = "38540f10-b2f7-11e9-35d8-d573e4eb0ff2"
@@ -23,8 +23,8 @@ ChainRulesCore = "1"
 CommonSolve = "0.2.4"
 ExplicitImports = "1.5"
 Hwloc = "3"
-LinearAlgebra = "1.10"
 InteractiveUtils = "<0.0.1, 1"
+LinearAlgebra = "1.10"
 NonlinearProblemLibrary = "0.1.2"
 NonlinearSolve = "4.8"
 NonlinearSolveBase = "2.1"

lib/SCCNonlinearSolve/ext/SCCNonlinearSolveChainRulesCoreExt.jl

@@ -1,9 +1,8 @@
 module SCCNonlinearSolveChainRulesCoreExt
 
-using SCCNonlinearSolve
-using SCCNonlinearSolve: SCCAlg, scc_solve_up
+import SCCNonlinearSolve: SCCAlg, scc_solve_up
 using SciMLBase: SCCNonlinearProblem, AbstractSensitivityAlgorithm, ChainRulesOriginator,
-    _concrete_solve_adjoint
+                 _concrete_solve_adjoint
 
 import ChainRulesCore
 

lib/SCCNonlinearSolve/src/SCCNonlinearSolve.jl

@@ -36,7 +36,8 @@ end
 
 function CommonSolve.solve(prob::SciMLBase.SCCNonlinearProblem, alg::SCCAlg;
         sensealg = nothing, u0 = nothing, p = nothing, kwargs...)
-    u0 = u0 !== nothing ? u0 : prob.u0
+    # Note: SCCNonlinearProblem does not have a u0 field - each subproblem has its own u0.
+    # The u0 parameter here is only used for AD hooks, not for actual solving.
     p = p !== nothing ? p : prob.p
     scc_solve_up(prob, sensealg, u0, p, alg; kwargs...)
 end

lib/SCCNonlinearSolve/test/core_tests.jl

@@ -76,6 +76,41 @@ end
     @test sol ≈ manualscc ≈ scc_sol_default
 end
 
+@testitem "SCCNonlinearProblem solve without explicit u0 (issue #758)" setup=[CoreRootfindTesting] tags=[:core] begin
+    # Regression test for https://github.com/SciML/NonlinearSolve.jl/issues/758
+    # SCCNonlinearProblem does not have a u0 field, so calling solve() without
+    # explicit u0 should not try to access prob.u0
+    using SciMLBase
+    using SCCNonlinearSolve
+    import NonlinearSolve
+
+    # Create simple nonlinear subproblems (OOP style, returning vectors)
+    f1(u, p) = [u[1]^2 - 2.0]
+    f2(u, p) = [u[1] - 1.0]
+
+    prob1 = NonlinearProblem(f1, [1.0])
+    prob2 = NonlinearProblem(f2, [1.0])
+
+    # Create explicit functions (identity - just pass through)
+    explicitfun1!(p, sols) = nothing
+    explicitfun2!(p, sols) = nothing
+
+    # Create the SCC problem using the same pattern as existing tests
+    scc_prob = SciMLBase.SCCNonlinearProblem(
+        (prob1, prob2),
+        SciMLBase.Void{Any}.([explicitfun1!, explicitfun2!])
+    )
+
+    # This should not throw an error about prob.u0 field
+    sol = SciMLBase.solve(scc_prob)
+
+    @test SciMLBase.successful_retcode(sol)
+    # First subproblem: u^2 = 2 → u = sqrt(2)
+    @test sol.u[1] ≈ sqrt(2.0)
+    # Second subproblem: u = 1
+    @test sol.u[2] ≈ 1.0
+end
+
 @testitem "SCC Residuals Transfer" setup=[CoreRootfindTesting] tags=[:core] begin
     using NonlinearSolveFirstOrder
     using LinearAlgebra