From 55bfcb1223a2299410bd82320ea90215754514cc Mon Sep 17 00:00:00 2001
From: Aayush Sabharwal <aayush.sabharwal@juliahub.com>
Date: Tue, 24 Mar 2026 18:51:12 +0530
Subject: [PATCH] feat: enable inline linear SCCs by default

---
 lib/ModelingToolkitTearing/src/reassemble.jl | 51 +++++++++++++++++++-
 1 file changed, 49 insertions(+), 2 deletions(-)

diff --git a/lib/ModelingToolkitTearing/src/reassemble.jl b/lib/ModelingToolkitTearing/src/reassemble.jl
index a9c1b97..95d0de5 100644
--- a/lib/ModelingToolkitTearing/src/reassemble.jl
+++ b/lib/ModelingToolkitTearing/src/reassemble.jl
@@ -426,6 +426,7 @@ function generate_system_equations!(state::TearingState, neweqs::Vector{Equation
     end
 
     digraph = DiCMOBiGraph{false}(graph, var_eq_matching)
+    has_inline_linsols = false
     for (i, scc) in enumerate(var_sccs)
         # note that the `vscc <-> escc` relation is a set-to-set mapping, and not
         # point-to-point.
@@ -447,6 +448,7 @@ function generate_system_equations!(state::TearingState, neweqs::Vector{Equation
             linsol_result = get_linear_scc_linsol(state, escc, vscc, neweqs, var_eq_matching, total_sub, analytical_linear_scc_limit, simplify)
         end
         if linsol_result isa Tuple{SymbolicT, BitVector, BitVector}
+            has_inline_linsols = true
             linsol, eqs_mask, vars_mask = linsol_result
             @assert length(eqs_mask) == length(escc)
             @assert length(vars_mask) == length(vscc)
@@ -501,6 +503,11 @@ function generate_system_equations!(state::TearingState, neweqs::Vector{Equation
         codegen_equation!(eq_generator, neweqs[eq], eq, var; simplify)
     end
 
+    if has_inline_linsols
+        sys = SU.setmetadata(sys, MTKBase.SymbolicADDisallowed, "Inline linear SCCs are enabled in `mtkcompile`")
+        state.sys = sys
+    end
+
     (; neweqs′, eq_ordering, var_ordering, solved_eqs, solved_vars) = eq_generator
 
     is_diff_eq = .!iszero.(var_ordering)
@@ -1264,14 +1271,44 @@ $TYPEDFIELDS
     """
     Whether SCCs which are linear systems of the associated variables should be
     handled using inline linear solves via `LinearSolve.jl`. By default, such
-    SCCs generate algebraic equations.
+    SCCs generate algebraic equations. Not supported for nonlinear systems - use
+    `SCCNonlinearProblem` instead. Also unsupported for discrete systems. The
+    default of `nothing` enables it for time-dependent non-discrete systems.
     """
-    inline_linear_sccs::Bool = false
+    inline_linear_sccs::Union{Bool, Nothing} = nothing
     """
     If `inline_linear_sccs == true`, this is the maximum size of a system of linear
     equations which is solved symbolically rather than using `LinearSolve.jl`.
     """
     analytical_linear_scc_limit::Int = 2
+
+    function DefaultReassembleAlgorithm(
+            simplify, array_hack, inline_linear_sccs, analytical_linear_scc_limit
+        )
+        if inline_linear_sccs && analytical_linear_scc_limit < 1
+            throw(ArgumentError("`analytical_linear_scc_limit` cannot be less than 1."))
+        end
+        return new(simplify, array_hack, inline_linear_sccs, analytical_linear_scc_limit)
+    end
+end
+
+struct InlineLinearSCCsUnsupportedError <: Exception
+    reason::String
+end
+
+function Base.showerror(io::IO, err::InlineLinearSCCsUnsupportedError)
+    print(io, """
+    Inline linear SCCs are not supported for this system for the following reason:
+
+    $(err.reason)
+
+    This can be fixed by not passing `reassemble_alg` to `mtkcompile`. Alternatively,
+    ensure to pass `inline_linear_sccs = false` to the `DefaultReassembleAlgorithm`
+    provided to `reassemble_alg`. For example:
+    ```julia
+    mtkcompile(sys; reassemble_alg = ModelingToolkitTearing.DefaultReassembleAlgorithm(; inline_linear_sccs = false))
+    ```
+    """)
 end
 
 function (alg::DefaultReassembleAlgorithm)(state::TearingState,
@@ -1306,6 +1343,16 @@ function (alg::DefaultReassembleAlgorithm)(state::TearingState,
         inline_linear_sccs = false
     end
 
+    # `nothing` enables this for supported systems.
+    inline_linear_sccs = something(inline_linear_sccs, iv isa SymbolicT && D isa Differential)
+    if inline_linear_sccs
+        if iv === nothing
+            throw(InlineLinearSCCsUnsupportedError("System is time-independent"))
+        elseif !(D isa Differential)
+            throw(InlineLinearSCCsUnsupportedError("System is discrete"))
+        end
+    end
+
     extra_unknowns = state.fullvars[extra_eqs_vars[2]]
     if StateSelection.is_only_discrete(state.structure)
         var_sccs = add_additional_history!(