fix: create specialized isdiag for symbolics in noise matrix

Author: AayushSabharwal

src/systems/diffeqs/sdesystem.jl

@@ -244,13 +244,20 @@ function Base.:(==)(sys1::SDESystem, sys2::SDESystem)
         all(s1 == s2 for (s1, s2) in zip(get_systems(sys1), get_systems(sys2)))
 end
 
+function __num_isdiag(mat)
+    for i in axes(mat, 1), j in axes(mat, 2)
+        i == j || isequal(mat[i, j], 0) || return false
+    end
+    return true
+end
+
 function generate_diffusion_function(sys::SDESystem, dvs = unknowns(sys),
         ps = full_parameters(sys); isdde = false, kwargs...)
     eqs = get_noiseeqs(sys)
     if isdde
         eqs = delay_to_function(sys, eqs)
     end
-    if eqs isa AbstractMatrix && isdiag(eqs)
+    if eqs isa AbstractMatrix && __num_isdiag(eqs)
         eqs = diag(eqs)
     end
     u = map(x -> time_varying_as_func(value(x), sys), dvs)

test/sdesystem.jl

@@ -722,3 +722,29 @@ let # test that diagonal noise is correctly handled
     # SOSRI only works for diagonal and scalar noise
     @test solve(prob, SOSRI()).retcode == ReturnCode.Success
 end
+
+@testset "Non-diagonal noise check" begin
+    @parameters σ ρ β
+    @variables x(t) y(t) z(t)
+    @brownian a b c
+    eqs = [D(x) ~ σ * (y - x) + 0.1a * x + 0.1b * y,
+        D(y) ~ x * (ρ - z) - y + 0.1b * y,
+        D(z) ~ x * y - β * z + 0.1c * z]
+    @mtkbuild de = System(eqs, t)
+
+    u0map = [
+        x => 1.0,
+        y => 0.0,
+        z => 0.0
+    ]
+
+    parammap = [
+        σ => 10.0,
+        β => 26.0,
+        ρ => 2.33
+    ]
+
+    prob = SDEProblem(de, u0map, (0.0, 100.0), parammap)
+    # SOSRI only works for diagonal and scalar noise
+    @test solve(prob, ImplicitEM()).retcode == ReturnCode.Success
+end