fix: update initials with non-symbolic u0 in remake

Author: AayushSabharwal

src/systems/nonlinear/initializesystem.jl

@@ -655,7 +655,17 @@ end
 function SciMLBase.late_binding_update_u0_p(
         prob, sys::AbstractSystem, u0, p, t0, newu0, newp)
     u0 === missing && return newu0, (p === missing ? copy(newp) : newp)
-    eltype(u0) <: Pair || return newu0, (p === missing ? copy(newp) : newp)
+    if !(eltype(u0) <: Pair)
+        p === missing || return newu0, newp
+        newu0 === nothing && return newu0, newp
+        newp = p === missing ? copy(newp) : newp
+        initials, repack, alias = SciMLStructures.canonicalize(
+            SciMLStructures.Initials(), newp)
+        initials = DiffEqBase.promote_u0(initials, newu0, t0)
+        newp = repack(initials)
+        setp(sys, Initial.(unknowns(sys)))(newp, newu0)
+        return newu0, newp
+    end
 
     newp = p === missing ? copy(newp) : newp
     newu0 = DiffEqBase.promote_u0(newu0, newp, t0)

test/initializationsystem.jl

@@ -1476,3 +1476,14 @@ end
         @test sol.ps[Γ[1]] ≈ 5.0
     end
 end
+
+@testset "Issue#3504: Update initials when `remake` called with non-symbolic `u0`" begin
+    @variables x(t) y(t)
+    @parameters c1 c2
+    @mtkbuild sys = ODESystem([D(x) ~ -c1 * x + c2 * y, D(y) ~ c1 * x - c2 * y], t)
+    prob1 = ODEProblem(sys, [1.0, 2.0], (0.0, 1.0), [c1 => 1.0, c2 => 2.0])
+    prob2 = remake(prob1, u0 = [2.0, 3.0])
+    integ1 = init(prob1, Tsit5())
+    integ2 = init(prob2, Tsit5())
+    @test integ2.u ≈ [2.0, 3.0]
+end