fix: ensure => nothing overrides defaults

Author: AayushSabharwal

src/systems/nonlinear/initializesystem.jl

@@ -42,7 +42,7 @@ function generate_initializesystem_timevarying(sys::AbstractSystem;
     op = anydict(op)
     u0map = anydict()
     pmap = anydict()
-    build_operating_point!(sys, op, u0map, pmap, defs, unknowns(sys),
+    build_operating_point!(sys, op, u0map, pmap, Dict(), unknowns(sys),
         parameters(sys; initial_parameters = true))
     for (k, v) in op
         if has_parameter_dependency_with_lhs(sys, k) && is_variable_floatingpoint(k)
@@ -113,7 +113,7 @@ function generate_initializesystem_timevarying(sys::AbstractSystem;
 
     # 3) process other variables
     for var in vars
-        if var ∈ keys(defs)
+        if var ∈ keys(op)
             push!(eqs_ics, var ~ defs[var])
         elseif var ∈ keys(guesses)
             push!(defs, var => guesses[var])
@@ -207,7 +207,7 @@ function generate_initializesystem_timeindependent(sys::AbstractSystem;
     op = anydict(op)
     u0map = anydict()
     pmap = anydict()
-    build_operating_point!(sys, op, u0map, pmap, defs, unknowns(sys),
+    build_operating_point!(sys, op, u0map, pmap, Dict(), unknowns(sys),
         parameters(sys; initial_parameters = true))
     for (k, v) in op
         if has_parameter_dependency_with_lhs(sys, k) && is_variable_floatingpoint(k)

test/initializationsystem.jl

@@ -1664,3 +1664,10 @@ end
     sol = solve(prob, Tsit5())
     @test SciMLBase.successful_retcode(sol)
 end
+
+@testset "Defaults removed with ` => nothing` aren't retained" begin
+    @variables x(t)[1:2]
+    @mtkbuild sys = System([D(x[1]) ~ -x[1], x[1] + x[2] ~ 3], t; defaults = [x[1] => 1])
+    prob = ODEProblem(sys, [x[1] => nothing, x[2] => 1], (0.0, 1.0))
+    @test SciMLBase.initialization_status(prob) == SciMLBase.FULLY_DETERMINED
+end