fix: ensure => nothing overrides defaults

src/systems/nonlinear/initializesystem.jl

@@ -61,6 +61,12 @@ function generate_initializesystem_timevarying(sys::AbstractSystem;
     isempty(trueobs) || filter_delay_equations_variables!(sys, trueobs)
     vars = unique([unknowns(sys); getfield.(trueobs, :lhs)])
     vars_set = Set(vars) # for efficient in-lookup
+    arrvars = Set()
+    for var in vars
+        if iscall(var) && operation(var) === getindex
+            push!(arrvars, first(arguments(var)))
+        end
+    end
 
     eqs_ics = Equation[]
     defs = copy(defaults(sys)) # copy so we don't modify sys.defaults
@@ -71,9 +77,13 @@ function generate_initializesystem_timevarying(sys::AbstractSystem;
 
     # PREPROCESSING
     op = anydict(op)
+    if isempty(op)
+        op = copy(defs)
+    end
+    scalarize_vars_in_varmap!(op, arrvars)
     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)
@@ -144,7 +154,7 @@ 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])
@@ -238,7 +248,7 @@ 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)

src/systems/problem_utils.jl

@@ -523,6 +523,24 @@ function scalarize_varmap!(varmap::AbstractDict)
     return varmap
 end
 
+"""
+    $(TYPEDSIGNATURES)
+
+For each array variable in `vars`, scalarize the corresponding entry in `varmap`.
+If a scalarized entry already exists, it is not overridden.
+"""
+function scalarize_vars_in_varmap!(varmap::AbstractDict, vars)
+    for var in vars
+        symbolic_type(var) == ArraySymbolic() || continue
+        is_sized_array_symbolic(var) || continue
+        haskey(varmap, var) || continue
+        for i in eachindex(var)
+            haskey(varmap, var[i]) && continue
+            varmap[var[i]] = varmap[var][i]
+        end
+    end
+end
+
 function get_temporary_value(p, floatT = Float64)
     stype = symtype(unwrap(p))
     return if stype == Real

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