fix: handle unscalarized defaults for scalarized observed variables

Author: AayushSabharwal

src/systems/problem_utils.jl

@@ -275,9 +275,9 @@ entry for `eq.lhs`, insert the reverse mapping if `eq.rhs` is not a number.
 """
 function add_observed_equations!(varmap::AbstractDict, eqs)
     for eq in eqs
-        if haskey(varmap, eq.lhs)
+        if var_in_varlist(eq.lhs, keys(varmap), nothing)
             eq.rhs isa Number && continue
-            haskey(varmap, eq.rhs) && continue
+            var_in_varlist(eq.rhs, keys(varmap), nothing) && continue
             !iscall(eq.rhs) || issym(operation(eq.rhs)) || continue
             varmap[eq.rhs] = eq.lhs
         else

test/initial_values.jl

@@ -183,3 +183,18 @@ end
     @test_throws ModelingToolkit.MissingParametersError ODEProblem(
         sys, [x => ones(2)], (0.0, 1.0))
 end
+
+@testset "Unscalarized default for scalarized observed variable" begin
+    @parameters p[1:4] = rand(4)
+    @variables x(t)[1:4] y(t)[1:2]
+    eqs = [
+        D(x) ~ x,
+        y[1] ~ x[3],
+        y[2] ~ x[4]
+    ]
+    @mtkbuild sys = ODESystem(eqs, t; defaults = [x => vcat(ones(2), y), y => x[1:2] ./ 2])
+    prob = ODEProblem(sys, [], (0.0, 1.0))
+    sol = solve(prob)
+    @test SciMLBase.successful_retcode(sol)
+    @test sol.u[1] ≈ [1.0, 1.0, 0.5, 0.5]
+end