Added test from issue (and a simplified version of it)

Author: hersle

test/odesystem.jl

@@ -1194,3 +1194,23 @@ end
     @test_nowarn obsfn(buffer, [1.0], ps..., 3.0)
     @test buffer ≈ [2.0, 3.0, 4.0]
 end
+
+# https://github.com/SciML/ModelingToolkit.jl/issues/2859
+@testset "Initialization with defaults from observed equations (edge case)" begin
+    @variables x(t) y(t) z(t)
+    eqs = [D(x) ~ 0, y ~ x, D(z) ~ 0]
+    defaults = [x => 1, z => y]
+    @named sys = ODESystem(eqs, t; defaults)
+    ssys = structural_simplify(sys)
+    prob = ODEProblem(ssys, [], (0.0, 1.0), [])
+    @test prob[x] == prob[y] == prob[z] == 1.0
+
+    @parameters y0
+    @variables x(t) y(t) z(t)
+    eqs = [D(x) ~ 0, y ~ y0 / x, D(z) ~ y]
+    defaults = [y0 => 1, x => 1, z => y]
+    @named sys = ODESystem(eqs, t; defaults)
+    ssys = structural_simplify(sys)
+    prob = ODEProblem(ssys, [], (0.0, 1.0), [])
+    @test prob[x] == prob[y] == prob[z] == 1.0
+end