preserve argument order for structural parameters

closes #2688

Author: baggepinnen

Project.toml

@@ -32,6 +32,7 @@ Libdl = "8f399da3-3557-5675-b5ff-fb832c97cbdb"
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
 MLStyle = "d8e11817-5142-5d16-987a-aa16d5891078"
 NaNMath = "77ba4419-2d1f-58cd-9bb1-8ffee604a2e3"
+OrderedCollections = "bac558e1-5e72-5ebc-8fee-abe8a469f55d"
 OrdinaryDiffEq = "1dea7af3-3e70-54e6-95c3-0bf5283fa5ed"
 PrecompileTools = "aea7be01-6a6a-4083-8856-8a6e6704d82a"
 RecursiveArrayTools = "731186ca-8d62-57ce-b412-fbd966d074cd"
@@ -89,6 +90,7 @@ Libdl = "1"
 LinearAlgebra = "1"
 MLStyle = "0.4.17"
 NaNMath = "0.3, 1"
+OrderedCollections = "1"
 OrdinaryDiffEq = "6.73.0"
 PrecompileTools = "1"
 RecursiveArrayTools = "2.3, 3"

src/systems/model_parsing.jl

@@ -50,7 +50,7 @@ function _model_macro(mod, name, expr, isconnector)
     ps, sps, vs, = [], [], []
     c_evts = []
     d_evts = []
-    kwargs = Set()
+    kwargs = OrderedCollections.OrderedSet()
     where_types = Expr[]
 
     push!(exprs.args, :(variables = []))

test/model_parsing.jl

@@ -793,3 +793,27 @@ end
     @test typeof(l_guess) == Num
     @test readable_code(l_guess) == "k(t)"
 end
+
+@testset "Argument order" begin
+    @mtkmodel OrderModel begin
+        @structural_parameters begin
+            b = 1 # reverse alphabetical order to test that the order is preserved
+            a = b
+        end
+        @parameters begin
+            c = a
+            d = b
+        end
+    end
+    @named ordermodel = OrderModel()
+    ordermodel = complete(ordermodel)
+    defs = ModelingToolkit.defaults(ordermodel)
+    @test defs[ordermodel.c] == 1
+    @test defs[ordermodel.d] == 1
+
+    @test_nowarn @named ordermodel = OrderModel(a = 2)
+    ordermodel = complete(ordermodel)
+    defs = ModelingToolkit.defaults(ordermodel)
+    @test defs[ordermodel.c] == 2
+    @test defs[ordermodel.d] == 1
+end