feat: improve error message when system contains array equations

Author: AayushSabharwal

src/systems/abstractsystem.jl

@@ -2859,6 +2859,9 @@ function check_eqs_u0(eqs, dvs, u0; check_length = true, kwargs...)
     if u0 !== nothing
         if check_length
             if !(length(eqs) == length(dvs) == length(u0))
+                if any(eq -> Symbolics.isarraysymbolic(eq.lhs), eqs)
+                    throw(ArgumentError("The system has array equations. Call `structural_simplify` to handle such equations."))
+                end
                 throw(ArgumentError("Equations ($(length(eqs))), unknowns ($(length(dvs))), and initial conditions ($(length(u0))) are of different lengths."))
             end
         elseif length(dvs) != length(u0)

test/odesystem.jl

@@ -1394,3 +1394,18 @@ end
     prob = @test_nowarn ODEProblem(sys, nothing, (0.0, 1.0))
     @test_nowarn solve(prob)
 end
+
+@testset "Issue #2597" begin
+    @mtkmodel Foo begin
+        @variables begin
+            x(t)[1:2]
+        end
+        @equations begin
+            D.(x) ~ .-x
+        end
+    end
+    @named foo = Foo()
+    foo = complete(foo)
+    @test_throws ["array equations", "structural_simplify"] ODEProblem(
+        foo, Dict(foo.x => [1.0, 1]), (0.0, 1.0))
+end