fix: error when simplified discrete system contains algebraic equations

Author: AayushSabharwal

Diff for: src/systems/systems.jl

@@ -26,6 +26,13 @@ function structural_simplify(
     else
         newsys = newsys′
     end
+    if newsys isa DiscreteSystem &&
+       any(eq -> symbolic_type(eq.lhs) == NotSymbolic(), equations(newsys))
+        error("""
+            Encountered algebraic equations when simplifying discrete system. This is \
+            not yet supported.
+        """)
+    end
     if newsys isa ODESystem || has_parent(newsys)
         @set! newsys.parent = complete(sys; split)
     end

Diff for: test/discrete_system.jl

@@ -265,3 +265,9 @@ function System(; name, buffer)
 end
 
 @test_nowarn @mtkbuild sys = System(; buffer = ones(10))
+
+# Ensure discrete systems with algebraic equations throw
+@variables x(t) y(t)
+k = ShiftIndex(t)
+@named sys = DiscreteSystem([x ~ x^2 + y^2, y ~ x(k - 1) + y(k - 1)], t)
+@test_throws ["algebraic equations", "not yet supported"] structural_simplify(sys)