Merge pull request #1582 from SciML/myb/aliasfix

Make sure we substitute both sides just in case

Author: YingboMa

src/structural_transformation/symbolics_tearing.jl

@@ -70,7 +70,7 @@ function full_equations(sys::AbstractSystem; simplify=false)
     solved = Dict(eq.lhs => eq.rhs for eq in subs)
     neweqs = map(equations(sys)) do eq
         if isdiffeq(eq)
-            return eq.lhs ~ tearing_sub(eq.rhs, solved, simplify)
+            return tearing_sub(eq.lhs, solved, simplify) ~ tearing_sub(eq.rhs, solved, simplify)
         else
             if !(eq.lhs isa Number && eq.lhs == 0)
                 eq = 0 ~ eq.rhs - eq.lhs

src/systems/alias_elimination.jl

@@ -52,7 +52,7 @@ function alias_elimination(sys)
 
     dict = Dict(subs)
     for (ieq, eq) in enumerate(eqs)
-        eqs[ieq] = eq.lhs ~ fixpoint_sub(eq.rhs, dict)
+        eqs[ieq] = fixpoint_sub(eq.lhs, dict) ~ fixpoint_sub(eq.rhs, dict)
     end
 
     newstates = []

test/odesystem.jl

@@ -756,3 +756,23 @@ let
     @test arr == sol
     @test typeof(arr) == typeof(sol)
 end
+
+
+let
+    @parameters t
+
+    u = collect(first(@variables u[1:4](t)))
+    Dt = Differential(t)
+
+    eqs = [
+           Differential(t)(u[2]) - 1.1u[1] ~ 0
+           Differential(t)(u[3]) - 1.1u[2] ~ 0
+           u[1] ~ 0.0
+           u[4] ~ 0.0
+          ]
+
+    ps = []
+
+    @named sys = ODESystem(eqs, t, u, ps)
+    @test_nowarn simpsys = structural_simplify(sys)
+end