fix: fix nested conditions in IfLifting

Author: AayushSabharwal

src/systems/if_lifting.jl

@@ -111,8 +111,8 @@ function (cw::CondRewriter)(expr, dep)
         # and ELSE branch is true
         # similarly for expression being false
         return (ifelse(rw_cond, rw_conda, rw_condb),
-            implies(ctrue, truea) | implies(cfalse, trueb),
-            implies(ctrue, falsea) | implies(cfalse, falseb))
+            ctrue & truea | cfalse & trueb,
+            ctrue & falsea | cfalse & falseb)
     elseif operation(expr) == Base.:(!) # NOT of expression
         (a,) = arguments(expr)
         (rw, ctrue, cfalse) = cw(a, dep)

test/if_lifting.jl

@@ -124,3 +124,43 @@ end
     end
     @test_nowarn @mtkbuild sys=SimpleAbs() additional_passes=[IfLifting]
 end
+
+@testset "Nested conditions are handled properly" begin
+    @mtkmodel RampModel begin
+        @variables begin
+            x(t)
+            y(t)
+        end
+        @parameters begin
+            start_time = 1.0
+            duration = 1.0
+            height = 1.0
+        end
+        @equations begin
+            y ~ ifelse(start_time < t,
+                ifelse(t < start_time + duration,
+                    (t - start_time) * height / duration, height),
+                0.0)
+            D(x) ~ y
+        end
+    end
+    @mtkbuild sys = RampModel()
+    @mtkbuild sys2=RampModel() additional_passes=[IfLifting]
+    prob = ODEProblem(sys, [sys.x => 1.0], (0.0, 3.0))
+    prob2 = ODEProblem(sys2, [sys.x => 1.0], (0.0, 3.0))
+    sol = solve(prob)
+    sol2 = solve(prob2)
+    @test sol(0.99)[1] > 1.0
+    @test sol2(0.99)[1] == 1.0
+    # During ramp
+    # D(x) ~ t - 1
+    # x ~ t^2 / 2 - t + C, and `x(1) ~ 1` => `C = 3/2`
+    # x(1.01) ~ 1.01^2 / 2 - 1.01 + 3/2 ~ 1.00005
+    @test sol2(1.01)[1] ≈ 1.00005
+    @test sol2(2)[1] ≈ 1.5
+    # After ramp
+    # D(x) ~ 1
+    # x ~ t + C and `x(2) ~ 3/2` => `C = -1/2`
+    # x(3) ~ 3 - 1/2
+    @test sol2(3)[1] ≈ 5 / 2
+end