Remove IfElse.jl dependency; use Base.ifelse

IfElse.jl's `ifelse` is `const ifelse = Base.ifelse` on modern Julia (0.1.1),
so the dependency is redundant: Symbolics registers a symbolic method on
`Base.ifelse`, so `ifelse(sym_cond, a, b)` traces to a symbolic expression
without IfElse. Removing IfElse also unblocks the strict Downgrade lane, which
previously could resolve IfElse 0.1.0 (whose greedy `ifelse(args...) =
Core.ifelse(args...)` broke symbolic conditions).

- Replace `IfElse.ifelse(...)` call sites with bare `ifelse(...)`.
- Drop every `using IfElse` / `import IfElse` line.
- Remove IfElse from [deps] and [compat].
- In Blocks, the removed `import IfElse: ifelse` had brought `ifelse` into
  scope so the `Parameter` overloads in sources.jl extended it. Replace with
  `import Base: ifelse` so those overloads extend `Base.ifelse` instead of
  creating a new `Blocks.ifelse` binding that would shadow the symbolic method.
- Drop the now-stale Downgrade.yml comment referencing the fntype regression.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>

Author: ChrisRackauckas

.github/workflows/Downgrade.yml

@@ -12,7 +12,7 @@ on:
       - 'docs/**'
 jobs:
   test:
-    # Runs strict (allow_reresolve=false); expected RED until fntype regression (#458/#452) is resolved.
+    # Runs strict (allow_reresolve=false).
     uses: "SciML/.github/.github/workflows/downgrade.yml@v1"
     with:
       julia-version: "lts"

Project.toml

@@ -6,7 +6,6 @@ authors = ["Chris Rackauckas and Julia Computing"]
 [deps]
 ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
 DiffEqBase = "2b5f629d-d688-5b77-993f-72d75c75574e"
-IfElse = "615f187c-cbe4-4ef1-ba3b-2fcf58d6d173"
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
 ModelingToolkitBase = "7771a370-6774-4173-bd38-47e70ca0b839"
 PreallocationTools = "d236fae5-4411-538c-8e31-a6e3d9e00b46"
@@ -20,7 +19,6 @@ DataFrames = "1.7"
 DataInterpolations = "8"
 DiffEqBase = "6.190, 7"
 ForwardDiff = "0.10, 1"
-IfElse = "0.1"
 LinearAlgebra = "1.10"
 ModelingToolkit = "11"
 ModelingToolkitBase = "1"

src/Blocks/Blocks.jl

@@ -6,7 +6,7 @@ using ModelingToolkitBase: ModelingToolkitBase, @component, @connector, @named,
     @parameters, @unpack, System, compose, connect, extend,
     getdefault, t_nounits as t, D_nounits as D
 using Symbolics: Symbolics, @register_symbolic, @variables, Differential, Equation
-import IfElse: ifelse
+import Base: ifelse
 import ..@symcheck
 
 export RealInput, RealInputArray, RealOutput, RealOutputArray, SISO

src/Electrical/Digital/sources.jl

@@ -84,7 +84,7 @@ function Pulse(; name, duty_cycle = 0.5, T = 1.0)
     @named d = DigitalPin()
 
     eqs = [
-        d.val ~ IfElse.ifelse(t % T > duty_cycle * T, 1, 0),
+        d.val ~ ifelse(t % T > duty_cycle * T, 1, 0),
     ]
     return System(eqs, t, [], [], systems = [d], name = name)
 end

src/Electrical/Electrical.jl

@@ -4,7 +4,6 @@ This library contains electrical components to build up analog circuits.
 """
 module Electrical
 
-using IfElse: IfElse
 using ModelingToolkitBase: ModelingToolkitBase, @component, @connector, @constants,
     @named, @parameters, @unpack, Flow, System, connect, extend,
     t_nounits as t, D_nounits as D

src/Electrical/utils.jl

@@ -105,9 +105,9 @@ end
 @connector function DigitalPin(; name)
     @variables val(t) v(t) i(t)
     eqs = [
-        val ~ IfElse.ifelse(
+        val ~ ifelse(
             (0.0 <= v) & (v <= 0.8) | (2.0 <= v) & (v <= 5.0),
-            IfElse.ifelse(v > 2.0, 1, 0), X
+            ifelse(v > 2.0, 1, 0), X
         ),
     ]
     System(

src/Hydraulic/IsothermalCompressible/IsothermalCompressible.jl

@@ -11,8 +11,6 @@ using Symbolics: Symbolics, @register_derivative, @register_symbolic, @variables
 using ...Blocks: RealInput
 using ...Mechanical.Translational: MechanicalPort, Mass
 
-using IfElse: ifelse
-
 export HydraulicPort, HydraulicFluid
 include("utils.jl")
 

src/Mechanical/MultiBody2D/MultiBody2D.jl

@@ -1,6 +1,5 @@
 module MultiBody2D
 
-using IfElse: IfElse
 using ModelingToolkitBase: ModelingToolkitBase, @component, @named, @parameters,
     System, t_nounits as t, D_nounits as D
 using Symbolics: Symbolics, @variables, Equation

src/Mechanical/Rotational/Rotational.jl

@@ -3,7 +3,6 @@ Library to model 1-dimensional, rotational mechanical systems
 """
 module Rotational
 
-using IfElse: IfElse
 using ModelingToolkitBase: ModelingToolkitBase, @component, @connector, @named,
     @parameters, @unpack, Flow, System, compose, extend, t_nounits as t, D_nounits as D
 using Symbolics: Symbolics, @variables, Equation

src/Mechanical/TranslationalModelica/TranslationalModelica.jl

@@ -3,7 +3,6 @@ Library to model 1-dimensional, translational mechanical components.
 """
 module TranslationalModelica
 
-using IfElse: IfElse
 using ModelingToolkitBase: ModelingToolkitBase, @component, @connector, @named,
     @parameters, @unpack, Flow, System, compose, extend, t_nounits as t, D_nounits as D
 using Symbolics: Symbolics, @variables, Equation