Add promote_symtype for ifelse

Author: YingboMa

src/methods.jl

@@ -141,6 +141,7 @@ end
 function ifelse(_if::Symbolic{Bool}, _then, _else)
     Term{Union{symtype(_then), symtype(_else)}}(ifelse, Any[_if, _then, _else])
 end
+promote_symtype(::typeof(ifelse), _, ::Type{T}, ::Type{S}) where {T,S} = Union{T, S}
 Base.@deprecate cond(_if, _then, _else) ifelse(_if, _then, _else)
 
 # Specially handle inv and literal pow

test/basics.jl

@@ -1,5 +1,6 @@
 using SymbolicUtils: Sym, FnType, Term, Add, Mul, Pow, symtype, operation, arguments
 using SymbolicUtils
+using IfElse: ifelse
 using Test
 
 @testset "@syms" begin
@@ -82,6 +83,7 @@ end
 
     @test symtype(ifelse(true, 4, 5)) == Int
     @test symtype(ifelse(a < 0, b, w)) == Union{Real, Complex}
+    @test SymbolicUtils.promote_symtype(ifelse, Bool, Int, Bool) == Union{Int, Bool}
     @test_throws MethodError w < 0
     @test isequal(w == 0, Term{Bool}(==, [w, 0]))
 end