Fix mapexponents_to! with same variables (#106)

Author: blegat

src/cmult.jl

@@ -111,7 +111,7 @@ function MP.mapexponents_to!(output::Monomial{true}, f::Function, x::Monomial{tr
             copyto!(output.vars, x.vars)
             resize!(output.z, n)
         end
-        _operate_exponents_to!(x.z, f, x.z, y.z)
+        _operate_exponents_to!(output.z, f, x.z, y.z)
     else
         _multdivmono!(output.z, output.vars, x.vars, y, f, x.z)
     end

test/mono.jl

@@ -1,3 +1,5 @@
+using Test
+
 @testset "PolyVar and Monomial tests" begin
     @testset "PolyVar macro index set" begin
         n = 3
@@ -136,5 +138,8 @@
     @testset "TODO remove when added to MP" begin
         @polyvar x y
         @test x == DynamicPolynomials.MP.mapexponents!(div, x^1, x * y^2)
+        for z in [x * y, x^2, y^2]
+            @test y == DynamicPolynomials.MP.mapexponents_to!(z, -, x * y^2, x * y)
+        end
     end
 end