Add test for custom type dimensional reduction

test/reducedim.jl

@@ -358,3 +358,18 @@ end
     @test B[argmax(B, dims=[2, 3])] == maximum(B, dims=[2, 3])
     @test B[argmin(B, dims=[2, 3])] == minimum(B, dims=[2, 3])
 end
+
+# issue #26709 (dimensional reduce with custom non-bitstype types)
+struct Variable
+    name::Symbol
+end
+struct AffExpr
+    vars::Vector{Variable}
+end
+Base.zero(::Union{Variable, Type{Variable}, AffExpr}) = AffExpr(Variable[])
+Base.:+(v::Variable, w::Variable) = AffExpr([v, w])
+Base.:+(aff::AffExpr, v::Variable) = AffExpr([aff.vars; v])
+Base.:+(aff1::AffExpr, aff2::AffExpr) = AffExpr([aff1.vars; aff2.vars])
+Base.:(==)(a::Variable, b::Variable) = a.name == b.name
+Base.:(==)(a::AffExpr, b::AffExpr) = a.vars == b.vars
+@test sum([Variable(:x), Variable(:y)], dims=1) == [AffExpr([Variable(:x), Variable(:y)])]