From b09145438ee2f63681376df0afba3794a3990ba7 Mon Sep 17 00:00:00 2001
From: Jishnu Bhattacharya <jishnub@users.noreply.github.com>
Date: Sun, 7 Aug 2022 16:30:22 +0530
Subject: [PATCH] UnsetSpace compatibility

---
 src/Space.jl       | 2 ++
 test/SpacesTest.jl | 5 +++++
 2 files changed, 7 insertions(+)

diff --git a/src/Space.jl b/src/Space.jl
index a5c9a9f2..843044a3 100644
--- a/src/Space.jl
+++ b/src/Space.jl
@@ -114,6 +114,8 @@ canonicaldomain(S::Space) = canonicaldomain(domain(S))
 # This is used in place of == to support AnyDomain
 spacescompatible(f::D,g::D) where D<:Space = error("Override spacescompatible for "*string(D))
 spacescompatible(::UnsetSpace,::UnsetSpace) = true
+spacescompatible(::Space, ::UnsetSpace) = true
+spacescompatible(::UnsetSpace, ::Space) = true
 spacescompatible(::NoSpace,::NoSpace) = true
 spacescompatible(f,g) = false
 ==(A::Space,B::Space) = spacescompatible(A,B) && domain(A) == domain(B)
diff --git a/test/SpacesTest.jl b/test/SpacesTest.jl
index 09c455f8..0e39593d 100644
--- a/test/SpacesTest.jl
+++ b/test/SpacesTest.jl
@@ -173,5 +173,10 @@ using ApproxFunOrthogonalPolynomials
         v = rand(4)
         v2 = transform(NormalizedChebyshev(), v)
         @test itransform(NormalizedChebyshev(), v2) ≈ v
+
+        @testset "ApproxFun.jl issue 616" begin
+            x, D = Fun(), Derivative(Chebyshev())
+            @test (D*Multiplication(x)*D)*x ≈ 1
+        end
     end
 end