Ultraspherical conversion error catching (#32)

* Ultraspherical conversion error catching

Closes #31

* Ultraspherical conversion fix error catch

* Add tests for Ultraspherical conversion argument error

Author: Gyoshi

src/Spaces/Ultraspherical/UltrasphericalOperators.jl

@@ -141,14 +141,14 @@ function Conversion(A::Ultraspherical,B::Ultraspherical)
     a=order(A); b=order(B)
     if b==a
         ConversionWrapper(Operator(I,A))
-    elseif a<b≤a+1  || b<a≤b+1
+    elseif -1 ≤ b-a ≤ 1 && (a,b) ≠ (2,1)
         ConcreteConversion(A,B)
-    elseif b ≠ 1
+    elseif b-a > 1
         d=domain(A)
         ConversionWrapper(TimesOperator(Conversion(Ultraspherical(b-1,d),B),
                                         Conversion(A,Ultraspherical(b-1,d))))
     else
-        error("Cannot convert from $A to $B")
+        throw(ArgumentError("Cannot convert from $A to $B"))
     end
 end
 

test/UltrasphericalTest.jl

@@ -0,0 +1,13 @@
+using ApproxFunOrthogonalPolynomials, ApproxFunBase, Test, SpecialFunctions, LinearAlgebra
+import ApproxFunBase: testbandedbelowoperator, testbandedoperator, testspace, testtransforms, Vec,
+                    maxspace, NoSpace, hasconversion, testfunctional,
+                    reverseorientation, ReverseOrientation
+import ApproxFunOrthogonalPolynomials: jacobip
+
+@testset "Ultraspherical" begin
+    @testset "Conversion" begin
+        # Tests whether invalid/unimplemented arguments correctly throws ArgumentError
+        @test_throws ArgumentError Conversion(Ultraspherical(2), Ultraspherical(1))
+        @test_throws ArgumentError Conversion(Ultraspherical(3), Ultraspherical(1.9))
+    end
+end