Add a test throws an error by invalid subtyping in definition (JuliaLang#20523)

Author: wookay

test/subtype.jl

@@ -911,3 +911,6 @@ ftwoparams(::TwoParams{<:Real,<:Real}) = 3
 @test TwoParams{Real,Complex}(3,0im) isa TwoParams{>:Int,<:Number}
 @test !(TwoParams(3.0,0im) isa TwoParams{>:Int,<:Number})
 @test !(TwoParams(3,'x') isa TwoParams{>:Int,<:Number})
+
+# Issue #20523
+@test_throws ErrorException immutable A20523 <: Base.AbstractSet; end