Add a test throws an error by invalid subtyping in definition (#19414) (#20548)

Author: wookay

test/subtype.jl

@@ -1071,6 +1071,11 @@ end
                Tuple{C20992{S, n, T, D, d} where d where D where T where n where S, Any},
                Tuple{C20992, Int})
 
+# Issue #19414
+let ex = try struct A19414 <: Base.AbstractSet end catch e; e end
+    @test isa(ex, ErrorException) && ex.msg == "invalid subtyping in definition of A19414"
+end
+
 # issue #20103, OP and comments
 struct TT20103{X,Y} end
 f20103{X,Y}(::Type{TT20103{X,Y}},x::X,y::Y) = 1