add a test for #11015, diagonal dispatch inference error fixed by jb/subtype

fixes #11015

Author: JeffBezanson

test/inference.jl

@@ -491,3 +491,13 @@ tpara18457{I}(::Type{AbstractMyType18457{I}}) = I
 tpara18457{A<:AbstractMyType18457}(::Type{A}) = tpara18457(supertype(A))
 @test tpara18457(MyType18457{true}) === true
 
+# issue #11015
+type AT11015
+    f::Union{Bool,Function}
+end
+
+g11015{S}(::Type{S}, ::S) = Int64(1)
+f11015(a::AT11015) = g11015(Base.fieldtype(typeof(a), :f), true)
+g11015(::Type{Bool}, ::Bool) = 2.0
+@test Int <: Base.return_types(f11015, (AT11015,))[1]
+@test f11015(AT11015(true)) === Int64(1)