fix issue 487 (#488)

Co-authored-by: Christopher Doris <github.com/cjdoris>

src/Utils/Utils.jl

@@ -90,8 +90,9 @@ module Utils
         mimes = copy(ALL_MIMES)
         # look for mimes on show methods for this type
         for meth in methods(show, Tuple{IO, MIME, typeof(x)}).ms
-            mimetype = _type_ub(meth.sig).parameters[3]
+            mimetype = _unwrap_unionall(meth.sig).parameters[3]
             mimetype isa DataType || continue
+            mimetype <: MIME || continue
             mime = string(mimetype.parameters[1])
             push!(mimes, mime)
         end
@@ -109,10 +110,7 @@ module Utils
     end
 
     @generated _type_lb(::Type{T}) where {T} = begin
-        R = T
-        while R isa UnionAll
-            R = R.body
-        end
+        R = _unwrap_unionall(T)
         if R isa DataType
             S = T
             while S isa UnionAll
@@ -124,6 +122,14 @@ module Utils
         end
     end
 
+    @generated function _unwrap_unionall(::Type{T}) where {T}
+        R = T
+        while R isa UnionAll
+            R = R.body
+        end
+        R
+    end
+
     @generated _promote_type_bounded(::Type{S}, ::Type{T}, ::Type{B}) where {S,T,B} = begin
         S <: B || error("require S <: B")
         T <: B || error("require T <: B")

test/Utils.jl

@@ -1,6 +1,17 @@
 @testitem "mimes_for" begin
-    for x in Any[1, "foo", [], 'z']
-        @test PythonCall.Utils.mimes_for(x) isa Vector{String}
+    # this example from https://github.com/JuliaPy/PythonCall.jl/issues/487
+    struct Test{T<:Number}
+        x::T
+    end
+    Base.show(io::IO, ::MIME"text/plain", x::Test{T}) where T = show(io, x.t)
+    Base.show(io::IO, ::MIME"text/x-test", x::Test) = show(io, x.t)
+
+    @testset for x in Any[1, "foo", [], 'z', Test(5)]
+        mimes = PythonCall.Utils.mimes_for(x)
+        @test mimes isa Vector{String}
+        @test "text/plain" in mimes
+        @test "text/html" in mimes
+        @test ("text/x-test" in mimes) == (x isa Test)
     end
 end