Test for a failing double-template with default

[stuglaser]

Integration test for #1413

autocxx appears to drop the first template, and then the C++ compiler cannot handle the resulting incompatible types.

[adetaylor]

Thanks very much for raising this.

I've taken a bit of a look, and for some reason the unused_template_parameter annotation isn't being applied to the typedef. I'll see if I can work out why.

[adetaylor]

Diagnosis:

• The typedef phase happens before the POD analysis phase (for reasons I can't remember, but I remember they were good reasons)
• The POD analysis phase replaces Holder with an IgnoredItem due to the unused template parameter.
• So, if these phases happened the other way round, then find_ignored_types would have marked Holder as troublesome, and convert_type_path would have (correctly) rejected the WidgetRefHolder with ConvertErrorFromCpp::ConcreteVersionOfIgnoredTemplate

We might need a POD pre-analysis phase to spot this annotation before the typedef phase.