Extended Union Type Isn't Supported

[engineerjoe440]

I stumbled across something where unions can extend other unions. Turns out that part of the grammar wasn't fully supported.

Here's an example of the broken logic:

TYPE THING EXTENDS THING_BASE:
UNION
	{attribute 'hide' := 'true'}
	As_THING_BASE : THING_BASE;
END_UNION
END_TYPE