BUG #15491: index on function not being used for full text searchwhen querying through a view

The following bug has been logged on the website:

Bug reference:      15491
Logged by:          Allen Wu
Email address:      allenwu1973@yahoo.co.jp
PostgreSQL version: 11.0
Operating system:   linux
Description:

Hi

I'm trying to implement full text search with PostgreSQL,
I found when using a function create index in conjunction with querying
through a view, will cause postgres not to use the index.
Below are example for each scenario.

When only using expression index, the index is used
https://dbfiddle.uk/?rdbms=postgres_11&fiddle=16ed7a04389ebf71dc5a1fb2daa34058

When only using view, the index is also used
https://dbfiddle.uk/?rdbms=postgres_11&fiddle=e8d50181b8612421773c2da39e45c9fd

view + expression, the index is NOT used 
https://dbfiddle.uk/?rdbms=postgres_11&fiddle=19d7b923514f100f6e402b54624cc972

I've avoid using function in my app, but I think this should have worked.
Thank you