Use index to estimate expression selectivity

Dear hackers,

After discussing the issue on irc, it looks like it could be possible 
for the planner to use a partial index matching an expression exactly to 
estimate its selectivity.

Here is a simplified version (thanks ysch) of the issue I am facing:

https://dbfiddle.uk/flPq8-pj

I have tried using CREATE STATISTICS as well but haven't found a way to 
improve the planner estimation for that query.

I have worked around the problem for my specific use case but that 
behavior is counter-intuitive, is there any interest in improving it?

Thank you!