Unique index using hash?

I was playing around with my latest compile and tried to make a unique
index on two columns, using a hash method. Both of these (more than
one column and unique) are currently not allowed for hash indexes.

I thought about this for a bit and realized that making a NESTED hash
index (index on a and b also serves as an index on a) would be a
trick, but allowing the unique clause should not be a problem.

Therefore, I would like to try implementing unique constraints on hash
indexes. Has this come up before? Are there any reasons not to
support this? As far as I understand, specifying an index method is
non standard (above and beyond standard) to begin with.

It is on the TODO list:

* add UNIQUE capability to non-btree indexes

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