Hi Cedric:

Check out rev 6f89cdc7c938. I think I fixed your reported cases:

  [issue1](issue1)
  [issue1](https://example.com/issue1)

I also tested to make sure that:

  [issue1] (https://example.com/issue1)

(incorrect markdown syntax) did replace the first issue1 with
a link. Note that mistune does process the url into a link.
The other two back ends just leave it as is.

I was not able to reproduce your test case where the input:

     [issue1](https://example.com/issue1)

tried to turn the issue1 in https://example.com/issue1 into a link.
From the regexp that matches designators (aka items) I don't
see why the issue1 isn't being returned but.....

Can you take a look and if it solves your issue please close this 
ticket.

Thanks.

-- rouilj

P.S. rev 316c2c32dace is needed to fix some test cases broken by my 
changes to the test harness.