Thanks,

vhdl-mode is using function 'comment-region' from the ‘newcomment.el’ 
package. With the default value of 1 for option 'comment-padding' the 
comment string "-- " is inserted, resulting in

-- -- this is a test comment

-- library ieee;
-- use        ieee.std_logic_1164.all

for which uncommenting works fine.  You might have changed 
'comment-padding' to 0, which is when you get

---- this is a test comment

--library ieee;
--use        ieee.std_logic_1164.all

For this, 'comment-region' doesn't work correctly for languages with 
comment starts that contain 2 of the same characters, like "--" in VHDL 
or "//" in C++. C++-mode doesn't honor the 'comment-padding' option 
though and always inserts "// ", so the problem doesn't occur.

Remedies:

 1. Use 1 for 'comment-padding'
 2. File a bug report against 'newcomment.el'
 3. Make vhdl-mode not honor option 'comment-padding'

Regards,
Reto


On 2020-08-18 13:39, Stefan Kangas wrote:
> Hi,
>
> Reto Zimmermann <reto@gnu.org> writes:
>
>> The file is missing.  Could you please re-attach?
> Sorry, it was missing in my reply.
>
> You can find the file here:
> https://debbugs.gnu.org/cgi/bugreport.cgi?bug=22368#8
>
> Best regards,
> Stefan Kangas