Problem/Motivation
The Simpletest module requires a PHP Curl library to be installed. This requirement only comes up in the error message when a user tries to install the module, and it consequently fails.
The error message itself only links to a page on php.net that gives the Drupal user who followed the link no information that is relevant for a Drupal installation.
Before
After
Proposed resolution
Disable the ability to enable Simpletest if Curl is not available.
Provide a useful inline message describing why the module is not able to be used.
Curl is a requirement of other modules, like aggregator.module, so it may make sense to centralise documentation about the Curl requirement and refer to the documentation page in core. This could be a follow up documentation issue.
Remaining tasks
Create follow up documentation issue to create documentation page on Drupal.org
User interface changes
This is a user interface change. Information about this should also go into the help text, because it would explain users why the module might be possible to install on one instance of the site, but not on another.
API changes
None.