When a role has the "post comments" permission but not the "access comments" permission he will see an "Add new comment" link at the bottom of the node page. It links to #comment-form, but the form is not there, i.e. nothing happens. In D6 the link generates a server hit, but only to show the error message "You are not authorized to view comments."
I don't know if it is intentional that the "access comments" permission prevent posting comments. It seems reasonable that a site admin will allow users to comment on nodes without showing the comments publicly. The error message in D6 doesn't really make sense from an end-user point of view.
This patch allows users to post comments without having permission to view comments.
PASSED: [[SimpleTest]]: [MySQL] 28,805 pass(es). View
PASSED: [[SimpleTest]]: [MySQL] 27,722 pass(es). View
PASSED: [[SimpleTest]]: [MySQL] 25,551 pass(es). View
PASSED: [[SimpleTest]]: [MySQL] 22,642 pass(es). View
FAILED: [[SimpleTest]]: [MySQL] 22,144 pass(es), 1 fail(s), and 0 exception(es). View