tree 3a3a158a668889023fa97484c04b36fcf077c927
parent a19461d9e114bea99186a4f0ea092a650a1650c2
author Richard Smith <richard@metafoo.co.uk> 1594745986 -0700
committer Richard Smith <richard@metafoo.co.uk> 1594746127 -0700

Fix merging of two arity-only pack deductions.

If we deduced the arity of a pack in two different ways, but didn't
deduce an element of the pack in either of those deductions, we'd merge
that element to produce a null template argument, which we'd incorrectly
interpret as the merge having failed.

Testcase based on one supplied by Hubert Tong.