[clang] [clang] fix assertion substituting decltype parameters during partially ordered deduction (PR #214130)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> These void_t examples comparing with GCC goes besides the point of the original issue.
> 

The problem I'm having is trying to make a decent test set for the cases you've mentioned and in doing so found these cases, and as far as I can make out we don't have existing test coverage here (I'll need to recheck but I don't believe _this_ PR breaks any tests) which is worrying for something as complex as this.

So I gave these test cases as a behavioural change and I couldn't work out if the change was _correct_ given it did change accept/reject but the change matched gcc. If the baseline test behavior is correct I'll make a testcase PR to add them to the repository.


https://github.com/llvm/llvm-project/pull/214130