[clang] [clang] Implement provisional wording for CWG2398 regarding packs (PR #90820)

[Vlad Serebrennikov via cfe-commits]

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Endilll wrote:

This file is not going to be picked up by the `make_cxx_dr_status`, because it's not in `clang/test/CXX/drs`. If we're implementing what Jason Merill speaks of (_In deduction we can determine that P is more specialized than B, then substitute B into P<T>, and then compare B<T,T> to B<int,int>_), then I agree with `// cwg2398: 19 drafting 2016-12-03` marking.

In the previous PR on this topic I asked for this test to be moved out of DR test suite, because it was testing with `-fno-relaxed-template-template-args`, which is not a conformance mode. This still holds, so this file can't be simply moved to DR test suite as is.

I also don't see why CWG2398 test can't be placed in `dr23xx.cpp`. We place DR tests in separate files only if they require special handling. Typical case is codegen tests. Note that special handling doesn't mean testing non-conformance modes, or reduced number of conformance modes.

https://github.com/llvm/llvm-project/pull/90820