[clang] [clang] CTAD: implement the missing IsDeducible constraint for alias templates (PR #89358)

[via cfe-commits]

cor3ntin wrote:

@hokein Independently of the direction taken I'd like to see a better diagnostic than "atomic constraint using an undocumented/cryptic trait that is not in the code is not satisfied". So when we try to print atomic constraints, we should do something more user friendly for is_deducible. (`note_atomic_constraint_evaluated_to_false` in `diagnoseWellFormedUnsatisfiedConstraintExpr` AFAICT).
It might be a bit ad-hoc, but I think it's worth doing

https://github.com/llvm/llvm-project/pull/89358