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This implements the same overload resolution behavior as GCC, as described in https://wg21.link/p3606 (section 1-2, not 3) If during overload resolution, there is a non-template candidate that would be always be picked - because each of the argument is a perfect match (ie the source and target types are the same), we do not perform deduction for any template candidate that might exists. The goal is to be able to merge https://github.com/llvm/llvm-project/pull/122423 without being too disruptive. This change means that the selection of the best viable candidate and template argument deduction become interleaved. To avoid rewriting half of Clang we store in OverloadCandidateSet enough information to be able to deduce template candidates from OverloadCandidateSet::BestViableFunction. Which means the lifetime of any object used by template argument must outlive a call to Add*Template*Candidate. This two phase resolution is not performed for some initialization as there are cases where template candidate are better match in these cases per the standard. It's also bypassed for code completion. The change has a nice impact on compile times https://llvm-compile-time-tracker.com/compare.php?from=719b029c16eeb1035da522fd641dfcc4cee6be74&to=bf7041045c9408490c395230047c5461de72fc39&stat=instructions%3Au Fixes https://github.com/llvm/llvm-project/issues/62096 Fixes https://github.com/llvm/llvm-project/issues/74581 Reapplies https://github.com/llvm/llvm-project/pull/133426