Commits on May 16, 2024

1. [clang] Implement provisional wording for CWG2398 regarding packs …

   This solves some ambuguity introduced in P0522 regarding how
   template template parameters are partially ordered, and should reduce
   the negative impact of enabling `-frelaxed-template-template-args`
   by default.
   
   When performing template argument deduction, a template template parameter
   containing no packs should be more specialized than one that does.
   
   Given the following example:
   ```C++
   template<class T2> struct A;
   template<template<class ...T3s> class TT1, class T4> struct A<TT1<T4>>; // #1
   template<template<class    T5 > class TT2, class T6> struct A<TT2<T6>>; // #2
   
   template<class T1> struct B;
   template struct A<B<char>>;
   ```
   
   Prior to P0522, candidate #2 would be more specialized.
   After P0522, neither is more specialized, so this becomes ambiguous.
   With this change, #2 becomes more specialized again,
   maintaining compatibility with pre-P0522 implementations.
   
   The problem is that in P0522, candidates are at least as specialized
   when matching packs to fixed-size lists both ways, whereas before,
   a fixed-size list is more specialized.
   
   This patch keeps the original behavior when checking template arguments
   outside deduction, but restores this aspect of pre-P0522 matching
   during deduction.
   
   ---
   
   Since this changes provisional implementation of CWG2398 which has
   not been released yet, and already contains a changelog entry,
   we don't provide a changelog entry here.

   

   mizvekov committed May 16, 2024
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