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### 2981. Remove redundant deduction guides from standard library

Section: 22.10.6 [refwrap], 33.6.5.2 [thread.lock.guard], 33.6.5.3 [thread.lock.scoped], 33.6.5.4 [thread.lock.unique], 33.6.5.5 [thread.lock.shared] Status: C++20 Submitter: Mike Spertus Opened: 2017-06-15 Last modified: 2021-02-25

Priority: 0

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Discussion:

There are several deduction guides added to the standard library by P0433R2 that have no effect probably because LWG had not considered late changes to core wording that automatically adds a "copy deduction candidate" (12.2.2.9 [over.match.class.deduct]) that renders these explicit guides moot.

[2017-07 Toronto Monday issue prioritization]

Priority 0; move to Ready

Proposed resolution:

This wording is relative to N4659.

1. Edit 22.10.6 [refwrap], end of class template `reference_wrapper` synopsis, as indicated:

   >     template<class T>
   >       reference_wrapper(reference_wrapper<T>) -> reference_wrapper<T>;

2. Edit 33.6.5.2 [thread.lock.guard], end of class template `lock_guard` synopsis, as indicated:

   >     template<class Mutex> lock_guard(lock_guard<Mutex>) -> lock_guard<Mutex>;

3. Edit 33.6.5.3 [thread.lock.scoped], end of class template `scoped_lock` synopsis, as indicated:

   >     template<class... MutexTypes>
   >       scoped_lock(scoped_lock<MutexTypes...>) -> scoped_lock<MutexTypes...>;

4. Edit 33.6.5.4 [thread.lock.unique], end of class template `unique_lock` synopsis, as indicated:

   >     template<class Mutex> unique_lock(unique_lock<Mutex>) -> unique_lock<Mutex>;

5. Edit 33.6.5.5 [thread.lock.shared], end of class template `shared_lock` synopsis, as indicated:

   >     template<class Mutex> shared_lock(shared_lock<Mutex>) -> shared_lock<Mutex>;