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### 1044. Empty tag types should be `constexpr` literals

Section: 32.5 [thread.mutex] Status: C++11 Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

Addresses UK 325 [CD1]

We believe `constexpr` literal values should be a more natural expression of empty tag types than extern objects as it should improve the compiler's ability to optimize the empty object away completely.

[ Summit: ]

> Move to review. The current specification is a "hack", and the proposed specification is a better "hack".

[ Batavia (2009-05): ]

> We agree with the proposed resolution. Move to Tentatively Ready.

Proposed resolution:

Change the synopsis in 32.5 [thread.mutex]:

>     struct defer_lock_t {};
>     struct try_to_lock_t {};
>     struct adopt_lock_t {};

>     extern constexpr defer_lock_t defer_lock {};
>     extern constexpr try_to_lock_t try_to_lock {};
>     extern constexpr adopt_lock_t adopt_lock {};