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Date: Sat, 14 Jun 2025 16:41:38 -0700 (PDT)
To: cygwin AT cygwin DOT com
Subject: PTHREAD_MUTEX_INITIALIZER and constinit
Message-ID: <87a87123-9ecb-ac15-ddb4-4cdfdf9faee4@jdrake.com>
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From: Jeremy Drake via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Jeremy Drake <cygwin AT jdrake DOT com>
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Sender: "Cygwin" <cygwin-bounces~archive-cygwin=delorie DOT com AT cygwin DOT com>

Consider the following -std=c++20:
#include <pthread.h>

constinit pthread_mutex_t foo = PTHREAD_MUTEX_INITIALIZER;

With g++ 12.4.0, this succeeds, but with clang 20.1.6 it fails:
pthreadtest.cpp:3:27: error: variable does not have a constant initializer
    3 | constinit pthread_mutex_t foo = PTHREAD_MUTEX_INITIALIZER;
      |                           ^     ~~~~~~~~~~~~~~~~~~~~~~~~~
pthreadtest.cpp:3:1: note: required by 'constinit' specifier here
    3 | constinit pthread_mutex_t foo = PTHREAD_MUTEX_INITIALIZER;
      | ^~~~~~~~~
pthreadtest.cpp:3:33: note: cast that performs the conversions of a reinterpret_cast is not allowed in a constant expression
    3 | constinit pthread_mutex_t foo = PTHREAD_MUTEX_INITIALIZER;
      |                                 ^
/usr/include/pthread.h:49:35: note: expanded from macro 'PTHREAD_MUTEX_INITIALIZER'
   49 | #define PTHREAD_MUTEX_INITIALIZER PTHREAD_NORMAL_MUTEX_INITIALIZER_NP
      |                                   ^
/usr/include/pthread.h:47:45: note: expanded from macro 'PTHREAD_NORMAL_MUTEX_INITIALIZER_NP'
   47 | #define PTHREAD_NORMAL_MUTEX_INITIALIZER_NP (pthread_mutex_t)19
      |                                             ^
1 error generated.

It seems the standard disallows this sort of cast pretty comprehensively.
Is there some way that Cygwin's pthread initializer macros can be made
compatible with constinit?

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