[v3] fix PR 53578 invalid narrowing conversion

[Jonathan Wakely]

This fixes an invalid narrowing conversion that causes problems when
libstdc++ is used with clang++ on non-pthreads targets.

        PR libstdc++/53578
        * include/ext/concurrence.h (__recursive_mutex::_S_destroy): Fix
        narrowing conversion.
        * include/std/mutex (__recursive_mutex_base::_S_destroy): Likewise.

Tested x86_64-linux with _GTHREAD_USE_RECURSIVE_MUTEX_INIT_FUNC
defined so the code is executed. Committed to trunk.

commit 583eb7d9fdcf5365c286fd85c0b43406a9ce46ef
Author: Jonathan Wakely <jwakely....@gmail.com>
Date:   Fri Jun 15 01:01:03 2012 +0100

        PR libstdc++/53578
        * include/ext/concurrence.h (__recursive_mutex::_S_destroy): Fix
        narrowing conversion.
        * include/std/mutex (__recursive_mutex_base::_S_destroy): Likewise.

diff --git a/libstdc++-v3/include/ext/concurrence.h 
b/libstdc++-v3/include/ext/concurrence.h
index 22c433b..25e218b 100644
--- a/libstdc++-v3/include/ext/concurrence.h
+++ b/libstdc++-v3/include/ext/concurrence.h
@@ -284,7 +284,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
     // matches a gthr-win32.h recursive mutex
     template<typename _Rm>
-      static typename __enable_if<sizeof(&_Rm::sema), void>::__type
+      static typename __enable_if<(bool)sizeof(&_Rm::sema), void>::__type
       _S_destroy(_Rm* __mx)
       {
         __gthread_mutex_t __tmp;
@@ -293,7 +293,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
     // matches a recursive mutex with a member 'actual'
     template<typename _Rm>
-      static typename __enable_if<sizeof(&_Rm::actual), void>::__type
+      static typename __enable_if<(bool)sizeof(&_Rm::actual), void>::__type
       _S_destroy(_Rm* __mx)
       { __gthread_mutex_destroy(&__mx->actual); }
 
diff --git a/libstdc++-v3/include/std/mutex b/libstdc++-v3/include/std/mutex
index a7ebace..34d64c5 100644
--- a/libstdc++-v3/include/std/mutex
+++ b/libstdc++-v3/include/std/mutex
@@ -116,13 +116,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
     // matches a recursive mutex with a member 'actual'
     template<typename _Rm>
-      static typename enable_if<sizeof(&_Rm::actual), void>::type
+      static typename enable_if<(bool)sizeof(&_Rm::actual), void>::type
       _S_destroy(_Rm* __mx)
       { __gthread_mutex_destroy(&__mx->actual); }
 
     // matches a gthr-win32.h recursive mutex
     template<typename _Rm>
-      static typename enable_if<sizeof(&_Rm::sema), void>::type
+      static typename enable_if<(bool)sizeof(&_Rm::sema), void>::type
       _S_destroy(_Rm* __mx)
       {
         __gthread_mutex_t __tmp;