[patch] Fix dangling pointer in future_error::what()

[patch] Fix dangling pointer in future_error::what()

[Jonathan Wakely]

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This bug didn't show up with COW strings, but is a real problem with
the new std::string. The fix is to call error_code::message() early
and store the result in the logic_error base class during
construction.

Tested x86_64-linux, committed to trunk.

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commit 3f7252dcc10eba698c12460c0a74bd313c7fef01
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Mar 6 11:04:13 2015 +0000

    	* include/std/future (future_error(error_code)): Construct base
    	class with error_code's message.
    	* src/c++11/future.cc (future_error::what()): Do not call c_str() on
    	temporary string.

diff --git a/libstdc++-v3/include/std/future b/libstdc++-v3/include/std/future
index cb0226d..fc3f816 100644
--- a/libstdc++-v3/include/std/future
+++ b/libstdc++-v3/include/std/future
@@ -98,7 +98,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   public:
     explicit future_error(error_code __ec)
-    : logic_error("std::future_error"), _M_code(__ec)
+    : logic_error("std::future_error: " + __ec.message()), _M_code(__ec)
     { }
 
     virtual ~future_error() noexcept;
diff --git a/libstdc++-v3/src/c++11/future.cc b/libstdc++-v3/src/c++11/future.cc
index c711a5f..3cf503b 100644
--- a/libstdc++-v3/src/c++11/future.cc
+++ b/libstdc++-v3/src/c++11/future.cc
@@ -75,7 +75,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   future_error::~future_error() noexcept { }
 
   const char*
-  future_error::what() const noexcept { return _M_code.message().c_str(); }
+  future_error::what() const noexcept { return logic_error::what(); }
 
 #if defined(_GLIBCXX_HAS_GTHREADS) && defined(_GLIBCXX_USE_C99_STDINT_TR1) \
   && (ATOMIC_INT_LOCK_FREE > 1)