[PATCH] PR c++/82039 suppress -Wzero-as-null-pointer-constant warning

[Jonathan Wakely]

This avoids an unhelpful warning from a system header. Users can't
change the code to avoid the warning, and we don't want to change it
(because it needs to work for C++98 as well as later versions).

	PR c++/82039
	* include/ext/new_allocator.h (__gnu_cxx::new_allocator::allocate):
	Adjust null pointer constant to avoid warning

Tested powerpc64le-linux, committed to trunk.

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commit 84cedda4ddf90070f8b5d9132be1bf401183cf1c
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Aug 31 12:25:34 2017 +0100

    PR c++/82039 suppress -Wzero-as-null-pointer-constant warning
    
            PR c++/82039
            * include/ext/new_allocator.h (__gnu_cxx::new_allocator::allocate):
            Adjust null pointer constant to avoid warning.

diff --git a/libstdc++-v3/include/ext/new_allocator.h b/libstdc++-v3/include/ext/new_allocator.h
index ee64b9c6447..e1e152c4bf0 100644
--- a/libstdc++-v3/include/ext/new_allocator.h
+++ b/libstdc++-v3/include/ext/new_allocator.h
@@ -96,7 +96,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       // NB: __n is permitted to be 0.  The C++ standard says nothing
       // about what the return value is when __n == 0.
       pointer
-      allocate(size_type __n, const void* = 0)
+      allocate(size_type __n, const void* = static_cast<const void*>(0))
       {
 	if (__n > this->max_size())
 	  std::__throw_bad_alloc();