[PATCH] Make cstddef / cstdarg more robust (PR 69881)

[Jonathan Wakely]

On 23/02/16 10:00 +0000, Bernd Edlinger wrote:
>On 23/02/15 10:42, Jonathan Wakely wrote:
>>On 23/02/16 07:15 +0000, Bernd Edlinger wrote:
>>>as described in the PR 69881 it happens quite often that cstddef is
>>>called with __need_size_t because we still support gmp-4.3.2 which
>>>is installed by contrib/download_prerequisites.  This causes a kind
>>>of undefined behavior.  It is just by chance that this does not cause
>>>the gcc-6 boot-strap to fail, but it causes gcc-4.9 bootstrap to fail
>>>when the gcc-6 version of cstddef is around.  So it looks like a
>>>regression, because the new cstddef is more fragile than before.
>>
>>Is it? cstddef hasn't been changed since 2013-06-11. cstdarg hasn't
>>been changed since 2011-01-30.
>>
>>What made them more fragile?
>
>Good question.
>
>The cstddef had this even in gcc.4.9 that's true, but it was not used
>by default:
>
>#if __cplusplus >= 201103L
>namespace std
>{
>  // We handle size_t, ptrdiff_t, and nullptr_t in c++config.h.
>  using ::max_align_t;
>}
>#endif
>
>
>Previously the g++ default was --std=gnu++98,
>but gcc-6 changed the default to --std=gnu++14.

Ah yes.

>And when building gcc-4.9, stage1 does not override that with
>--std=gnu++98.
>
>That has changed, and that triggers the latent bug.

Alright then, the patch is OK for trunk.

I might revert it once we stop using the buggy GMP in
contrib/download_prerequisites.