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From: "noloader at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/53431] C++ preprocessor ignores #pragma GCC diagnostic
Date: Wed, 22 Jul 2015 00:00:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53431

--- Comment #13 from Jeffrey Walton <noloader at gmail dot com> ---
This issued caused Crypto++ to remove -Wall (and above) under GCC. Crypto++ is
C++ with lots of interfaces, and it performs a fair amount of intermediate
calculations used in an assert. It really needed the following to work as
expected:

// GCC diagnostcs available after GCC 4.2
(https://gcc.gnu.org/ml/gcc-help/2015-07/msg00063.html)
#define GCC_DIAGNOSTIC_AWARE ((__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__
>= 2)) || defined(__clang__))

#if GCC_DIAGNOSTIC_AWARE
# pragma GCC diagnostic push
# pragma GCC diagnostic ignored "-Wunused-value"
# pragma GCC diagnostic ignored "-Wunused-variable"
#endif

Crypto++ is fortunate in that its multi-compiler friendly and cross-platform.
It can use Clang, ICC, Comeau or, say MSVC with /W4. Other projects are not so
lucky. They include Asterisk (the PBX software package). Asterisk uses
trampolines, so it cannot compile under Clang. And it never supported MSVC.