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From: "r_new at rambler dot ru" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug preprocessor/94753] New: -undef, c++20 and feature-test macros
Date: Fri, 24 Apr 2020 23:22:31 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: preprocessor
X-Bugzilla-Version: 10.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94753

            Bug ID: 94753
           Summary: -undef, c++20 and feature-test macros
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: preprocessor
          Assignee: unassigned at gcc dot gnu.org
          Reporter: r_new at rambler dot ru
  Target Milestone: ---

In c++20 feature-test macros (_=C2=AD_=C2=ADcpp_=C2=ADattributes, _=C2=AD_=
=C2=ADcpp_=C2=ADconstexpr, ...) are
standard predefined macros, must be predefined even with -undef.

g++ -undef -std=3Dc++2a -E -dM -x c++ - < /dev/null=20
#define __STDC_HOSTED__ 1
#define __STDC_IEC_559__ 1
#define __STDC_ISO_10646__ 201706L
#define __STDC_UTF_16__ 1
#define __cplusplus 201709L
#define _STDC_PREDEF_H 1
#define __STDC_IEC_559_COMPLEX__ 1
#define __STDC_UTF_32__ 1
#define __STDC__ 1
#define _GNU_SOURCE 1

gcc version 10.0.1 20200423 (experimental) (Compiler-Explorer-Build) and gcc
version 9.3.0 (Arch Linux 9.3.0-1)=