[Bug 1810695] [NEW] g++: M_PI is visible in conforming mode

[Keith Thompson]

Public bug reported:

The macro M_PI, which expands to a constant approximating the value
of pi, is defined by POSIX, but not by ISO C or ISO C++.  It is
not a reserved identifier, so it should be available for use as a
user-defined identifier.

The problem: Including <cmath> causes M_PI to be defined, even when
the C++ compiler is invoked in what should be a conforming mode.

EXPECTED: Program compiles without error and prints "3".
OBSERVED: Program is rejected at compile time.

(The program is rejected when g++ or clang++ is invoked without
options.  That's not a bug.)

Using <math.h> rather than <cmath> doesn't change the symptom.

A similar program in C does not exhibit the problem.

I *think* the problem is in the "math.h" header, which should arrange
for M_PI and similar macros not to be defined in conforming mode.
It's also possible that a fix might involve updates to g++ and/or
clang++.  I haven't fully investigated the twisty maze of macro
definitions and nested #includes in math.h.

(I acknowledge that writing your own definition of M_PI is not a
good idea, and that the definition in this program is particularly
poor style.)

Output illustrating the problem follows:

$ lsb_release -rd
Description:    Ubuntu 18.04.1 LTS
Release:        18.04

$ uname -a
Linux bomb20 4.15.0-42-generic #45-Ubuntu SMP Thu Nov 15 19:32:57 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

$ dpkg -l g++ clang libc6-dev:amd64 | grep '^ii'
ii  clang           1:6.0-41~exp5~ubuntu1 amd64        C, C++ and Objective-C compiler (LLVM based)
ii  g++             4:7.3.0-3ubuntu2.1    amd64        GNU C++ compiler
ii  libc6-dev:amd64 2.27-3ubuntu1         amd64        GNU C Library: Development Libraries and Header Files

$ g++ --version | head -1
g++ (Ubuntu 7.3.0-27ubuntu1~18.04) 7.3.0

$ clang++ --version
clang version 6.0.0-1ubuntu2 (tags/RELEASE_600/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

$ cat M_PI_bug.cpp 
#include <iostream>
#include <cmath>
int main() {
    const int M_PI = 22/7;
    std::cout << M_PI << '\n';
}

// This is not reasonable code.
// The issue is that a conforming C++ compiler must accept it.

// Expected output: 3

$ g++ -std=c++17 -pedantic-errors -c M_PI_bug.cpp 
In file included from /usr/include/c++/7/cmath:45:0,
                 from M_PI_bug.cpp:2:
M_PI_bug.cpp: In function ‘int main()’:
M_PI_bug.cpp:4:15: error: expected unqualified-id before numeric constant
     const int M_PI = 22/7;
               ^

$ clang++ -std=c++17 -pedantic-errors -c M_PI_bug.cpp 
M_PI_bug.cpp:4:15: error: expected unqualified-id
    const int M_PI = 22/7;
              ^
/usr/include/math.h:777:16: note: expanded from macro 'M_PI'
# define M_PI           3.14159265358979323846  /* pi */
                        ^
1 error generated.

** Affects: glibc (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1810695

Title:
  g++: M_PI is visible in conforming mode

Status in glibc package in Ubuntu:
  New

Bug description:
  The macro M_PI, which expands to a constant approximating the value
  of pi, is defined by POSIX, but not by ISO C or ISO C++.  It is
  not a reserved identifier, so it should be available for use as a
  user-defined identifier.

  The problem: Including <cmath> causes M_PI to be defined, even when
  the C++ compiler is invoked in what should be a conforming mode.

  EXPECTED: Program compiles without error and prints "3".
  OBSERVED: Program is rejected at compile time.

  (The program is rejected when g++ or clang++ is invoked without
  options.  That's not a bug.)

  Using <math.h> rather than <cmath> doesn't change the symptom.

  A similar program in C does not exhibit the problem.

  I *think* the problem is in the "math.h" header, which should arrange
  for M_PI and similar macros not to be defined in conforming mode.
  It's also possible that a fix might involve updates to g++ and/or
  clang++.  I haven't fully investigated the twisty maze of macro
  definitions and nested #includes in math.h.

  (I acknowledge that writing your own definition of M_PI is not a
  good idea, and that the definition in this program is particularly
  poor style.)

  Output illustrating the problem follows:

  $ lsb_release -rd
  Description:    Ubuntu 18.04.1 LTS
  Release:        18.04

  $ uname -a
  Linux bomb20 4.15.0-42-generic #45-Ubuntu SMP Thu Nov 15 19:32:57 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

  $ dpkg -l g++ clang libc6-dev:amd64 | grep '^ii'
  ii  clang           1:6.0-41~exp5~ubuntu1 amd64        C, C++ and Objective-C compiler (LLVM based)
  ii  g++             4:7.3.0-3ubuntu2.1    amd64        GNU C++ compiler
  ii  libc6-dev:amd64 2.27-3ubuntu1         amd64        GNU C Library: Development Libraries and Header Files

  $ g++ --version | head -1
  g++ (Ubuntu 7.3.0-27ubuntu1~18.04) 7.3.0

  $ clang++ --version
  clang version 6.0.0-1ubuntu2 (tags/RELEASE_600/final)
  Target: x86_64-pc-linux-gnu
  Thread model: posix
  InstalledDir: /usr/bin

  $ cat M_PI_bug.cpp 
  #include <iostream>
  #include <cmath>
  int main() {
      const int M_PI = 22/7;
      std::cout << M_PI << '\n';
  }

  // This is not reasonable code.
  // The issue is that a conforming C++ compiler must accept it.

  // Expected output: 3

  $ g++ -std=c++17 -pedantic-errors -c M_PI_bug.cpp 
  In file included from /usr/include/c++/7/cmath:45:0,
                   from M_PI_bug.cpp:2:
  M_PI_bug.cpp: In function ‘int main()’:
  M_PI_bug.cpp:4:15: error: expected unqualified-id before numeric constant
       const int M_PI = 22/7;
                 ^

  $ clang++ -std=c++17 -pedantic-errors -c M_PI_bug.cpp 
  M_PI_bug.cpp:4:15: error: expected unqualified-id
      const int M_PI = 22/7;
                ^
  /usr/include/math.h:777:16: note: expanded from macro 'M_PI'
  # define M_PI           3.14159265358979323846  /* pi */
                          ^
  1 error generated.

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