[Bug c++/80648] New: Valid C++11 null pointer constant (1-1) is rejected

[Keith.S.Thompson at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80648

            Bug ID: 80648
           Summary: Valid C++11 null pointer constant (1-1) is rejected
           Product: gcc
           Version: 7.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Keith.S.Thompson at gmail dot com
  Target Milestone: ---

I'm using g++ 7.1.0, built from source, on Ubuntu 16.10 x86_64.

$ g++ --version
g++ (GCC) 7.1.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ uname -a
Linux bomb20 4.8.0-46-generic #49-Ubuntu SMP Fri Mar 31 13:57:14 UTC 2017
x86_64 x86_64 x86_64 GNU/Linux
$

Test case:

int main() {
    void *p = 1-1;
}

When compiled with "g++ -std=c++03 -pedantic c.cpp", the compiler correctly
doesn't complain; likewise with -std=c++98.

With "g++ -std=c++11 -pedantic", it produces an error message:

c.cpp: In function ‘int main()’:
c.cpp:2:16: error: invalid conversion from ‘int’ to ‘void*’ [-fpermissive]
     void *p = 1-1;
               ~^~

And the same message with "g++ -std=c++14 -pedantic".

C++14 restricted the definition of a null pointer constant.  In the
N4296 draft, 4.10p1 [conv.ptr] says:

"A null pointer constant is an integer literal (2.13.2) with value
zero or a prvalue of type std::nullptr_t."

1-1 is not an integer literal, so the error message is correct for C++14.

But C++11 had not yet made that change.  The C++11 standard,
ISO/IEC 14882:2011(E), in the corresponding section, says:

"A null pointer constant is an integral constant expression (5.19)
prvalue of integer type that evaluates to zero or a prvalue of type
std::nullptr_t."

g++ correctly accepts 1-1 as a null pointer constant in C++98
and C++03 modes, and correctly rejects it in C++14 mode, but it
should accept it in C++11 mode.

(I do not of course suggest that using 1-1 as a null pointer constant
is a good idea.)