[Bug c++/86598] New: Incorrect lexing of pp-numbers in C++11 due to hexfloat extension

[zhonghao at pku dot org.cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86598

            Bug ID: 86598
           Summary: Incorrect lexing of pp-numbers in C++11 due to
                    hexfloat extension
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zhonghao at pku dot org.cn
  Target Milestone: ---

g++ support C99 hexadecimal literals as an extension in its C++ modes. This is
*almost* a conforming extension in C++98 mode, but it's far from being one in
C++11 mode. For instance, in C++98:

#define PREFIX(x) foo ## x
void f() {
  int PREFIX(1p);
  int PREFIX(2p) = PREFIX(1p+5);
}

g++ rejects the above code:

code2.cpp:1:19: error: pasting "foo" and "1p+5" does not give a valid
preprocessing token
 #define PREFIX(x) foo ## x
                   ^~~
code2.cpp:4:19: note: in expansion of macro 'PREFIX'
  int PREFIX(2p) = PREFIX(1p+5); 
                   ^~~~~~
code2.cpp: In function 'void f()':
code2.cpp:1:19: error: 'foo' was not declared in this scope
 #define PREFIX(x) foo ## x
                   ^~~
code2.cpp:4:19: note: in expansion of macro 'PREFIX'
  int PREFIX(2p) = PREFIX(1p+5); 
                   ^~~~~~
code2.cpp:1:19: note: suggested alternative: 'foo2p'
 #define PREFIX(x) foo ## x
                   ^~~
code2.c.cpp:4:19: note: in expansion of macro 'PREFIX'
  int PREFIX(2p) = PREFIX(1p+5); 
                   ^~~~~~

In the rules of C++, "1p+5" is three tokens, not one.

In C++11, this goes wrong in much less contrived situations:

Units<Amp> operator"" _amp(unsigned long long d);
int k = 4_amp+1; // incorrect error: no literal operator "" _amp+1

BTW, clang++ accepts the above code.