[Bug c++/77462] New: Error message prints source from wrong file after #line

[d.frey at gmx dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77462

            Bug ID: 77462
           Summary: Error message prints source from wrong file after
                    #line
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: d.frey at gmx dot de
  Target Milestone: ---

When I use #line, I can make the compiler print a source line from another file
when it encounters an error. This is totally misleading.

Consider two files, first fool.cc:

  #include "liar.hh"

  int main()
  {
    static_assert( 2 + 2 == 4, "oops" );
  }

and a second file liar.hh:

  #line 5 "fool.cc"
    static_assert( 2 + 2 == 5, "oops" );

If I now compile with GCC, I get:

frey@vbox::~/work/test/lie_to_me$ g++-6 -std=c++11 fool.cc 
In file included from fool.cc:1:0:
fool.cc:5:3: error: static assertion failed: oops
   static_assert( 2 + 2 == 4, "oops" );
   ^~~~~~~~~~~~~

Note the line says "2 + 2 == 4", which is the line from "fool.cc", not the one
where the error *actually* occurs, namely the one saying "2 + 2 == 5" from
"liar.hh". Clang gets it right:

frey@vbox:1:~/work/test/lie_to_me$ clang++-3.9 -std=c++11 fool.cc 
In file included from fool.cc:1:
fool.cc:5:3: error: static_assert failed "oops"
  static_assert( 2 + 2 == 5, "oops" );
  ^              ~~~~~~~~~~
1 error generated.

Yes, #line leads to "fool.cc:5:3", but that is fine and what is intended by
#line is AFAICT *not* to read the source line for the error message from that
file and line.