[Bug c/70791] New: -Wnested-externs prints inconsistent column number

["egall at gwmail dot gwu.edu" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70791

            Bug ID: 70791
           Summary: -Wnested-externs prints inconsistent column number
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: egall at gwmail dot gwu.edu
  Target Milestone: ---
              Host: i386-apple-darwin9.8.0
            Target: i386-apple-darwin9.8.0
             Build: i386-apple-darwin9.8.0

Say I have the following file:

$ cat undeclared.c
char *foo(char *);

int bar(void) {
        char *baz = foo(UNDECLARED_MACRO(""));
}

When I compile it like this, gcc says:

$ /usr/local/bin/gcc -c -Wimplicit -Wnested-externs -Wint-conversion
undeclared.c
undeclared.c: In function âbarâ:
undeclared.c:4:18: warning: implicit declaration of function âUNDECLARED_MACROâ
[-Wimplicit-function-declaration]
  char *baz = foo(UNDECLARED_MACRO(""));
                  ^
undeclared.c:4:2: warning: nested extern declaration of âUNDECLARED_MACROâ
[-Wnested-externs]
  char *baz = foo(UNDECLARED_MACRO(""));
  ^
undeclared.c:4:18: warning: passing argument 1 of âfooâ makes pointer from
integer without a cast [-Wint-conversion]
  char *baz = foo(UNDECLARED_MACRO(""));
                  ^
undeclared.c:1:7: note: expected âchar *â but argument is of type âintâ
 char *foo(char *);
       ^

I would expect the warning from -Wnested-externs to point to the same column as
the other 2 warnings. 
(Of course, ideally the 3 would be combined into a single warning, but that's a
separate issue)

My gcc version info:

$ /usr/local/bin/gcc -v
Using built-in specs.
COLLECT_GCC=/usr/local/bin/gcc
COLLECT_LTO_WRAPPER=/usr/local/libexec/gcc/i386-apple-darwin9.8.0/6.0.0/lto-wrapper
Target: i386-apple-darwin9.8.0
Configured with: ../configure --disable-werror
--enable-languages=c,c++,lto,objc,obj-c++ --enable-stage1-checking=release -C
--with-system-libunwind --enable-secureplt --enable-frame-pointer
--enable-version-specific-runtime-libs --enable-assert --enable-fat
--enable-portable-binary --enable-browser-plugin --enable-gconf-peer
--enable-libgcj-debug --enable-gc-debug --enable-interpreter
--enable-aot-compile-rpm --enable-java-home --with-x --enable-collections
--enable-gstreamer-peer --enable-xmlj --enable-qt-peer --enable-debug
--enable-local-sockets --without-isl --enable-objc-gc --enable-maintainer-mode
--enable-host-shared CC=/usr/local/bin/gcc CXX=/usr/local/bin/g++
AUTOCONF=/usr/local/bin/autoconf AUTOHEADER=/usr/local/bin/autoheader
AUTOM4TE=/usr/local/bin/autom4te AUTORECONF=/usr/local/bin/autoreconf
AUTOSCAN=/usr/local/bin/autoscan AUTOUPDATE=/usr/local/bin/autoupdate
IFNAMES=/usr/local/bin/ifnames
Thread model: posix
gcc version 6.0.0 20151007 (experimental) (GCC)