Why "-flto" silent GCC's warning "stringop-truncation"

I understand that newer GCC versions introduced a warning for possibly wrong string operations "stringop-truncation"

And here's the sample code that I can easily trigger this warning:

$ cat strncpy-warning.cxx
#include <cstring>

extern char g_buf[16];

void mycopy ( const char* src_c_str )
{
    strncpy ( g_buf, src_c_str, sizeof ( g_buf ) );
}

Compile it with the following flags triggers this warning:

$ g++ --version
g++ (GCC) 8.3.0
Copyright (C) 2018 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.

$ g++ -Wall -std=c++14 -Wextra -Werror -O3 -c strncpy-warning.cxx
strncpy-warning.cxx: In function ‘void mycopy(const char*)’:
strncpy-warning.cxx:7:13: error: ‘char* strncpy(char*, const char*, size_t)’ specified bound 16 equals destination size [-Werror=stringop-truncation]
     strncpy ( g_buf, src_c_str, sizeof ( g_buf ) );
     ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors

Which is expected, however, if I added an -flto to the flags, this warning is gone and the object file is generated:

$ g++ -Wall -std=c++14 -Wextra -Werror -O3 -c strncpy-warning.cxx -flto
$ ls strncpy-warning.o
strncpy-warning.o

Apparently, -flto WILL catch some compile time warnings so not sure why that stringop-truncation is not caught, e.g.:

$ cat strncpy-warning.cxx
#include <cstring>

extern char g_buf[16];

void mycopy ( const char* src_c_str, const char* unused )
{
    strncpy ( g_buf, src_c_str, sizeof ( g_buf ) );
}

$ g++ -Wall -std=c++14 -Wextra -Werror -O3 -c strncpy-warning.cxx -flto
strncpy-warning.cxx: In function ‘void mycopy(const char*, const char*)’:
strncpy-warning.cxx:5:50: error: unused parameter ‘unused’ [-Werror=unused-parameter]
 void mycopy ( const char* src_c_str, const char* unused )
                                      ~~~~~~~~~~~~^~~~~~
cc1plus: all warnings being treated as errors                                               

So the question is:

1. Is this expected behavior?
2. I understand that with -flto specified for a source file, GCC will write special things into the resulting object file ( *.o ), but the question is why the warning is skipped by GCC? Is that intentional?
3. Is there a way to enable this warning even in case of -flto is given?
4. Or does the point-3 above make any sense?

Many thanks!