Bug with LTO optimizations in G++?

[Vasco Alexandre da Silva Costa]

This code shows the bug:
---test.cc---
#include <stdio.h>

typedef __attribute__ ((__aligned__ (16))) struct _TEST_FLOAT128 {
    __extension__ unsigned long long Part[2];
} TEST_FLOAT128;

int main( void )
{
    TEST_FLOAT128 tf;
    printf( "alignof(tf) = %d\n",(int)__alignof__(tf) );

    printf( "alignof(struct _TEST_FLOAT128) = %d\n",(int)__alignof__(struct _TEST_FLOAT128) );
    printf( "alignof(TEST_FLOAT128) = %d\n",(int)__alignof__(TEST_FLOAT128) );

    return 0;
}
---

$ g++-4.9 -O3 -flto -oalign test.cc -Wall -Wextra -g
# compiles without warnings.

$ g++-5 -O3 -flto -oalign test.cc -Wall -Wextra -g
test.cc:3:51: warning: type ‘struct _TEST_FLOAT128’ violates one definition rule [-Wodr]
 typedef __attribute__ ((__aligned__ (16))) struct _TEST_FLOAT128 {
                                                   ^
test.cc:5:3: note: a type with different alignment is defined in another translation unit
 } TEST_FLOAT128;
   ^
# triggers a warning.

[Vasco Alexandre da Silva Costa (vasco-costa)]

gcc version 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.4)