Code:

template <class>
void tf()
{
  extern void (*qt_memfill32)();
  qt_memfill32();
}

void f() {
  tf<int>();
}

When compiled with GCC 5.0, the object file has a reference to "_Z12qt_memfill32", which doesn't seem to me to be a valid C++ mangled name. At the very least, it's an ABI compatibility break with previous versions of GCC, other compilers and with itself.

This symbol is defined in another .o as "qt_memfill32", even when compiled with GCC 5.0.

Tests:

gcc-5.0 -O2 -S -o - test.cpp:
        jmp     *_Z12qt_memfill32(%rip)

gcc-4.9 -O2 -S -o - test.cpp:
        movq    qt_memfill32(%rip), %rax
        jmp     *%rax

clang-3.6 -O2 -S -o - test.cpp:
        jmpq    *qt_memfill32(%rip)  # TAILCALL

icpc -O2 -S -o - test.cpp:
        movq      qt_memfill32(%rip), %rax
        jmp       *%rax