https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86138
--- Comment #5 from Christian Franke <franke at computer dot org> ---
(In reply to Jonathan Wakely from comment #4)
> (In reply to Christian Franke from comment #3)
> > > The extern templates are disabled because std::basic_string has additional
> > > member functions in C++17 mode, and they're not instantiated in the
> > > library.
> > > By disabling the explicit instantiation declarations the compiler will
> > > emit
> > > definitions for the C++17-only member functions.
> > This has no effect due to the bogus specialization in basic_string.h (see
> > patch below).
>
> What's bogus about it?
Here a simple example which demonstrates the same situation:
$ cat bug.cpp
template<typename T> void f(T & x) { x = T(); }
#ifndef FIXED
template<> void f(int &);
// no visible implementation => same effect as extern template below!
#endif
#if __cplusplus <= 201402L
extern template void f(int &);
#endif
void call_f(int & i) {
f(i);
}
$ g++ -std=c++14 -c -o bug-14.o bug.cpp
$ g++ -std=c++17 -c -o bug-17.o bug.cpp
$ g++ -std=c++17 -DFIXED -c -o bug-17-fixed.o bug.cpp
$ diff -qs bug-14.o bug-17.o ; diff -qs bug-17.o bug-17-fixed.o
Files bug-14.o and bug-17.o are identical
Files bug-17.o and bug-17-fixed.o differ
$ nm -C bug-17.o | grep ' [TU] '
U void f<int>(int&)
0000000000000000 T call_f(int&)
$ nm -C bug-17-fixed.o | grep ' [TU] '
0000000000000000 T void f<int>(int&)
0000000000000000 T call_f(int&)
> This patch won't be accepted, it's papering over some other problem not
> fixing anything.
I disagree. This should be fixed anyway, see above.
