Re: std::basic_string<_Tp> constructor point of instantiation woes?

[Jonathan Wakely <jwakely@redhat.com>]

On Mon, 22 Nov 2021 at 16:31, Stephan Bergmann via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> When using recent libstc++ trunk with Clang in C++20 mode,
> std::u16string literals as in
>
> > #include <string>
> > int main() {
> >   using namespace std::literals;
> >   u""s;
> > }
>
> started to cause linker failures due to undefined
>
> > _ZNSt7__cxx1112basic_stringIDsSt11char_traitsIDsESaIDsEE12_M_constructIPKDsEEvT_S8_St20forward_iterator_tag
>
> After some head scratching, I found the more insightful
>
> > $ cat test.cc
> > #include <string>
> > constexpr std::string s("", 0);
>
> > $ clang++ -std=c++20 -fsyntax-only test.cc
> > test.cc:2:23: error: constexpr variable 's' must be initialized by a constant expression
> > constexpr std::string s("", 0);
> >                       ^~~~~~~~
> > ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/12.0.0/../../../../include/c++/12.0.0/bits/basic_string.h:620:2: note: undefined function '_M_construct<const char *>' cannot be used in a constant expression
> >         _M_construct(__s, __s + __n, std::forward_iterator_tag());
> >         ^
> > test.cc:2:23: note: in call to 'basic_string(&""[0], 0, std::allocator<char>())'
> > constexpr std::string s("", 0);
> >                       ^
> > ~/gcc/trunk/inst/lib/gcc/x86_64-pc-linux-gnu/12.0.0/../../../../include/c++/12.0.0/bits/basic_string.h:331:9: note: declared here
> >         _M_construct(_FwdIterator __beg, _FwdIterator __end,
> >         ^
> > 1 error generated.
>
> and after some more head scratching found Clang to complain about the
> reduced
>
> > template<typename> struct S {
> >     constexpr void f();
> >     constexpr S() { f(); };
> > };
> > S<void> s1;
> > template<typename T> constexpr void S<T>::f() {}
> > constexpr S<void> s2;
>
> (about which GCC does not complain).  Not entirely sure who is right,
> but what would help Clang is to move the definitions of the literal
> operators in basic_string.h (which implicitly instantiate the
> corresponding std::basic_string<_Tp> constructor) past the definition of
> _M_construct (which is called from the constructor) in basic_string.tcc;
> something like

The .tcc files are something of an anachronism, as I think they were
supposed to have the non-inline function definitions which might be
subject to 'export' for separate compilation. Except that feature was
removed from C++11, and so now it's just a fairly pointless separation
between inline and non-inline functions ... except where we're muddied
the waters by changing some to 'inline' without moving them to the
other file (because why bother).

That said, all the one- or two-line inline functions like the literal
operators and to_string are all in basic_string.h and having to move
some arbitrary subset of them into the other file, after the
non-inline definitions, is a bit annoying.

I think this is https://bugs.llvm.org/show_bug.cgi?id=24128