[Bug c++/120349] New: [16 regression] std::println link fail with std module

[c at cyano dot cn via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120349

            Bug ID: 120349
           Summary: [16 regression] std::println link fail with std module
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: c at cyano dot cn
  Target Milestone: ---

`test.cc`:
```
import std;

int main() {
  std::println("Hello, {0}!", "world");
  return 0;
}
```

Compiled with:
```
g++ -std=c++23 -fmodules -O2 -fmodule-only -fsearch-include-path -c bits/std.cc
g++ -std=c++23 -fmodules -O2 test.cc
```

Error message:
```
/usr/bin/ld: /tmp/ccml92so.o: in function `std::format_error::~format_error()':
test.cc:(.text._ZNSt12format_errorD2Ev[_ZNSt12format_errorD5Ev]+0x3): undefined
reference to `vtable for std::format_error'
/usr/bin/ld: /tmp/ccml92so.o: in function `std::__throw_format_error(char
const*)':
test.cc:(.text.unlikely._ZSt20__throw_format_errorPKc[_ZSt20__throw_format_errorPKc]+0x21):
undefined reference to `vtable for std::format_error'
/usr/bin/ld: /tmp/ccml92so.o: in function `std::format_error::~format_error()':
test.cc:(.text._ZNSt12format_errorD0Ev[_ZNSt12format_errorD5Ev]+0x7): undefined
reference to `vtable for std::format_error'
/usr/bin/ld: /tmp/ccml92so.o: in function `main.cold':
test.cc:(.text.unlikely+0x2f): undefined reference to `vtable for
std::__format::_Seq_sink<std::__cxx11::basic_string<char,
std::char_traits<char>, std::allocator<char> > >'
/usr/bin/ld: test.cc:(.text.unlikely+0x85): undefined reference to `vtable for
std::__format::_Seq_sink<std::__cxx11::basic_string<char,
std::char_traits<char>, std::allocator<char> > >'
/usr/bin/ld: /tmp/ccml92so.o: in function `.LC25':
test.cc:(.rodata.cst8+0x0): undefined reference to `vtable for
std::__format::_Seq_sink<std::__cxx11::basic_string<char,
std::char_traits<char>, std::allocator<char> > >'
/usr/bin/ld: /tmp/ccml92so.o: in function `.LC26':
test.cc:(.rodata.cst8+0x8): undefined reference to `vtable for
std::__format::_Formatting_scanner<std::__format::_Sink_iter<char>, char>'
collect2: error: ld returned 1 exit status
```

Bisection points to
https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=c875748cdc468ec604c904e19a688998aa4d7a1f.

`gcc -v`:
```
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/home/cyano/gcc-git/bin/../libexec/gcc/x86_64-pc-linux-gnu/16.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc/configure --prefix= --enable-languages=c,c++
--enable-static --disable-shared --disable-bootstrap --disable-multilib
CFLAGS=-O2 CXXFLAGS=-O2 LDFLAGS=-s
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 16.0.0 20250516 (experimental) (GCC) 
```