[libc++][modules] Attach declarations to the global module.

Declarations in the global module fragment (the included headers) are attached
to the global module. The using declarations in the module std and std.compat
were attached to their module.

This violates

  [basic.link]/10
  If two declarations of an entity are attached to different modules, the
  program is ill-formed; no diagnostic is required if neither is reachable from
  the other.

Instead attach the delarations in the module to the global module by using a
linkage-specification. ([module.unit]/7)

Thanks to @DanielaE for pointing out this issue.

libcxx/include/__config

@@ -837,6 +837,17 @@ typedef __char32_t char32_t;
 #  define _LIBCPP_END_NAMESPACE_FILESYSTEM }} _LIBCPP_END_NAMESPACE_STD
 // clang-format on
 
+// Named declarations included in the headers are part of the global module
+// fragment. These declarations are attached to the global module. The using
+// declarations in the module should also attach their declarations to the
+// global module. This is done by using a linkage specification.
+#  if defined(_LIBCPP_CLANG_VER) && _LIBCPP_CLANG_VER < 1800
+// Clang-17 does not support extern "C++" export
+#    define _LIBCPP_MODULE_EXPORT export
+#  else
+#    define _LIBCPP_MODULE_EXPORT extern "C++" export
+#  endif
+
 #  if __has_attribute(__enable_if__)
 #    define _LIBCPP_PREFERRED_OVERLOAD __attribute__((__enable_if__(true, "")))
 #  endif

libcxx/modules/std.compat/cassert.inc

@@ -7,6 +7,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-export {
+_LIBCPP_MODULE_EXPORT {
   // This module exports nothing.
-} // export
+} // _LIBCPP_MODULE_EXPORT

libcxx/modules/std.compat/cctype.inc

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-export {
+_LIBCPP_MODULE_EXPORT {
   using ::isalnum _LIBCPP_USING_IF_EXISTS;
   using ::isalpha _LIBCPP_USING_IF_EXISTS;
   using ::isblank _LIBCPP_USING_IF_EXISTS;
@@ -22,4 +22,4 @@ export {
   using ::isxdigit _LIBCPP_USING_IF_EXISTS;
   using ::tolower _LIBCPP_USING_IF_EXISTS;
   using ::toupper _LIBCPP_USING_IF_EXISTS;
-} // export
+} // _LIBCPP_MODULE_EXPORT

libcxx/modules/std.compat/cerrno.inc

@@ -7,6 +7,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-export {
+_LIBCPP_MODULE_EXPORT {
   // This module exports nothing.
-} // export
+} // _LIBCPP_MODULE_EXPORT

libcxx/modules/std.compat/cfenv.inc

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-export {
+_LIBCPP_MODULE_EXPORT {
   // types
   using ::fenv_t _LIBCPP_USING_IF_EXISTS;
   using ::fexcept_t _LIBCPP_USING_IF_EXISTS;
@@ -26,4 +26,4 @@ export {
   using ::feholdexcept _LIBCPP_USING_IF_EXISTS;
   using ::fesetenv _LIBCPP_USING_IF_EXISTS;
   using ::feupdateenv _LIBCPP_USING_IF_EXISTS;
-} // export
+} // _LIBCPP_MODULE_EXPORT

libcxx/modules/std.compat/cfloat.inc

@@ -7,6 +7,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-export {
+_LIBCPP_MODULE_EXPORT {
   // This module exports nothing.
-} // export
+} // _LIBCPP_MODULE_EXPORT

libcxx/modules/std.compat/cinttypes.inc

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-export {
+_LIBCPP_MODULE_EXPORT {
   using ::imaxdiv_t _LIBCPP_USING_IF_EXISTS;
 
   using ::imaxabs _LIBCPP_USING_IF_EXISTS;
@@ -22,4 +22,4 @@ export {
 
   // div is conditionally here, but always present in cstdlib.cppm. To avoid
   // conflicing declarations omit the using here.
-} // export
+} // _LIBCPP_MODULE_EXPORT

libcxx/modules/std.compat/climits.inc

@@ -7,6 +7,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-export {
+_LIBCPP_MODULE_EXPORT {
   // This module exports nothing.
-} // export
+} // _LIBCPP_MODULE_EXPORT

libcxx/modules/std.compat/clocale.inc

@@ -7,11 +7,11 @@
 //
 //===----------------------------------------------------------------------===//
 
-export {
+_LIBCPP_MODULE_EXPORT {
 #ifndef _LIBCPP_HAS_NO_LOCALIZATION
   using ::lconv _LIBCPP_USING_IF_EXISTS;
 
   using ::localeconv _LIBCPP_USING_IF_EXISTS;
   using ::setlocale _LIBCPP_USING_IF_EXISTS;
 #endif // _LIBCPP_HAS_NO_LOCALIZATION
-} // export
+} // _LIBCPP_MODULE_EXPORT

libcxx/modules/std.compat/cmath.inc

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-export {
+_LIBCPP_MODULE_EXPORT {
   using ::double_t _LIBCPP_USING_IF_EXISTS;
   using ::float_t _LIBCPP_USING_IF_EXISTS;
 
@@ -265,4 +265,4 @@ export {
   using ::signbit _LIBCPP_USING_IF_EXISTS;
 
   // [sf.cmath], mathematical special functions
-} // export
+} // _LIBCPP_MODULE_EXPORT

libcxx/modules/std.compat/csetjmp.inc

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-export {
+_LIBCPP_MODULE_EXPORT {
   using ::jmp_buf _LIBCPP_USING_IF_EXISTS;
   using ::longjmp _LIBCPP_USING_IF_EXISTS;
-} // export
+} // _LIBCPP_MODULE_EXPORT

libcxx/modules/std.compat/csignal.inc

@@ -7,11 +7,11 @@
 //
 //===----------------------------------------------------------------------===//
 
-export {
+_LIBCPP_MODULE_EXPORT {
   using ::sig_atomic_t _LIBCPP_USING_IF_EXISTS;
 
   // [support.signal], signal handlers
   using ::signal _LIBCPP_USING_IF_EXISTS;
 
   using ::raise _LIBCPP_USING_IF_EXISTS;
-} // export
+} // _LIBCPP_MODULE_EXPORT

libcxx/modules/std.compat/cstdarg.inc

@@ -7,4 +7,4 @@
 //
 //===----------------------------------------------------------------------===//
 
-export { using ::va_list _LIBCPP_USING_IF_EXISTS; } // export
+_LIBCPP_MODULE_EXPORT { using ::va_list _LIBCPP_USING_IF_EXISTS; } // _LIBCPP_MODULE_EXPORT

libcxx/modules/std.compat/cstddef.inc

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-export {
+_LIBCPP_MODULE_EXPORT {
   using ::max_align_t _LIBCPP_USING_IF_EXISTS;
   using ::nullptr_t;
   using ::ptrdiff_t _LIBCPP_USING_IF_EXISTS;
@@ -19,4 +19,4 @@ export {
   // function templates described in [support.types.byteops]. ...
 
   // [support.types.byteops], byte type operations
-} // export
+} // _LIBCPP_MODULE_EXPORT

libcxx/modules/std.compat/cstdint.inc

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-export {
+_LIBCPP_MODULE_EXPORT {
   // signed
   using ::int8_t _LIBCPP_USING_IF_EXISTS;
   using ::int16_t _LIBCPP_USING_IF_EXISTS;
@@ -47,4 +47,4 @@ export {
   using ::uintmax_t _LIBCPP_USING_IF_EXISTS;
 
   using ::uintptr_t _LIBCPP_USING_IF_EXISTS;
-} // export
+} // _LIBCPP_MODULE_EXPORT

libcxx/modules/std.compat/cstdio.inc

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-export {
+_LIBCPP_MODULE_EXPORT {
   using ::FILE _LIBCPP_USING_IF_EXISTS;
   using ::fpos_t _LIBCPP_USING_IF_EXISTS;
   using ::size_t _LIBCPP_USING_IF_EXISTS;
@@ -58,4 +58,4 @@ export {
   using ::vsprintf _LIBCPP_USING_IF_EXISTS;
   using ::vsscanf _LIBCPP_USING_IF_EXISTS;
 
-} // export
+} // _LIBCPP_MODULE_EXPORT

libcxx/modules/std.compat/cstdlib.inc

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-export {
+_LIBCPP_MODULE_EXPORT {
   using ::div_t _LIBCPP_USING_IF_EXISTS;
   using ::ldiv_t _LIBCPP_USING_IF_EXISTS;
   using ::lldiv_t _LIBCPP_USING_IF_EXISTS;
@@ -69,4 +69,4 @@ export {
   using ::ldiv _LIBCPP_USING_IF_EXISTS;
   using ::lldiv _LIBCPP_USING_IF_EXISTS;
 
-} // export
+} // _LIBCPP_MODULE_EXPORT

libcxx/modules/std.compat/cstring.inc

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-export {
+_LIBCPP_MODULE_EXPORT {
   using ::size_t _LIBCPP_USING_IF_EXISTS;
 
   using ::memchr _LIBCPP_USING_IF_EXISTS;
@@ -33,4 +33,4 @@ export {
   using ::strtok _LIBCPP_USING_IF_EXISTS;
   using ::strxfrm _LIBCPP_USING_IF_EXISTS;
 
-} // export
+} // _LIBCPP_MODULE_EXPORT

libcxx/modules/std.compat/ctime.inc

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-export {
+_LIBCPP_MODULE_EXPORT {
   using ::clock_t _LIBCPP_USING_IF_EXISTS;
   using ::size_t _LIBCPP_USING_IF_EXISTS;
   using ::time_t _LIBCPP_USING_IF_EXISTS;
@@ -25,4 +25,4 @@ export {
   using ::strftime _LIBCPP_USING_IF_EXISTS;
   using ::time _LIBCPP_USING_IF_EXISTS;
   using ::timespec_get _LIBCPP_USING_IF_EXISTS;
-} // export
+} // _LIBCPP_MODULE_EXPORT

libcxx/modules/std.compat/cuchar.inc

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-export {
+_LIBCPP_MODULE_EXPORT {
   // Note the Standard does not mark these symbols optional, but libc++'s header
   // does. So this seems strictly not to be conforming.
 
@@ -25,4 +25,4 @@ export {
   using ::c16rtomb _LIBCPP_USING_IF_EXISTS;
   using ::mbrtoc32 _LIBCPP_USING_IF_EXISTS;
   using ::c32rtomb _LIBCPP_USING_IF_EXISTS;
-} // export
+} // _LIBCPP_MODULE_EXPORT

libcxx/modules/std.compat/cwchar.inc

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-export {
+_LIBCPP_MODULE_EXPORT {
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
   using ::mbstate_t _LIBCPP_USING_IF_EXISTS;
   using ::size_t _LIBCPP_USING_IF_EXISTS;
@@ -77,4 +77,4 @@ export {
   using ::wcrtomb _LIBCPP_USING_IF_EXISTS;
   using ::wcsrtombs _LIBCPP_USING_IF_EXISTS;
 #endif // _LIBCPP_HAS_NO_WIDE_CHARACTERS
-} // export
+} // _LIBCPP_MODULE_EXPORT

libcxx/modules/std.compat/cwctype.inc

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-export {
+_LIBCPP_MODULE_EXPORT {
 #ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
   using ::wctrans_t _LIBCPP_USING_IF_EXISTS;
   using ::wctype_t _LIBCPP_USING_IF_EXISTS;
@@ -32,4 +32,4 @@ export {
   using ::wctrans _LIBCPP_USING_IF_EXISTS;
   using ::wctype _LIBCPP_USING_IF_EXISTS;
 #endif // _LIBCPP_HAS_NO_WIDE_CHARACTERS
-} // export
+} // _LIBCPP_MODULE_EXPORT

libcxx/modules/std/algorithm.inc

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-export namespace std {
+_LIBCPP_MODULE_EXPORT namespace std {
   namespace ranges {
     // [algorithms.results], algorithm result types
     using std::ranges::in_found_result;

libcxx/modules/std/any.inc

@@ -7,8 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-export namespace std {
-
+_LIBCPP_MODULE_EXPORT namespace std {
   // [any.bad.any.cast], class bad_any_cast
   using std::bad_any_cast;
 

libcxx/modules/std/array.inc

@@ -7,8 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-export namespace std {
-
+_LIBCPP_MODULE_EXPORT namespace std {
   // [array], class template array
   using std::array;
 

libcxx/modules/std/atomic.inc

@@ -7,8 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-export namespace std {
-
+_LIBCPP_MODULE_EXPORT namespace std {
   // [atomics.order], order and consistency
   using std::memory_order _LIBCPP_USING_IF_EXISTS;
   using std::memory_order_acq_rel _LIBCPP_USING_IF_EXISTS;

libcxx/modules/std/barrier.inc

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-export namespace std {
+_LIBCPP_MODULE_EXPORT namespace std {
 #ifndef _LIBCPP_HAS_NO_THREADS
   using std::barrier;
 #endif // _LIBCPP_HAS_NO_THREADS

libcxx/modules/std/bit.inc

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-export namespace std {
+_LIBCPP_MODULE_EXPORT namespace std {
   // [bit.cast], bit_cast
   using std::bit_cast;
 

libcxx/modules/std/bitset.inc

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-export namespace std {
+_LIBCPP_MODULE_EXPORT namespace std {
   using std::bitset;
 
   // [bitset.operators], bitset operators

libcxx/modules/std/cassert.inc

@@ -7,6 +7,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-export namespace std {
+_LIBCPP_MODULE_EXPORT namespace std {
   // This module exports nothing.
 } // namespace std

libcxx/modules/std/cctype.inc

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-export namespace std {
+_LIBCPP_MODULE_EXPORT namespace std {
   using std::isalnum _LIBCPP_USING_IF_EXISTS;
   using std::isalpha _LIBCPP_USING_IF_EXISTS;
   using std::isblank _LIBCPP_USING_IF_EXISTS;

libcxx/modules/std/cerrno.inc

@@ -7,6 +7,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-export namespace std {
+_LIBCPP_MODULE_EXPORT namespace std {
   // This module exports nothing.
 } // namespace std

libcxx/modules/std/cfenv.inc

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-export namespace std {
+_LIBCPP_MODULE_EXPORT namespace std {
   // types
   using std::fenv_t _LIBCPP_USING_IF_EXISTS;
   using std::fexcept_t _LIBCPP_USING_IF_EXISTS;