[libc++][syncbuf] Implement LWG3253 (llvm#99778)

Closes llvm#100264

Author: mordante

libcxx/docs/Status/Cxx20Issues.csv

@@ -172,7 +172,7 @@
 "`LWG3221 <https://wg21.link/LWG3221>`__","Result of ``year_month``\  arithmetic with ``months``\  is ambiguous","2019-11 (Belfast)","|Complete|","8.0",""
 "`LWG3235 <https://wg21.link/LWG3235>`__","``parse``\  manipulator without abbreviation is not callable","2019-11 (Belfast)","","",""
 "`LWG3246 <https://wg21.link/LWG3246>`__","LWG3246: What are the constraints on the template parameter of `basic_format_arg`?","2019-11 (Belfast)","|Nothing To Do|","",""
-"`LWG3253 <https://wg21.link/LWG3253>`__","``basic_syncbuf::basic_syncbuf()``\  should not be explicit","2019-11 (Belfast)","","",""
+"`LWG3253 <https://wg21.link/LWG3253>`__","``basic_syncbuf::basic_syncbuf()``\  should not be explicit","2019-11 (Belfast)","|Complete|","20.0",""
 "`LWG3245 <https://wg21.link/LWG3245>`__","Unnecessary restriction on ``'%p'``\  parse specifier","2019-11 (Belfast)","","",""
 "`LWG3244 <https://wg21.link/LWG3244>`__","Constraints for ``Source``\  in |sect|\ [fs.path.req] insufficiently constrainty","2019-11 (Belfast)","","",""
 "`LWG3241 <https://wg21.link/LWG3241>`__","``chrono-spec``\  grammar ambiguity in |sect|\ [time.format]","2019-11 (Belfast)","|Complete|","16.0",""

libcxx/include/syncstream

@@ -46,7 +46,9 @@ namespace std {
         using streambuf_type = basic_streambuf<charT, traits>;
 
         // [syncstream.syncbuf.cons], construction and destruction
-        explicit basic_syncbuf(streambuf_type* obuf = nullptr)
+        basic_syncbuf()
+          : basic_syncbuf(nullptr) {}
+        explicit basic_syncbuf(streambuf_type* obuf)
           : basic_syncbuf(obuf, Allocator()) {}
         basic_syncbuf(streambuf_type*, const Allocator&);
         basic_syncbuf(basic_syncbuf&&);
@@ -253,7 +255,10 @@ public:
 
   // [syncstream.syncbuf.cons], construction and destruction
 
-  _LIBCPP_HIDE_FROM_ABI explicit basic_syncbuf(streambuf_type* __obuf = nullptr)
+  _LIBCPP_HIDE_FROM_ABI basic_syncbuf()
+      : basic_syncbuf(nullptr) {}
+
+  _LIBCPP_HIDE_FROM_ABI explicit basic_syncbuf(streambuf_type* __obuf)
       : basic_syncbuf(__obuf, _Allocator()) {}
 
   _LIBCPP_HIDE_FROM_ABI basic_syncbuf(streambuf_type* __obuf, _Allocator const& __alloc)

libcxx/test/std/input.output/syncstream/syncbuf/syncstream.syncbuf.cons/cons.default.pass.cpp

@@ -25,8 +25,15 @@
 #include "constexpr_char_traits.h"
 #include "test_allocator.h"
 
+template <class CharT>
+std::basic_syncbuf<CharT> lwg3253_default_constructor_is_not_explicit() {
+  return {};
+}
+
 template <class CharT>
 void test() {
+  lwg3253_default_constructor_is_not_explicit<CharT>();
+
   {
     using Buf = std::basic_syncbuf<CharT>;
     static_assert(std::default_initializable<Buf>);