Implement LWG-3956 chrono::parse uses from_stream as a customization point (#5334)

Co-authored-by: Stephan T. Lavavej <stl@microsoft.com>

Author: frederick-vs-ja

stl/inc/chrono

@@ -4608,6 +4608,22 @@ namespace chrono {
         return _Istr;
     }
 
+    namespace _From_stream_adl_only {
+#if defined(__clang__) || defined(__EDG__) // TRANSITION, VSO-1681199
+        void from_stream() = delete; // Block unqualified name lookup
+#else // ^^^ no workaround / workaround vvv
+        void from_stream();
+#endif // ^^^ workaround ^^^
+
+        template <class _Parsable, class _CharT, class _Traits, class... _Rest>
+        concept _Can_from_stream = requires(
+            basic_istream<_CharT, _Traits>& __istr, const _CharT* __s, _Parsable& __parsed, _Rest&&... __rest_args) {
+            from_stream(__istr, +__s, __parsed, _STD forward<_Rest>(__rest_args)...); // intentional ADL
+        };
+    } // namespace _From_stream_adl_only
+
+    using _From_stream_adl_only::_Can_from_stream;
+
     template <class _CharT, class _Traits, class _Alloc, class _Parsable>
     struct _Time_parse_iomanip_c_str {
         _Time_parse_iomanip_c_str(const _CharT* _Fmt_, _Parsable& _Tp_,
@@ -4618,6 +4634,7 @@ namespace chrono {
 
         friend basic_istream<_CharT, _Traits>& operator>>(
             basic_istream<_CharT, _Traits>& _Is, _Time_parse_iomanip_c_str&& _Tpi) {
+            using _From_stream_adl_only::from_stream;
             from_stream(_Is, _Tpi._Fmt, _Tpi._Tp, _Tpi._Abbrev, _Tpi._Offset); // intentional ADL
             return _Is;
         }
@@ -4638,6 +4655,7 @@ namespace chrono {
 
         friend basic_istream<_CharT, _Traits>& operator>>(
             basic_istream<_CharT, _Traits>& _Is, _Time_parse_iomanip&& _Tpi) {
+            using _From_stream_adl_only::from_stream;
             from_stream(_Is, _Tpi._Fmt.c_str(), _Tpi._Tp, _Tpi._Abbrev, _Tpi._Offset); // intentional ADL
             return _Is;
         }
@@ -4648,12 +4666,6 @@ namespace chrono {
         minutes* _Offset;
     };
 
-    template <class _Parsable, class _CharT, class _Traits, class... _Rest>
-    concept _Can_from_stream = requires(
-        basic_istream<_CharT, _Traits>& __istr, const _CharT* __s, _Parsable& __parsed, _Rest&&... __rest_args) {
-        from_stream(__istr, +__s, __parsed, _STD forward<_Rest>(__rest_args)...); // intentional ADL
-    };
-
     _EXPORT_STD template <class _CharT, _Can_from_stream<_CharT, char_traits<_CharT>> _Parsable>
     _NODISCARD auto parse(const _CharT* _Fmt, _Parsable& _Tp) {
         return _Time_parse_iomanip_c_str<_CharT, char_traits<_CharT>, allocator<_CharT>, _Parsable>{_Fmt, _Tp};

tests/std/tests/P0355R7_calendars_and_time_zones_io/test.cpp

@@ -6,6 +6,7 @@
 #include <charconv>
 #include <chrono>
 #include <cstdint>
+#include <istream>
 #include <iterator>
 #include <limits>
 #include <locale>
@@ -1247,8 +1248,52 @@ void test_io_manipulator() {
     fail_parse(WIDEN(CharT, "a  b"), CStringOrStdString{WIDEN(CharT, "a%nb")}, time);
 }
 
+namespace lwg_3956 {
+    struct has_adl_from_stream {
+        int value = 0;
+
+        template <class CharT, class Traits, class... ArgTypes>
+        friend basic_istream<CharT, Traits>& from_stream(
+            basic_istream<CharT, Traits>& istr, const CharT*, has_adl_from_stream& parsed, ArgTypes&&...) {
+            parsed.value = 42;
+            return istr;
+        }
+    };
+
+    struct has_no_adl_from_stream {
+        operator year&() &;
+    };
+
+    template <class... ArgTypes>
+    concept can_parse = requires(ArgTypes&&... args) { parse(forward<ArgTypes>(args)...); };
+
+    static_assert(can_parse<const char*, has_adl_from_stream&>);
+    static_assert(can_parse<const string&, has_adl_from_stream&>);
+    static_assert(can_parse<const wchar_t*, has_adl_from_stream&>);
+    static_assert(can_parse<const wstring&, has_adl_from_stream&>);
+
+    static_assert(!can_parse<const char*, has_no_adl_from_stream&>);
+    static_assert(!can_parse<const string&, has_no_adl_from_stream&>);
+    static_assert(!can_parse<const wchar_t*, has_no_adl_from_stream&>);
+    static_assert(!can_parse<const wstring&, has_no_adl_from_stream&>);
+} // namespace lwg_3956
+
+void test_lwg_3956() {
+    {
+        lwg_3956::has_adl_from_stream parsed{};
+        test_parse("", "", parsed);
+        assert(parsed.value == 42);
+    }
+    {
+        lwg_3956::has_adl_from_stream parsed{};
+        test_parse(L"", L"", parsed);
+        assert(parsed.value == 42);
+    }
+}
+
 void test_parse() {
     test_lwg_3536();
+    test_lwg_3956();
     parse_seconds();
     parse_minutes();
     parse_hours();