[PATCH RFA] libstdc++: add ADL friends

[Jason Merrill]

On 11/3/25 5:26 PM, Patrick Palka wrote:
> 
> On Sun, 2 Nov 2025, Jason Merrill wrote:
> 
>> Tested x86_64-pc-linux-gnu, OK for trunk?
>>
>> -- 8< --
>>
>> Since the implementation namespaces __detail and __exception_ptr aren't
>> exported from std, ADL can't find these functions there.  Adding friend
>> declarations makes it work.
> 
> I thought Nathaniel's module ADL fixes (for 121705 and/or 117658) would
> make this unnecessary?

I don't think they help here, because e.g. in
18_support/exception_ptr/rethrow_exception.cc there is no instantiation 
context for the call

     rethrow_exception(make_exception_ptr(0));

which happens in a non-template function.  I don't see anything in 
[basic.lookup.argdep] that would cause us to look into the interface 
unit of std in this case.
>> libstdc++-v3/ChangeLog:
>>
>> 	* include/bits/quoted_string.h: Add ADL friends.
>> 	* libsupc++/exception_ptr.h: Add ADL friend.
>> ---
>>   libstdc++-v3/include/bits/quoted_string.h | 23 +++++++++++++++++++++++
>>   libstdc++-v3/libsupc++/exception_ptr.h    |  4 ++++
>>   2 files changed, 27 insertions(+)
>>
>> diff --git a/libstdc++-v3/include/bits/quoted_string.h b/libstdc++-v3/include/bits/quoted_string.h
>> index 3a828d5eab9..9a51aa367fe 100644
>> --- a/libstdc++-v3/include/bits/quoted_string.h
>> +++ b/libstdc++-v3/include/bits/quoted_string.h
>> @@ -61,6 +61,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>>   	_Quoted_string&
>>   	operator=(_Quoted_string&) = delete;
>>   
>> +	// Friends for ADL with module std.
>> +	template<typename _CharT2, typename _Traits>
>> +	friend std::basic_ostream<_CharT2, _Traits>&
>> +	operator<<(std::basic_ostream<_CharT2, _Traits>& __os,
>> +		   const _Quoted_string<const _CharT2*, _CharT2>& __str);
>> +
>> +	template<typename _CharT2, typename _Traits, typename _String2>
>> +	friend std::basic_ostream<_CharT2, _Traits>&
>> +	operator<<(std::basic_ostream<_CharT2, _Traits>& __os,
>> +		   const _Quoted_string<_String2, _CharT2>& __str);
>> +
>> +	template<typename _CharT2, typename _Traits, typename _Alloc>
>> +	friend std::basic_istream<_CharT2, _Traits>&
>> +	operator>>(std::basic_istream<_CharT2, _Traits>& __is,
>> +		   const _Quoted_string<basic_string<_CharT2, _Traits, _Alloc>&,
>> +		   _CharT2>& __str);
>> +
>>   	_String _M_string;
>>   	_CharT _M_delim;
>>   	_CharT _M_escape;
>> @@ -78,6 +95,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>>   	_Quoted_string&
>>   	operator=(_Quoted_string&) = delete;
>>   
>> +	// Friend for ADL with module std.
>> +	template<typename _CharT2, typename _Traits2, typename _String2>
>> +	friend std::basic_ostream<_CharT2, _Traits2>&
>> +	operator<<(std::basic_ostream<_CharT2, _Traits2>& __os,
>> +		   const _Quoted_string<_String2, _CharT2>& __str);
>> +
>>   	basic_string_view<_CharT, _Traits> _M_string;
>>   	_CharT _M_delim;
>>   	_CharT _M_escape;
>> diff --git a/libstdc++-v3/libsupc++/exception_ptr.h b/libstdc++-v3/libsupc++/exception_ptr.h
>> index f673a334333..76482db3756 100644
>> --- a/libstdc++-v3/libsupc++/exception_ptr.h
>> +++ b/libstdc++-v3/libsupc++/exception_ptr.h
>> @@ -214,6 +214,10 @@ namespace std _GLIBCXX_VISIBILITY(default)
>>         { return __x._M_exception_object != __y._M_exception_object; }
>>   #endif
>>   
>> +      // Friend for ADL with module std.
>> +      friend _GLIBCXX26_CONSTEXPR void
>> +      swap(exception_ptr& __lhs, exception_ptr& __rhs);
>> +
>>         const class std::type_info*
>>         __cxa_exception_type() const _GLIBCXX_USE_NOEXCEPT
>>   	__attribute__ ((__pure__));
>>
>> base-commit: 2be505ace7e5e29a29ab53025762cf1a100a6d76
>> -- 
>> 2.51.0
>>
>>
>