[PATCH 3/3] libstdc++: Implement C++20 range adaptors

[Stephan Bergmann]

On 04/02/2020 03:07, Patrick Palka wrote:
> This patch implements [range.adaptors].  It also includes the changes from P3280
> and P3278 and P3323, without which many standard examples won't work.

I see that with this 
<https://gcc.gnu.org/git/gitweb.cgi?p=gcc.git;a=commit;h=cba9ef069e58eac00f30489d3ef21390caee6e45> 
"libstdc++: Implement C++20 range adaptors", compiling <ranges> with 
recent Clang trunk (which appears to mostly implement C++20 concepts 
now) in -std=c++2a mode fails as below (besides also failing due to some 
"missing" typenames, where Clang apparently doesn't yet implement 
P0634R3).  And I'm not sure which of Clang vs. GCC is right here.

The failure is

> gcc/trunk/inst/include/c++/10.0.1/ranges:1512:47: error: ambiguous deduction for template arguments of '_RangeAdaptor'
>     inline constexpr __adaptor::_RangeAdaptor filter
>                                               ^
> gcc/trunk/inst/include/c++/10.0.1/ranges:1073:2: note: candidate function [with _Callable = std::ranges::views::(lambda at gcc/trunk/inst/include/c++/10.0.1/ranges:1513:9)]
>         _RangeAdaptor(const _Callable& = {})
>         ^
> gcc/trunk/inst/include/c++/10.0.1/ranges:1078:2: note: candidate function [with _Callable = std::ranges::views::(lambda at gcc/trunk/inst/include/c++/10.0.1/ranges:1513:9)]
>         _RangeAdaptor(_Callable __callable)
>         ^

and a stripped-down reproducer is

> template<typename T> struct S {
>   S(T const &) requires true;
>   S(T) requires false;
> };
> S s = 0;

(Clang accepts this when the last line is replaced with

> S<int> s = 0;

and thus no class template argument deduction needs to be done.)

I think what is relevant here is [over.match.class.deduct]/1 in the 
current spec, which specifies a helper set of hypothetical function 
templates based on a class' constructors for class template argument 
deduction.  It details the function templates' template parameters, 
function parameters, and return types, but does not mention 
requires-clauses.  From my superficial understanding of concepts and 
class template argument deduction it would thus look like the 
constructors' requires-clauses should indeed not be taken into account here?