libstdc++: Give ranges::empty() a concrete return type (PR 93978)

This works around PR 93978 by avoiding having to instantiate the body of
ranges::empty() when checking the constraints of view_interface::operator
bool().  When ranges::empty() has an auto return type, then we must instantiate
its body in order to determine whether the requires expression {
ranges::empty(_M_derived()); } is well-formed.  But this means instantiating
view_interface::empty() and hence view_interface::_M_derived(), all before we've
yet deduced the return type of join_view::end().  (The reason
view_interface::operator bool() is needed in join_view::end() in the first place
is because in this function we perform direct initialization of
join_view::_Sentinel from a join_view, and so we try to find a conversion
sequence from the latter to the former that goes through this conversion
operator.)

Giving ranges::empty() a concrete return type of bool should be safe according
to [range.prim.empty]/4 which says "whenever ranges::empty(E) is a valid
expression, it has type bool."

This fixes the test case in PR 93978 when compiling without -Wall, but with -Wall
the test case still fails due to the issue described in PR c++/94038, I think.
I still don't quite understand why the test case doesn't fail without -O.

libstdc++-v3/ChangeLog:

	PR libstdc++/93978
	* include/bits/range_access.h (__cust_access::_Empty::operator()):
	Declare return type to be bool instead of auto.
	* testsuite/std/ranges/adaptors/93978.cc: New test.

1 files changed, 1 insertions, 1 deletions

diff --git a/libstdc++-v3/include/bits/range_access.h b/libstdc++-v3/include/bits/range_access.h
index 8b276fd..4dd48bb 100644
--- a/libstdc++-v3/include/bits/range_access.h
+++ b/libstdc++-v3/include/bits/range_access.h
@@ -793,7 +793,7 @@ namespace ranges
       template<typename _Tp>
 	requires __member_empty<_Tp> || __size0_empty<_Tp>
 	|| __eq_iter_empty<_Tp>
-	constexpr auto
+	constexpr bool
 	operator()(_Tp&& __e) const noexcept(_S_noexcept<_Tp>())
 	{
 	  if constexpr (__member_empty<_Tp>)