6d082cd901
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.
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