[libc++] [ranges] Fix `decltype(auto) ranges::iter_move`

[• Quuxplusone]

Summary

The new test fails before this patch.
See

• https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92894#c3
• https://reviews.llvm.org/D119589#inline-1151299

[Mordante]

The GCC bug mentions Fixed for 10.2 and we use GCC 11 in our CI.

[philnik]

In D120417#3340764, @Mordante wrote:

The GCC bug mentions Fixed for 10.2 and we use GCC 11 in our CI.

The bug was a libstdc++ bug.

[jwakely]

In D120417#3340764, @Mordante wrote:

The GCC bug mentions Fixed for 10.2 and we use GCC 11 in our CI.

Fixed by changing libstdc++, which is irrelevant to libc++ because you're not using the libstdc++ code.

[• Quuxplusone]

Riiight. Actually use a projection type with non-external linkage, this time.

[• Quuxplusone]

GCC 11 was still happy with the previous test. Try one basically-not-reduced-at-all from the ranges::min review.

[• Quuxplusone]

I finally hit a reproducing case locally, while writing tests for C++20 move_iterator!

[• Quuxplusone]

https://reviews.llvm.org/harbormaster/unit/view/2832828/ — filed https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104792 for what I consider a bogus GCC warning, but will fix anyway.

[• Quuxplusone]

Fix GCC buildkite warning by removing unused typedefs.

libcxx/test/std/iterators/iterator.requirements/indirectcallable/projected/projected.compile.pass.cpp

 
 #include "test_iterators.h"
 
 using IntPtr = std::projected<int const*, std::identity>;
 static_assert(std::same_as<IntPtr::value_type, int>);
 static_assert(std::same_as<decltype(*std::declval<IntPtr>()), int const&>);
 static_assert(std::same_as<std::iter_difference_t<IntPtr>, std::ptrdiff_t>);
 
+void test() {
+  {
+    auto projection = [](int x) { return x + 1; };
+    using P = std::projected<int*, decltype(projection)>;
+    static_assert(std::same_as<std::iter_value_t<P>, int>);
+    static_assert(std::same_as<std::iter_reference_t<P>, int>);
+    static_assert(std::same_as<std::iter_difference_t<P>, std::ptrdiff_t>);
+    static_assert(std::same_as<std::iter_rvalue_reference_t<P>, int>);
+    static_assert(std::same_as<std::iter_common_reference_t<P>, int>);
+  }
+  {
+    auto projection = [](int& x) -> int& { return x; };
+    using P = std::projected<int*, decltype(projection)>;
+    static_assert(std::same_as<std::iter_value_t<P>, int>);
+    static_assert(std::same_as<std::iter_reference_t<P>, int&>);
+    static_assert(std::same_as<std::iter_difference_t<P>, std::ptrdiff_t>);
+    static_assert(std::same_as<std::iter_rvalue_reference_t<P>, int&&>);
+    static_assert(std::same_as<std::iter_common_reference_t<P>, int&>);
+  }
+}
+
 struct S { };
 
 using Cpp17InputIterator = std::projected<cpp17_input_iterator<S*>, int S::*>;
 static_assert(std::same_as<Cpp17InputIterator::value_type, int>);
 static_assert(std::same_as<decltype(*std::declval<Cpp17InputIterator>()), int&>);
 static_assert(std::same_as<std::iter_difference_t<Cpp17InputIterator>, std::ptrdiff_t>);
 
 using Cpp20InputIterator = std::projected<cpp20_input_iterator<S*>, int S::*>;