From 20869c5ba0692974a3ec76b935360aaa67d0218b Mon Sep 17 00:00:00 2001
From: Hui Xie <hui.xie1990@gmail.com>
Date: Wed, 22 Jun 2022 09:24:09 +0100
Subject: [PATCH] [libc++] fix views::all hard error on lvalue move only views
 instead of SFINAE

For an lvalue reference to a move only view x, views::all(x) gives hard error because the expression inside noexcept is not well formed and it is not SFINAE friendly.

Given a move only view type `V`, and a concept

```
template <class R>
concept can_all = requires {
    std::views::all(std::declval<R>());
};
```

The expression `can_all<V&>` returns
libstdc++: false
msvc stl : false
libc++   : error: static_cast from 'V' to 'typename decay<decltype((std::forward<V &>(__t)))>::type' (aka 'V') uses deleted function
      noexcept(noexcept(_LIBCPP_AUTO_CAST(std::forward<_Tp>(__t))))

The standard spec has its own problem, the spec says it is expression equivalent to `decay-copy(E)` but the spec of `decay-copy` does not have any constraint, which means the expression `decay-copy(declval<V&>())` is well-formed and the concept `can_all<V&>` should return true and should error when instantiating the function body of decay-copy. This is clearly wrong behaviour in the spec and we will probably create an LWG issue. But the libc++'s behaviour is clearly not correct. The `noexcept` is an "extension" in libc++ which is not in the spec, but the expression inside `noexpect` triggers hard error, which is not right.

Reviewed By: #libc, ldionne, var-const

Differential Revision: https://reviews.llvm.org/D128281
---
 libcxx/include/__ranges/all.h                    |  1 +
 .../ranges/range.adaptors/range.all/all.pass.cpp | 16 ++++++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/libcxx/include/__ranges/all.h b/libcxx/include/__ranges/all.h
index 5f9bf7e277ed7..181477419c850 100644
--- a/libcxx/include/__ranges/all.h
+++ b/libcxx/include/__ranges/all.h
@@ -39,6 +39,7 @@ namespace __all {
     [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
     constexpr auto operator()(_Tp&& __t) const
       noexcept(noexcept(_LIBCPP_AUTO_CAST(std::forward<_Tp>(__t))))
+      -> decltype(_LIBCPP_AUTO_CAST(std::forward<_Tp>(__t)))
     {
       return _LIBCPP_AUTO_CAST(std::forward<_Tp>(__t));
     }
diff --git a/libcxx/test/std/ranges/range.adaptors/range.all/all.pass.cpp b/libcxx/test/std/ranges/range.adaptors/range.all/all.pass.cpp
index 0e3d5abb11edc..4d4d1ebc0b226 100644
--- a/libcxx/test/std/ranges/range.adaptors/range.all/all.pass.cpp
+++ b/libcxx/test/std/ranges/range.adaptors/range.all/all.pass.cpp
@@ -49,6 +49,17 @@ struct CopyableView : std::ranges::view_base {
 static_assert(std::ranges::view<CopyableView<true>>);
 static_assert(std::ranges::view<CopyableView<false>>);
 
+struct MoveOnlyView : std::ranges::view_base{
+  MoveOnlyView() = default;
+  MoveOnlyView(const MoveOnlyView&) = delete;
+  MoveOnlyView& operator=(const MoveOnlyView&) = delete;
+  MoveOnlyView(MoveOnlyView&&) = default;
+  MoveOnlyView& operator=(MoveOnlyView&&) = default;
+
+  int* begin() const;
+  int* end() const;
+};
+
 struct Range {
   int start_;
   constexpr explicit Range(int start) noexcept : start_(start) {}
@@ -139,6 +150,11 @@ constexpr bool test() {
   {
     static_assert(!std::is_invocable_v<decltype(std::views::all)>);
     static_assert(!std::is_invocable_v<decltype(std::views::all), RandomAccessRange, RandomAccessRange>);
+
+    // `views::all(v)` is expression equivalent to `decay-copy(v)` if the decayed type 
+    // of `v` models `view`. If `v` is an lvalue-reference to a move-only view, the 
+    // expression should be ill-formed because `v` is not copyable
+    static_assert(!std::is_invocable_v<decltype(std::views::all), MoveOnlyView&>);
   }
 
   // Test that std::views::all is a range adaptor