[libc++][test] Add const and reference tests for enable_view. NFC.

[jloser]

Summary

As discussed in https://reviews.llvm.org/D117714, there is missing test coverage
for the behavior of enable_view when given a const or reference qualified
type. Add such tests showing the current behavior.

[philnik]

libcxx/test/std/ranges/range.req/range.view/enable_view.compile.pass.cpp

42

46

This seems weird. It looks to me like the standard requires it that way, but I don't know why.

61

[• Quuxplusone]

Thanks! This is an improvement as-is, but I think you should consider merging all your new tests from D117714 into this review. They should all work today, because view_interface<T> already inherits from view_base. Then your D117714 will become a code-only change, with no diffs to any of the tests except for the empty-base-sizeof test (and I guess if merging the tests over here discovers any bugs in the existing implementation!).

[• Quuxplusone]

libcxx/test/std/ranges/range.req/range.view/enable_view.compile.pass.cpp

46

Yikes. Good catch. This reminds me of D116388, but unlike bind, this stuff is so new that I think this deserves an LWG issue. I'll request one.

[jloser]

Merge tests from LWG3549 PR into this review.

[jloser]

libcxx/test/std/ranges/range.req/range.view/enable_view.compile.pass.cpp

46

Exactly my thoughts as well. It's clear from a technical perspective, but surely this isn't what the user intended. They likely intended to disable cv-qual of their type as well and expected us to do it for them basically by opting in for their type only (and not aalso
for cv-qual of their type).

Generally speaking, this is likely to come up with other customization points in the standard library, but I'd have to search for other "broken" ones.

[• Quuxplusone]

libcxx/test/std/ranges/range.req/range.view/enable_view.compile.pass.cpp

46

Initial reply from LWG is: "Who cares?"

The only person who should be querying enable_view<X> is concept view itself; no user-programmer should ever be querying it. And concept view will care about enable_view<X> only when movable<X>, which means

• enable_view<T&> is never relevant
• enable_view<const T> is relevant only when assignable_from<const V&, const V>; such a V must have begin() and end() and const V& operator=(const V&) const.

Here are the test cases from my attempted-LWG-issue-filing, for the record.

struct V1 : std::ranges::view_base {};
struct V2 : std::ranges::view_interface<V2> {};
struct V3 {}; template<> constexpr bool std::ranges::enable_view<V3> = true;
struct V4 : std::view_base {}; template<> constexpr bool std::ranges::enable_view<V4> = false;

static_assert(std::ranges::enable_view<V1> == true);
static_assert(std::ranges::enable_view<V2> == true);
static_assert(std::ranges::enable_view<V3> == true);
static_assert(std::ranges::enable_view<V4> == false);

static_assert(std::ranges::enable_view<const V1> == true);
LIBCPP_STATIC_ASSERT(std::ranges::enable_view<const V2> == true);
static_assert(std::ranges::enable_view<const V3> == false);
static_assert(std::ranges::enable_view<const V4> == true);

libcxx/test/std/ranges/range.req/range.view/enable_view.compile.pass.cpp

 
 #include <ranges>
 
 #include "test_macros.h"
 
 // Doesn't derive from view_base
 struct Empty { };
 static_assert(!std::ranges::enable_view<Empty>);
+static_assert(!std::ranges::enable_view<Empty&>);
+static_assert(!std::ranges::enable_view<Empty&&>);
+static_assert(!std::ranges::enable_view<const Empty>);
+static_assert(!std::ranges::enable_view<const Empty&>);
+static_assert(!std::ranges::enable_view<const Empty&&>);
 
 // Derives from view_base, but privately
 struct PrivateViewBase : private std::ranges::view_base { };
 static_assert(!std::ranges::enable_view<PrivateViewBase>);
+static_assert(!std::ranges::enable_view<PrivateViewBase&>);
+static_assert(!std::ranges::enable_view<PrivateViewBase&&>);
+static_assert(!std::ranges::enable_view<const PrivateViewBase>);
+static_assert(!std::ranges::enable_view<const PrivateViewBase&>);
+static_assert(!std::ranges::enable_view<const PrivateViewBase&&>);
 
 // Derives from view_base, but specializes enable_view to false
 struct EnableViewFalse : std::ranges::view_base { };
-namespace std::ranges { template <> constexpr bool enable_view<EnableViewFalse> = false; }
+template <> constexpr bool std::ranges::enable_view<EnableViewFalse> = false;
 static_assert(!std::ranges::enable_view<EnableViewFalse>);
-
+static_assert(!std::ranges::enable_view<EnableViewFalse&>);
+static_assert(!std::ranges::enable_view<EnableViewFalse&&>);
+static_assert(std::ranges::enable_view<const EnableViewFalse>);
+static_assert(!std::ranges::enable_view<const EnableViewFalse&>);
+static_assert(!std::ranges::enable_view<const EnableViewFalse&&>);
 
 // Derives from view_base
 struct PublicViewBase : std::ranges::view_base { };
 static_assert(std::ranges::enable_view<PublicViewBase>);
+static_assert(!std::ranges::enable_view<PublicViewBase&>);
+static_assert(!std::ranges::enable_view<PublicViewBase&&>);
+static_assert(std::ranges::enable_view<const PublicViewBase>);
+static_assert(!std::ranges::enable_view<const PublicViewBase&>);
+static_assert(!std::ranges::enable_view<const PublicViewBase&&>);
 
 // Does not derive from view_base, but specializes enable_view to true
 struct EnableViewTrue { };
-namespace std::ranges { template <> constexpr bool enable_view<EnableViewTrue> = true; }
+template <> constexpr bool std::ranges::enable_view<EnableViewTrue> = true;
 static_assert(std::ranges::enable_view<EnableViewTrue>);
-
+static_assert(!std::ranges::enable_view<EnableViewTrue&>);
+static_assert(!std::ranges::enable_view<EnableViewTrue&&>);
+static_assert(!std::ranges::enable_view<const EnableViewTrue>);
+static_assert(!std::ranges::enable_view<const EnableViewTrue&>);
+static_assert(!std::ranges::enable_view<const EnableViewTrue&&>);
 
 // Make sure that enable_view is a bool, not some other contextually-convertible-to-bool type.
 ASSERT_SAME_TYPE(decltype(std::ranges::enable_view<Empty>), const bool);
 ASSERT_SAME_TYPE(decltype(std::ranges::enable_view<PublicViewBase>), const bool);
+
+struct V1 : std::ranges::view_interface<V1> {};
+static_assert(std::ranges::enable_view<V1>);
+static_assert(!std::ranges::enable_view<V1&>);
+static_assert(!std::ranges::enable_view<V1&&>);
+static_assert(std::ranges::enable_view<const V1>);
+static_assert(!std::ranges::enable_view<const V1&>);
+static_assert(!std::ranges::enable_view<const V1&&>);
+
+struct V2 : std::ranges::view_interface<V1>, std::ranges::view_interface<V2> {};
+static_assert(!std::ranges::enable_view<V2>);
+static_assert(!std::ranges::enable_view<V2&>);
+static_assert(!std::ranges::enable_view<V2&&>);
+static_assert(!std::ranges::enable_view<const V2>);
+static_assert(!std::ranges::enable_view<const V2&>);
+static_assert(!std::ranges::enable_view<const V2&&>);
+
+struct V3 : std::ranges::view_interface<V1> {};
+static_assert(std::ranges::enable_view<V3>);
+static_assert(!std::ranges::enable_view<V3&>);
+static_assert(!std::ranges::enable_view<V3&&>);
+static_assert(std::ranges::enable_view<const V3>);
+static_assert(!std::ranges::enable_view<const V3&>);
+static_assert(!std::ranges::enable_view<const V3&&>);
+
+struct PrivateInherit : private std::ranges::view_interface<PrivateInherit> {};
+static_assert(!std::ranges::enable_view<PrivateInherit>);
+
+// ADL-proof
+struct Incomplete;
+template<class T> struct Holder { T t; };
+static_assert(!std::ranges::enable_view<Holder<Incomplete>*>);
+
+static_assert(!std::ranges::enable_view<void>);