[libc++] Extension: Make move and forward constexpr in C++11.

Summary:
`std::move` and `std::forward` were not marked constexpr in C++11.  This can be very damaging because it makes otherwise constant expressions non-constant. For example:

```
#include <utility>
template <class T>
struct Foo {
  constexpr Foo(T&& tx) :  t(std::move(tx)) {}
  T t;
};
[[clang::require_constant_initialization]] Foo<int> f(42); // Foo should be constant initialized but C++11 move is not constexpr. As a result `f` is an unsafe global.
```

This patch applies `constexpr` to `move` and `forward` as an extension in C++11. Normally the library is not allowed to add `constexpr` because it may be observable to the user. In particular adding constexpr may cause valid code to stop compiling. However these problems only happen in more complex situations, like making `__invoke(...)` constexpr. `forward` and `move` are simply enough that applying `constexpr` is safe. 

Note that libstdc++ has offered this extension since at least 4.8.1.

Most of the changes in this patch are simply test cleanups or additions. The main changes in the tests are:

* Fold all `forward_N.fail.cpp` tests into a single `forward.fail.cpp` test using -verify.
* Delete most `move_only_N.fail.cpp` tests because they weren't actually testing anything.
* Fold `move_copy.pass.cpp` and `move_only.pass.cpp` into a single `move.pass.cpp` test.
* Add return type and noexcept tests for `forward` and `move`.




Reviewers: rsmith, mclow.lists, EricWF

Subscribers: K-ballo, loladiro

Differential Revision: https://reviews.llvm.org/D24637

llvm-svn: 282439

libcxx/include/type_traits

@@ -2155,7 +2155,7 @@ template <class _Tp> _LIBCPP_CONSTEXPR bool is_destructible_v
 #ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
 
 template <class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
 typename remove_reference<_Tp>::type&&
 move(_Tp&& __t) _NOEXCEPT
 {
@@ -2164,20 +2164,20 @@ move(_Tp&& __t) _NOEXCEPT
 }
 
 template <class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
 _Tp&&
 forward(typename remove_reference<_Tp>::type& __t) _NOEXCEPT
 {
     return static_cast<_Tp&&>(__t);
 }
 
 template <class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
 _Tp&&
 forward(typename remove_reference<_Tp>::type&& __t) _NOEXCEPT
 {
     static_assert(!is_lvalue_reference<_Tp>::value,
-                  "Can not forward an rvalue as an lvalue.");
+                  "can not forward an rvalue as an lvalue");
     return static_cast<_Tp&&>(__t);
 }
 

libcxx/test/std/utilities/utility/forward/forward.fail.cpp

@@ -0,0 +1,53 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// test forward
+
+#include <utility>
+
+#include "test_macros.h"
+
+struct A
+{
+};
+
+A source() {return A();}
+const A csource() {return A();}
+
+int main()
+{
+#if TEST_STD_VER >= 11
+    {
+        std::forward<A&>(source());  // expected-note {{requested here}}
+        // expected-error@type_traits:* 1 {{static_assert failed "can not forward an rvalue as an lvalue"}}
+    }
+#else
+    {
+        std::forward<A&>(source()); // expected-error {{no matching function for call to 'forward'}}
+    }
+#endif
+    {
+        const A ca = A();
+        std::forward<A&>(ca); // expected-error {{no matching function for call to 'forward'}}
+    }
+    {
+        std::forward<A&>(csource());  // expected-error {{no matching function for call to 'forward'}}
+    }
+    {
+        const A ca = A();
+        std::forward<A>(ca); // expected-error {{no matching function for call to 'forward'}}
+    }
+    {
+        std::forward<A>(csource()); // expected-error {{no matching function for call to 'forward'}}
+    }
+    {
+        A a;
+        std::forward(a); // expected-error {{no matching function for call to 'forward'}}
+    }
+}

libcxx/test/std/utilities/utility/forward/forward.pass.cpp

@@ -7,32 +7,40 @@
 //
 //===----------------------------------------------------------------------===//
 
+// UNSUPPORTED: c++98, c++03
+
 // test forward
 
 #include <utility>
+#include <type_traits>
 #include <cassert>
 
+#include "test_macros.h"
+
 struct A
 {
 };
 
-A source() {return A();}
-const A csource() {return A();}
-
-typedef char one;
-struct two {one _[2];};
-struct four {one _[4];};
-struct eight {one _[8];};
-
-one test(A&);
-two test(const A&);
-
-#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
-
-four test(A&&);
-eight test(const A&&);
-
-#endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
+A source() noexcept {return A();}
+const A csource() noexcept {return A();}
+
+
+constexpr bool test_constexpr_forward() {
+#if TEST_STD_VER > 11
+    int x = 42;
+    const int cx = 101;
+    return std::forward<int&>(x)        == 42
+        && std::forward<int>(x)         == 42
+        && std::forward<const int&>(x)  == 42
+        && std::forward<const int>(x)   == 42
+        && std::forward<int&&>(x)       == 42
+        && std::forward<const int&&>(x) == 42
+        && std::forward<const int&>(cx) == 101
+        && std::forward<const int>(cx)  == 101;
+#else
+    return true;
+#endif
+}
 
 int main()
 {
@@ -42,42 +50,42 @@ int main()
     ((void)a); // Prevent unused warning
     ((void)ca); // Prevent unused warning
 
-#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
-    static_assert(sizeof(test(std::forward<A&>(a))) == 1, "");
-    static_assert(sizeof(test(std::forward<A>(a))) == 4, "");
-    static_assert(sizeof(test(std::forward<A>(source()))) == 4, "");
-
-    static_assert(sizeof(test(std::forward<const A&>(a))) == 2, "");
-//    static_assert(sizeof(test(std::forward<const A&>(source()))) == 2, "");
-    static_assert(sizeof(test(std::forward<const A>(a))) == 8, "");
-    static_assert(sizeof(test(std::forward<const A>(source()))) == 8, "");
-
-    static_assert(sizeof(test(std::forward<const A&>(ca))) == 2, "");
-//    static_assert(sizeof(test(std::forward<const A&>(csource()))) == 2, "");
-    static_assert(sizeof(test(std::forward<const A>(ca))) == 8, "");
-    static_assert(sizeof(test(std::forward<const A>(csource()))) == 8, "");
-
-#else  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
-
-    static_assert(sizeof(test(std::forward<A&>(a))) == 1, "");
-    static_assert(sizeof(test(std::forward<A>(a))) == 1, "");
-//    static_assert(sizeof(test(std::forward<A>(source()))) == 2, "");
-
-    static_assert(sizeof(test(std::forward<const A&>(a))) == 2, "");
-    static_assert(sizeof(test(std::forward<const A&>(source()))) == 2, "");
-    static_assert(sizeof(test(std::forward<const A>(a))) == 2, "");
-    static_assert(sizeof(test(std::forward<const A>(source()))) == 2, "");
-
-    static_assert(sizeof(test(std::forward<const A&>(ca))) == 2, "");
-    static_assert(sizeof(test(std::forward<const A&>(csource()))) == 2, "");
-    static_assert(sizeof(test(std::forward<const A>(ca))) == 2, "");
-    static_assert(sizeof(test(std::forward<const A>(csource()))) == 2, "");
-#endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
-
-#if _LIBCPP_STD_VER > 11
-    constexpr int i1 = std::move(23);
-    static_assert(i1 == 23, "" );
+    static_assert(std::is_same<decltype(std::forward<A&>(a)), A&>::value, "");
+    static_assert(std::is_same<decltype(std::forward<A>(a)), A&&>::value, "");
+    static_assert(std::is_same<decltype(std::forward<A>(source())), A&&>::value, "");
+    static_assert(noexcept(std::forward<A&>(a)), "");
+    static_assert(noexcept(std::forward<A>(a)), "");
+    static_assert(noexcept(std::forward<A>(source())), "");
+
+    static_assert(std::is_same<decltype(std::forward<const A&>(a)), const A&>::value, "");
+    static_assert(std::is_same<decltype(std::forward<const A>(a)), const A&&>::value, "");
+    static_assert(std::is_same<decltype(std::forward<const A>(source())), const A&&>::value, "");
+    static_assert(noexcept(std::forward<const A&>(a)), "");
+    static_assert(noexcept(std::forward<const A>(a)), "");
+    static_assert(noexcept(std::forward<const A>(source())), "");
+
+    static_assert(std::is_same<decltype(std::forward<const A&>(ca)), const A&>::value, "");
+    static_assert(std::is_same<decltype(std::forward<const A>(ca)), const A&&>::value, "");
+    static_assert(std::is_same<decltype(std::forward<const A>(csource())), const A&&>::value, "");
+    static_assert(noexcept(std::forward<const A&>(ca)), "");
+    static_assert(noexcept(std::forward<const A>(ca)), "");
+    static_assert(noexcept(std::forward<const A>(csource())), "");
+
+#if TEST_STD_VER > 11
+    {
+    constexpr int i2 = std::forward<int>(42);
+    static_assert(std::forward<int>(42) == 42, "");
+    static_assert(std::forward<const int&>(i2) == 42, "");
+    static_assert(test_constexpr_forward(), "");
+    }
+#endif
+#if TEST_STD_VER == 11 && defined(_LIBCPP_VERSION)
+    // Test that std::forward is constexpr in C++11. This is an extension
+    // provided by both libc++ and libstdc++.
+    {
     constexpr int i2 = std::forward<int>(42);
-    static_assert(i2 == 42, "" );
+    static_assert(std::forward<int>(42) == 42, "" );
+    static_assert(std::forward<const int&>(i2) == 42, "");
+    }
 #endif
 }