[libcxx] Library support for contracts (C++2a)

[hamzasood]

Summary

This patch adds the library components needed for contracts in C++2a.

The wording says that a contract_violation object is populated in an implementation-defined manner. A private constructor that the compiler can call seems like the most sensible (and obvious) solution.

[mclow.lists]

Thanks for doing this, but it needs tests before it can land.
If I were to define class contract_violation {}; that would pass the tests you've provided.
(Heck, if I removed the entire definition of contract_violation, it would still pass.)

include/contract

42

Needs _LIBCPP_TYPE_VIS

[mclow.lists]

I get that lib++ will not make these things, but we still need some tests showing that the whole machinery works.

[hamzasood]

• Added a missing visibility attribute.
• Added a simple test that type-checks the expected public interface.

Testing whether it's actually hooked up correctly is quite problematic; it isn't meant to be instantiable. What's the best way to proceed with this?

[mclow.lists]

Testing whether it's actually hooked up correctly is quite problematic; it isn't meant to be instantiable. What's the best way to proceed with this?

The compiler is supposed to create these - right?
Does clang currently do this?

• If not, we should wait for them to catch up.
• If so, then we should have tests that have a contract violation, and pass these around.

Since the method for hooking up a CV proc is implementation-defined, those tests will probably have to go into test/libcxx.

test/std/language.support/support.contract/contract_violation.pass.cpp

28

You can do this better with static_assert and declval:

static_assert(noexcept(std::declval<const contract_violation&>().line_number()), "");

and same with the types:

static_assert(std::is_same_v<decltype(std::declval<const contract_violation&>().line_number()>, std::uint_least32_t>, "");

(It's shorter to write using CV = const contract_violation; and then declval<CV&>())

[mclow.lists]

I'd like to be clear that I'm not against using ASSERT_NOEXCEPT or ASSERT_SAME_TYPE - that's fine.
But I'd rather you use declval<> - which is only useful in an unevaluated context, rather than having a function that takes a parameter - which someone might actually call.

Also, then you can move all the tests into main and get rid of the function.

[mclow.lists]

test/libcxx/language.support/support.contract/version.pass.cpp

10

All the tests need // UNSUPPORTED: c++03, c++11, c++14, c++17

[hamzasood]

• Made the suggested test changes.

The compiler is meant to create these; Clang doesn't do this yet but I'm working on the implementation.

I was under the impression that the compiler support would require the library stuff to land first, but I just checked Clang's coroutine tests and it looks like they include their own coroutine header so it doesn't depend on the library. I suppose the same thing can happen here, in which case I agree it's probably best to put this on hold in the meantime.

include/contract

+// -*- C++ -*-
+//===------------------------------ contract ------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP_CONTRACT
+#define _LIBCPP_CONTRACT
+
+/*
+    contract synopsis
+
+namespace std {
+  class contract_violation {
+  public:
+    uint_least32_t line_number() const noexcept;
+    string_view file_name() const noexcept;
+    string_view function_name() const noexcept;
+    string_view comment() const noexcept;
+    string_view assertion_level() const noexcept;
+  };
+}
+*/
+
+#include <__config>
+#include <cstdint>
+#include <string_view>
+
+#ifndef _LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER
+#pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER > 17
+
+class _LIBCPP_TYPE_VIS contract_violation {
+public:
+  _LIBCPP_INLINE_VISIBILITY uint_least32_t line_number() const noexcept { return __line_number; }
+  _LIBCPP_INLINE_VISIBILITY string_view file_name() const noexcept { return __file_name; }
+  _LIBCPP_INLINE_VISIBILITY string_view function_name() const noexcept { return __function_name; }
+  _LIBCPP_INLINE_VISIBILITY string_view comment() const noexcept { return __comment; }
+  _LIBCPP_INLINE_VISIBILITY string_view assertion_level() const noexcept { return __assertion_level; }
+
+private:
+  _LIBCPP_INLINE_VISIBILITY
+  constexpr contract_violation(uint_least32_t __line, string_view __file, string_view __fn,
+                               string_view __comment, string_view __lvl) noexcept
+      : __line_number(__line), __file_name(__file), __function_name(__fn),
+        __comment(__comment), __assertion_level(__lvl) {}
+
+  uint_least32_t __line_number;
+  string_view __file_name;
+  string_view __function_name;
+  string_view __comment;
+  string_view __assertion_level;
+};
+
+#endif // _LIBCPP_STD_VER > 17
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP_CONTRACT

include/module.modulemap

     header "condition_variable"
     export *
   }
+  module contract {
+    header "contract"
+    export *
+  }
   module deque {
     header "deque"
     export initializer_list

test/libcxx/double_include.sh.cpp

 #include <complex>
 #include <complex.h>
 #include <condition_variable>
+#include <contract>
 #include <csetjmp>
 #include <csignal>
 #include <cstdarg>

test/libcxx/language.support/support.contract/version.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// <contract>
+
+#include <contract>
+
+#ifndef _LIBCPP_VERSION
+#error _LIBCPP_VERSION not defined
+#endif
+
+int main()
+{
+}

test/std/language.support/support.contract/contract_violation.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14, c++17
+
+// <contract>
+// class contract_violation
+
+#include <contract>
+#include <cstdint>
+#include <string_view>
+#include <type_traits>
+#include <utility>
+
+using CV = std::contract_violation;
+
+// Check for noexcept.
+static_assert(noexcept(std::declval<const CV &>().line_number()));
+static_assert(noexcept(std::declval<const CV &>().file_name()));
+static_assert(noexcept(std::declval<const CV &>().function_name()));
+static_assert(noexcept(std::declval<const CV &>().comment()));
+static_assert(noexcept(std::declval<const CV &>().assertion_level()));
+
+// Check return types.
+static_assert(std::is_same_v<decltype(std::declval<const CV &>().line_number()),
+                             std::uint_least32_t>);
+static_assert(std::is_same_v<decltype(std::declval<const CV &>().file_name()),
+                             std::string_view>);
+static_assert(std::is_same_v<decltype(std::declval<const CV &>().function_name()),
+                             std::string_view>);
+static_assert(std::is_same_v<decltype(std::declval<const CV &>().comment()),
+                             std::string_view>);
+static_assert(std::is_same_v<decltype(std::declval<const CV &>().assertion_level()),
+                             std::string_view>);
+
+int main()
+{
+}