From 82b9af71dd5aa3ad6fee808e91e9b1024ebaeafb Mon Sep 17 00:00:00 2001
From: Karen Arutyunov <karen@codesynthesis.com>
Date: Tue, 19 May 2020 21:19:58 +0300
Subject: Improve std::optional to better deal with lack of copy/move
 constructors

---
 libbutl/optional.mxx      | 83 +++++++++++++++++++++++++++++++++++++++++++----
 tests/optional/buildfile  |  6 ++++
 tests/optional/driver.cxx | 43 ++++++++++++++++++++++++
 3 files changed, 126 insertions(+), 6 deletions(-)
 create mode 100644 tests/optional/buildfile
 create mode 100644 tests/optional/driver.cxx

diff --git a/libbutl/optional.mxx b/libbutl/optional.mxx
index fe533c5..d32e14b 100644
--- a/libbutl/optional.mxx
+++ b/libbutl/optional.mxx
@@ -60,7 +60,7 @@
 #else
 #  include <utility>     // move()
 #  include <functional>  // hash
-#  include <type_traits> // is_trivially_destructible
+#  include <type_traits> // is_*
 #endif
 #endif
 
@@ -91,9 +91,6 @@ LIBBUTL_MODEXPORT namespace butl
   // Simple optional class template while waiting for std::optional.
   //
   struct nullopt_t {constexpr explicit nullopt_t (int) {}};
-#if defined(__cpp_modules_ts) && defined(__clang__) //@@ MOD Clang duplicate sym.
-  inline
-#endif
   constexpr nullopt_t nullopt (1);
 
   namespace details
@@ -184,12 +181,86 @@ LIBBUTL_MODEXPORT namespace butl
       optional_data& operator= (const optional_data&);
       optional_data& operator= (optional_data&&);
     };
+
+    template <typename T,
+              bool = std::is_copy_constructible<T>::value,
+              bool = std::is_move_constructible<T>::value>
+    struct optional_ctors: optional_data<T>
+    {
+      using optional_data<T>::optional_data;
+    };
+
+    template <typename T>
+    struct optional_ctors<T, false, true>: optional_ctors<T, true, true>
+    {
+      using optional_ctors<T, true, true>::optional_ctors;
+
+#if !defined(_MSC_VER) || _MSC_VER > 1900
+      constexpr optional_ctors () = default;
+#else
+      optional_ctors () = default;
+#endif
+
+      optional_ctors (const optional_ctors&) = delete;
+
+#if (!defined(_MSC_VER) || _MSC_VER > 1900) &&                  \
+    (!defined(__GNUC__) || __GNUC__ > 4 || defined(__clang__))
+      constexpr optional_ctors (optional_ctors&&) = default;
+#else
+      optional_ctors (optional_ctors&&) = default;
+#endif
+
+      optional_ctors& operator= (const optional_ctors&) = default;
+      optional_ctors& operator= (optional_ctors&&) = default;
+    };
+
+    template <typename T>
+    struct optional_ctors<T, true, false>: optional_ctors<T, true, true>
+    {
+      using optional_ctors<T, true, true>::optional_ctors;
+
+#if !defined(_MSC_VER) || _MSC_VER > 1900
+      constexpr optional_ctors () = default;
+#else
+      optional_ctors () = default;
+#endif
+
+#if (!defined(_MSC_VER) || _MSC_VER > 1900) &&                  \
+    (!defined(__GNUC__) || __GNUC__ > 4 || defined(__clang__))
+      constexpr optional_ctors (const optional_ctors&) = default;
+#else
+      optional_ctors (const optional_ctors&) = default;
+#endif
+
+      optional_ctors (optional_ctors&&) = delete;
+
+      optional_ctors& operator= (const optional_ctors&) = default;
+      optional_ctors& operator= (optional_ctors&&) = default;
+    };
+
+    template <typename T>
+    struct optional_ctors<T, false, false>: optional_ctors<T, true, true>
+    {
+      using optional_ctors<T, true, true>::optional_ctors;
+
+#if !defined(_MSC_VER) || _MSC_VER > 1900
+      constexpr optional_ctors () = default;
+#else
+      optional_ctors () = default;
+#endif
+
+      optional_ctors (const optional_ctors&) = delete;
+      optional_ctors (optional_ctors&&) = delete;
+
+      optional_ctors& operator= (const optional_ctors&) = default;
+      optional_ctors& operator= (optional_ctors&&) = default;
+    };
   }
 
   template <typename T>
-  class optional: private details::optional_data<T>
+  class optional: private details::optional_ctors<T>
   {
-    using base = details::optional_data<T>;
+    using base = details::optional_ctors<T>;
 
   public:
     using value_type = T;
diff --git a/tests/optional/buildfile b/tests/optional/buildfile
new file mode 100644
index 0000000..68188d7
--- /dev/null
+++ b/tests/optional/buildfile
@@ -0,0 +1,6 @@
+# file      : tests/optional/buildfile
+# license   : MIT; see accompanying LICENSE file
+
+import libs = libbutl%lib{butl}
+
+exe{driver}: {hxx cxx}{*} $libs
diff --git a/tests/optional/driver.cxx b/tests/optional/driver.cxx
new file mode 100644
index 0000000..5d72f08
--- /dev/null
+++ b/tests/optional/driver.cxx
@@ -0,0 +1,43 @@
+// file      : tests/optional/driver.cxx -*- C++ -*-
+// license   : MIT; see accompanying LICENSE file
+
+#include <cassert>
+
+#ifndef __cpp_lib_modules_ts
+#include <vector>
+#include <utility>    // move()
+#endif
+
+// Other includes.
+
+#ifdef __cpp_modules_ts
+#ifdef __cpp_lib_modules_ts
+import std.core;
+#endif
+import butl.optional;
+#else
+#include <libbutl/optional.mxx>
+#endif
+
+using namespace std;
+
+struct move_only
+{
+  move_only () = default;
+
+  move_only (move_only&&) = default;
+  move_only& operator= (move_only&&) = default;
+
+  move_only (const move_only&) = delete;
+  move_only& operator= (const move_only&) = delete;
+};
+
+int
+main ()
+{
+  using butl::optional;
+
+  optional<move_only> r;
+  vector<optional<move_only>> rs;
+  rs.emplace_back (move (r));
+}
-- 
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