[libc++] [P0935] [C++20] Eradicating unnecessarily explicit default constructors from the standard library.

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0935r0.html

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D91292

Author: mkurdej

libcxx/docs/Cxx2aStatusPaperStatus.csv

@@ -44,7 +44,7 @@
 "`P0887R1 <https://wg21.link/P0887R1>`__","LWG","The identity metafunction","Rapperswil","|Complete|","8.0"
 "`P0892R2 <https://wg21.link/P0892R2>`__","CWG","explicit(bool)","Rapperswil","",""
 "`P0898R3 <https://wg21.link/P0898R3>`__","LWG","Standard Library Concepts","Rapperswil","",""
-"`P0935R0 <https://wg21.link/P0935R0>`__","LWG","Eradicating unnecessarily explicit default constructors from the standard library","Rapperswil","",""
+"`P0935R0 <https://wg21.link/P0935R0>`__","LWG","Eradicating unnecessarily explicit default constructors from the standard library","Rapperswil","|Complete|","12.0"
 "`P0941R2 <https://wg21.link/P0941R2>`__","CWG","Integrating feature-test macros into the C++ WD","Rapperswil","|In Progress|",""
 "`P1023R0 <https://wg21.link/P1023R0>`__","LWG","constexpr comparison operators for std::array","Rapperswil","|Complete|","8.0"
 "`P1025R1 <https://wg21.link/P1025R1>`__","CWG","Update The Reference To The Unicode Standard","Rapperswil","",""

libcxx/include/algorithm

@@ -3029,9 +3029,17 @@ private:
 
 public:
     // constructors and reset functions
-    explicit uniform_int_distribution(result_type __a = 0,
-                                      result_type __b = numeric_limits<result_type>::max())
+#ifndef _LIBCPP_CXX03_LANG
+    uniform_int_distribution() : uniform_int_distribution(0) {}
+    explicit uniform_int_distribution(
+        result_type __a, result_type __b = numeric_limits<result_type>::max())
+        : __p_(param_type(__a, __b)) {}
+#else
+    explicit uniform_int_distribution(
+        result_type __a = 0,
+        result_type __b = numeric_limits<result_type>::max())
         : __p_(param_type(__a, __b)) {}
+#endif
     explicit uniform_int_distribution(const param_type& __p) : __p_(__p) {}
     void reset() {}
 

libcxx/include/locale

@@ -92,7 +92,11 @@ public:
     typedef typename Codecvt::state_type                      state_type;
     typedef typename wide_string::traits_type::int_type       int_type;
 
-    explicit wstring_convert(Codecvt* pcvt = new Codecvt);          // explicit in C++14
+    wstring_convert(Codecvt* pcvt = new Codecvt);          // before C++14
+    explicit wstring_convert(Codecvt* pcvt = new Codecvt); // before C++20
+    wstring_convert() : wstring_convert(new Codecvt) {}    // C++20
+    explicit wstring_convert(Codecvt* pcvt);               // C++20
+
     wstring_convert(Codecvt* pcvt, state_type state);
     explicit wstring_convert(const byte_string& byte_err,           // explicit in C++14
                     const wide_string& wide_err = wide_string());
@@ -121,8 +125,14 @@ class wbuffer_convert
 public:
     typedef typename Tr::state_type state_type;
 
-    explicit wbuffer_convert(streambuf* bytebuf = 0, Codecvt* pcvt = new Codecvt,
-                    state_type state = state_type());       // explicit in C++14
+    wbuffer_convert(streambuf* bytebuf = 0, Codecvt* pcvt = new Codecvt,
+                    state_type state = state_type());          // before C++14
+    explicit wbuffer_convert(streambuf* bytebuf = nullptr, Codecvt* pcvt = new Codecvt,
+                            state_type state = state_type()); // before C++20
+    wbuffer_convert() : wbuffer_convert(nullptr) {} // C++20
+    explicit wbuffer_convert(streambuf* bytebuf, Codecvt* pcvt = new Codecvt,
+                            state_type state = state_type()); // C++20
+
     wbuffer_convert(const wbuffer_convert&) = delete;               // C++14
     wbuffer_convert & operator=(const wbuffer_convert &) = delete;  // C++14
     ~wbuffer_convert();                                             // C++14
@@ -3650,8 +3660,17 @@ private:
     wstring_convert(const wstring_convert& __wc);
     wstring_convert& operator=(const wstring_convert& __wc);
 public:
+#ifndef _LIBCPP_CXX03_LANG
     _LIBCPP_INLINE_VISIBILITY
-    _LIBCPP_EXPLICIT_AFTER_CXX11 wstring_convert(_Codecvt* __pcvt = new _Codecvt);
+    wstring_convert() : wstring_convert(new _Codecvt) {}
+    _LIBCPP_INLINE_VISIBILITY
+    explicit wstring_convert(_Codecvt* __pcvt);
+#else
+    _LIBCPP_INLINE_VISIBILITY
+    _LIBCPP_EXPLICIT_AFTER_CXX11
+    wstring_convert(_Codecvt* __pcvt = new _Codecvt);
+#endif
+
     _LIBCPP_INLINE_VISIBILITY
     wstring_convert(_Codecvt* __pcvt, state_type __state);
     _LIBCPP_EXPLICIT_AFTER_CXX11 wstring_convert(const byte_string& __byte_err,
@@ -3918,9 +3937,20 @@ private:
 
     wbuffer_convert(const wbuffer_convert&);
     wbuffer_convert& operator=(const wbuffer_convert&);
+
 public:
-    _LIBCPP_EXPLICIT_AFTER_CXX11 wbuffer_convert(streambuf* __bytebuf = nullptr,
-            _Codecvt* __pcvt = new _Codecvt, state_type __state = state_type());
+#ifndef _LIBCPP_CXX03_LANG
+    wbuffer_convert() : wbuffer_convert(nullptr) {}
+    explicit wbuffer_convert(streambuf* __bytebuf,
+                             _Codecvt* __pcvt = new _Codecvt,
+                             state_type __state = state_type());
+#else
+    _LIBCPP_EXPLICIT_AFTER_CXX11
+    wbuffer_convert(streambuf* __bytebuf = nullptr,
+                    _Codecvt* __pcvt = new _Codecvt,
+                    state_type __state = state_type());
+#endif
+
     ~wbuffer_convert();
 
     _LIBCPP_INLINE_VISIBILITY

libcxx/include/queue

@@ -112,18 +112,11 @@ protected:
     Compare comp;
 
 public:
-    priority_queue() = default;
-    ~priority_queue() = default;
-
-    priority_queue(const priority_queue& q) = default;
-    priority_queue(priority_queue&& q) = default;
-
-    priority_queue& operator=(const priority_queue& q) = default;
-    priority_queue& operator=(priority_queue&& q) = default;
-
-    explicit priority_queue(const Compare& comp);
-    priority_queue(const Compare& comp, const container_type& c);
-    explicit priority_queue(const Compare& comp, container_type&& c);
+    priority_queue() : priority_queue(Compare()) {} // C++20
+    explicit priority_queue(const Compare& x) : priority_queue(x, Container()) {}
+    priority_queue(const Compare& x, const Container&);
+    explicit priority_queue(const Compare& x = Compare(), Container&&= Container()); // before C++20
+    priority_queue(const Compare& x, Container&&); // C++20
     template <class InputIterator>
         priority_queue(InputIterator first, InputIterator last,
                        const Compare& comp = Compare());
@@ -474,7 +467,7 @@ public:
     priority_queue(const value_compare& __comp, const container_type& __c);
 #ifndef _LIBCPP_CXX03_LANG
     _LIBCPP_INLINE_VISIBILITY
-    explicit priority_queue(const value_compare& __comp, container_type&& __c);
+    priority_queue(const value_compare& __comp, container_type&& __c);
 #endif
     template <class _InputIter>
         _LIBCPP_INLINE_VISIBILITY