[[nodiscard]] for constructors

stl/inc/mutex

@@ -125,7 +125,7 @@ _INLINE_VAR constexpr try_to_lock_t try_to_lock{};
 
 // CLASS TEMPLATE unique_lock
 template <class _Mutex>
-class unique_lock { // whizzy class with destructor that unlocks mutex
+class _NODISCARD_CTOR unique_lock { // whizzy class with destructor that unlocks mutex
 public:
     using mutex_type = _Mutex;
 
@@ -421,7 +421,7 @@ void lock(_Lock0& _Lk0, _Lock1& _Lk1, _LockN&... _LkN) { // lock multiple locks,
 
 // CLASS TEMPLATE lock_guard
 template <class _Mutex>
-class lock_guard { // class with destructor that unlocks a mutex
+class _NODISCARD_CTOR lock_guard { // class with destructor that unlocks a mutex
 public:
     using mutex_type = _Mutex;
 
@@ -445,7 +445,7 @@ private:
 #if _HAS_CXX17
 // CLASS TEMPLATE scoped_lock
 template <class... _Mutexes>
-class scoped_lock { // class with destructor that unlocks mutexes
+class _NODISCARD_CTOR scoped_lock { // class with destructor that unlocks mutexes
 public:
     explicit scoped_lock(_Mutexes&... _Mtxes) : _MyMutexes(_Mtxes...) { // construct and lock
         _STD lock(_Mtxes...);
@@ -465,7 +465,7 @@ private:
 };
 
 template <class _Mutex>
-class scoped_lock<_Mutex> {
+class _NODISCARD_CTOR scoped_lock<_Mutex> {
 public:
     using mutex_type = _Mutex;
 

stl/inc/shared_mutex

@@ -227,7 +227,7 @@ private:
 
 // CLASS TEMPLATE shared_lock
 template <class _Mutex>
-class shared_lock { // shareable lock
+class _NODISCARD_CTOR shared_lock { // shareable lock
 public:
     using mutex_type = _Mutex;
 

stl/inc/thread

@@ -85,7 +85,7 @@ private:
 
 public:
     template <class _Fn, class... _Args, enable_if_t<!is_same_v<_Remove_cvref_t<_Fn>, thread>, int> = 0>
-    explicit thread(_Fn&& _Fx, _Args&&... _Ax) {
+    _NODISCARD_CTOR explicit thread(_Fn&& _Fx, _Args&&... _Ax) {
         _Start(_STD forward<_Fn>(_Fx), _STD forward<_Args>(_Ax)...);
     }
 
@@ -282,7 +282,7 @@ public:
     jthread() noexcept : _Impl{}, _Ssource{nostopstate} {}
 
     template <class _Fn, class... _Args, enable_if_t<!is_same_v<remove_cvref_t<_Fn>, jthread>, int> = 0>
-    explicit jthread(_Fn&& _Fx, _Args&&... _Ax) {
+    _NODISCARD_CTOR explicit jthread(_Fn&& _Fx, _Args&&... _Ax) {
         if constexpr (is_invocable_v<decay_t<_Fn>, stop_token, decay_t<_Args>...>) {
             _Impl._Start(_STD forward<_Fn>(_Fx), _Ssource.get_token(), _STD forward<_Args>(_Ax)...);
         } else {

stl/inc/yvals_core.h

@@ -351,6 +351,14 @@
 // _HAS_NODISCARD (in vcruntime.h) controls:
 // [[nodiscard]] attributes on STL functions
 
+#ifndef __has_cpp_attribute
+#define _NODISCARD_CTOR
+#elif __has_cpp_attribute(nodiscard) >= 201907L
+#define _NODISCARD_CTOR _NODISCARD
+#else
+#define _NODISCARD_CTOR
+#endif
+
 // Determine if we should use [[msvc::known_semantics]] to communicate to the compiler
 // that certain type trait specializations have the standard-mandated semantics
 #ifndef __has_cpp_attribute