diff --git a/stl/inc/condition_variable b/stl/inc/condition_variable
index e4304da8d1a..e5b917a87c0 100644
--- a/stl/inc/condition_variable
+++ b/stl/inc/condition_variable
@@ -224,16 +224,8 @@ public:
             // but unfortunately our ABI speaks struct xtime, which is relative to the system clock.
             _CSTD xtime _Tgt;
             (void) _To_xtime_10_day_clamped(_Tgt, _Rel_time);
-            const int _Res = _Cnd_timedwait(_Mycnd(), _Myptr->_Mymtx(), &_Tgt);
+            (void) _Cnd_timedwait(_Mycnd(), _Myptr->_Mymtx(), &_Tgt);
             _Guard_unlocks_before_locking_outer.unlock();
-
-            switch (_Res) {
-            case _Thrd_timedout:
-            case _Thrd_success:
-                break;
-            default:
-                _Throw_C_error(_Res);
-            }
         } // relock
 
         return _Pred();
@@ -262,13 +254,10 @@ private:
         const int _Res = _Cnd_timedwait(_Mycnd(), _Ptr->_Mymtx(), _Abs_time);
         _Guard.unlock();
 
-        switch (_Res) {
-        case _Thrd_success:
+        if (_Res == _Thrd_success) {
             return cv_status::no_timeout;
-        case _Thrd_timedout:
+        } else {
             return cv_status::timeout;
-        default:
-            _Throw_C_error(_Res);
         }
     }
 };
diff --git a/stl/inc/mutex b/stl/inc/mutex
index 46a75447559..393d9dff2c5 100644
--- a/stl/inc/mutex
+++ b/stl/inc/mutex
@@ -746,13 +746,11 @@ public:
 
         // Nothing to do to comply with LWG-2135 because std::mutex lock/unlock are nothrow
         const int _Res = _Cnd_timedwait(_Mycnd(), _Lck.mutex()->_Mymtx(), _Abs_time);
-        switch (_Res) {
-        case _Thrd_success:
+
+        if (_Res == _Thrd_success) {
             return cv_status::no_timeout;
-        case _Thrd_timedout:
+        } else {
             return cv_status::timeout;
-        default:
-            _Throw_C_error(_Res);
         }
     }
 
diff --git a/stl/inc/thread b/stl/inc/thread
index 70cd8fb6e6c..18f862c5ff3 100644
--- a/stl/inc/thread
+++ b/stl/inc/thread
@@ -137,7 +137,10 @@ public:
             _Throw_Cpp_error(_INVALID_ARGUMENT);
         }
 
-        _Check_C_return(_Thrd_detach(_Thr));
+        if (_Thrd_detach(_Thr) != _Thrd_success) {
+            _Throw_Cpp_error(_INVALID_ARGUMENT);
+        }
+
         _Thr = {};
     }
 
diff --git a/stl/inc/xthreads.h b/stl/inc/xthreads.h
index 81576850978..83c04c3fb50 100644
--- a/stl/inc/xthreads.h
+++ b/stl/inc/xthreads.h
@@ -129,16 +129,7 @@ enum { // constants for error codes
     _RESOURCE_UNAVAILABLE_TRY_AGAIN
 };
 
-extern "C++" [[noreturn]] _CRTIMP2_PURE void __cdecl _Throw_C_error(int _Code);
 extern "C++" [[noreturn]] _CRTIMP2_PURE void __cdecl _Throw_Cpp_error(int _Code);
-
-inline int _Check_C_return(int _Res) { // throw exception on failure
-    if (_Res != _Thrd_success) {
-        _Throw_C_error(_Res);
-    }
-
-    return _Res;
-}
 _STD_END
 #pragma pop_macro("new")
 _STL_RESTORE_CLANG_WARNINGS
diff --git a/stl/inc/xtimec.h b/stl/inc/xtimec.h
index 64741469db9..12419359719 100644
--- a/stl/inc/xtimec.h
+++ b/stl/inc/xtimec.h
@@ -27,7 +27,6 @@ struct xtime { // store time with nanosecond resolution
 
 _CRTIMP2_PURE int __cdecl xtime_get(xtime*, int);
 
-_CRTIMP2_PURE long __cdecl _Xtime_diff_to_millis(const xtime*);
 _CRTIMP2_PURE long __cdecl _Xtime_diff_to_millis2(const xtime*, const xtime*);
 _CRTIMP2_PURE long long __cdecl _Xtime_get_ticks();
 
diff --git a/stl/src/thread0.cpp b/stl/src/thread0.cpp
index bffe7faf63b..d3c8bbecf05 100644
--- a/stl/src/thread0.cpp
+++ b/stl/src/thread0.cpp
@@ -35,6 +35,7 @@ static constexpr errc codes[] = {
     _THROW(system_error(static_cast<int>(codes[code]), _STD generic_category(), msgs[code]));
 }
 
+// TRANSITION, ABI: preserved for binary compatibility
 [[noreturn]] _CRTIMP2_PURE void __cdecl _Throw_C_error(int code) { // throw error object for C error
     switch (code) { // select the exception
     case _Thrd_nomem:
diff --git a/stl/src/xtime.cpp b/stl/src/xtime.cpp
index 1f7109b76f1..cc1c5c5c111 100644
--- a/stl/src/xtime.cpp
+++ b/stl/src/xtime.cpp
@@ -65,6 +65,7 @@ _CRTIMP2_PURE long __cdecl _Xtime_diff_to_millis2(const xtime* xt1, const xtime*
     return static_cast<long>(diff.sec * _Msec_per_sec + (diff.nsec + _Nsec_per_msec - 1) / _Nsec_per_msec);
 }
 
+// TRANSITION, ABI: preserved for binary compatibility
 _CRTIMP2_PURE long __cdecl _Xtime_diff_to_millis(const xtime* xt) { // convert time to milliseconds
     xtime now;
     xtime_get(&now, TIME_UTC);