[ub,intro.progress] Added entry for UB due to lack of forward progress

source/basic.tex

@@ -6912,8 +6912,8 @@
 \rSec3[intro.progress]{Forward progress}
 
 \pnum
-The implementation may assume that any thread will eventually do one of the
-following:
+The implementation may assume\ubdef{intro.progress.stops} that any thread
+will eventually do one of the following:
 \begin{itemize}
 \item terminate,
 \item invoke the function \tcode{std::this_thread::yield}\iref{thread.thread.this},

source/ub.tex

 undefined behavior. Undefined behavior that is implicit is not covered by this annex. Each entry contains 
 This Annex documents undefined behavior explicitly called out in the main standard text using the 
 following phrases: the behavior of the program is undefined, has undefined behavior, results in 
 undefined behavior, the behavior is undefined, have undefined behavior, is undefined, result has 
 undefined behavior. Undefined behavior that is implicit is not covered by this annex. Each entry contains 
@@ -502,6 +502,29 @@
 \end{codeblock}
 \end{example}
 
+\rsec2[ub.intro.progress]{Forward progress}
+
+\pnum
+\ubxref{intro.progress.stops} \\
+
+The behavior is undefined if a thread of execution that has not terminated stops
+making execution steps.
+
+\pnum
+\begin{example}
+\begin{codeblock}
+bool stop() { return false; }
+
+void busy_wait_thread() {
+  while (!stop()); // undefined behavior, thread makes no progress but the loop
+}                  // is not trivial because `stop()` is not a constant expression
+
+int main() {
+  std::thread t(busy_wait_thread);
+  t.join();
+}
+\end{codeblock}
+\end{example}
 
 \rSec2[ub.basic.start.main]{main function}