[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

@@ -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}