See https://sourceforge.net/p/tclap/bugs/33/

The temporary fix is to patch MultiArg.h and ValueArg.h:

--- MultiArg.h.old	2021-05-28 10:03:24.259444977 +0200
+++ MultiArg.h	2021-05-28 10:03:36.645101249 +0200
@@ -225,8 +225,8 @@
 	/**
 	 * Prevent accidental copying
 	 */
-	MultiArg<T>(const MultiArg<T>& rhs);
-	MultiArg<T>& operator=(const MultiArg<T>& rhs);
+	MultiArg(const MultiArg<T>& rhs);
+	MultiArg& operator=(const MultiArg<T>& rhs);

 
--- ValueArg.h.old	2021-05-28 10:03:27.202544809 +0200
+++ ValueArg.h	2021-05-28 10:03:43.847815033 +0200
@@ -240,8 +240,8 @@
        /**
         * Prevent accidental copying
         */
-       ValueArg<T>(const ValueArg<T>& rhs);
-       ValueArg<T>& operator=(const ValueArg<T>& rhs);
+       ValueArg(const ValueArg<T>& rhs);
+       ValueArg& operator=(const ValueArg<T>& rhs);
 };