https://bugs.gentoo.org/958161
https://sources.debian.org/patches/openmsx/20.0%2Bdfsg-1.2/c%2B%2B20-iterator.diff/

This is fixed properly upstream in >20.0 but this is a quick, minimal fix for now.
--- a/src/utils/view.hh
+++ b/src/utils/view.hh
@@ -206,6 +206,7 @@ public:
 	}
 
 	[[nodiscard]] constexpr friend difference_type operator-(const TransformIterator& x, const TransformIterator& y)
+	requires std::sized_sentinel_for<Iterator, Iterator>
 	{
 		return x.it - y.it;
 	}