The current text of the 'note' diagnostic for bad conversions is confusing in the presence of user-defined conversion operations: https://godbolt.org/z/zrgeHH
For the first error, the conversion function is simply pointed at in a note without any further explanation. For the second error, the final note claims there is no conversion from X2 to Y2, even though it is plainly visible in the source code. The reason for the error is that only one implicit user-defined conversion may be applied in these circumstances, but the error message is misleading to those who may not know this rule.
This patch clarifies this situation with a better diagnostic message. It does so by finding user conversions even where they are not allowed, and then discarding them afterward as invalid, but keeping a record that they existed. There is also a stronger mode (UDC_Skip) for skipping the search for user-defined conversions outright, which matches the old behavior and is retained to prevent infinite recursions.