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 is determined by searching for these illegal conversions in CompleteNonViableCandidate after overload resolution fails.