Both ARM 13.9.1 and the GNAT User Guide (in Section 3.2.4 Validity
Checking) require that such reads are NOT erroneous.It depends what "such reads" mean. 13.9.1(12) clearly says that the result of an Unchecked_Conversion is erroneous if it isn't a valid representation. There are some cases, however, where an out-of-range value is a bounded error instead of being erroneous. However, note 20 (13.9.2) says that 'Valid is not considered a "read" and hence its use is not erroneous.
