In explaining the relationship between preprocessing tokens and tokens, 5.5 [lex.pptoken] paragraph 4 contains the following example:

[Example: The program fragment 1Ex is parsed as a preprocessing number token (one that is not a valid floating or integer literal token), even though a parse as the pair of preprocessing tokens 1 and Ex might produce a valid expression (for example, if Ex were a macro defined as +1).

This analysis does not take into account the addition of user-defined literals. In fact, 1Ex matches the rule for a user-defined-integer-literal, which is then ill-formed because it uses a reserved ud-suffix (5.13.9 [lex.ext] paragraph 10), as well as (presumably) because of a lookup failure for a matching literal operator, raw literal operator, or literal operator template.

More generally, it might be preferable to eliminate the restriction on the use of a reserved ud-suffix and rely simply on the fact that it is ill-formed to declare a literal operator, raw literal operator, or literal operator template with a reserved literal suffix identifier (16.4.5.3.6 [usrlit.suffix], cf 12.6 [over.literal] paragraph 1).