Compile error when using a member of a user-defined literal

Question

When compiling this code (without any header)

template 
struct Temperature {
    T temp;

    explicit Temperature(T t)
        : temp(t)
          


        

Answer 1

Preprocessing numbers are odd beasts, specified mostly to make the preprocessor easier to write.

pp-number:
    digit
    . digit
    pp-number digit
    pp-number identifier-nondigit
    pp-number ' digit
    pp-number ' nondigit
    pp-number e sign
    pp-number E sign
    pp-number p sign
    pp-number P sign
    pp-number .

12 is a valid pp-number token, so is 0xe+foo (see the example in [lex.pptoken]/4), and so is .12.CA'TS_RULE..56.me+owp-urr. If the latter two make it past translation phase 6, then the program is ill-formed because it cannot be converted to a valid token in phase 7. Until then, however, it is valid, so maximal munch says we parse 0xe+foo or 100.0_f.temp as a single preprocessing token.