Is every “normal” use of user-defined literals undefined behavior?

Question

User defined literals must start with an underscore.

This is a more or less universally well-known rule that you can find on every layman-worded site talkin

Answer 1

Is every “normal” use of user-defined literals undefined behavior?

Clearly not.

The following is the idiomatic (and thus definitely “normal”) use of UDLs, and it’s well-defined according to the rule you’ve just listed:

namespace si {
    struct metre { … };

    constexpr metre operator ""_m(long double value) { return metre{value}; }
}

You’ve listed problematic cases and I agree with your assessment about their validity but they’re easily avoided in idiomatic C++ code so I don’t entirely see the problem with the current wording, even if it was potentially accidental.

According to the example in [over.literal]/8, we can even use capital letters after the underscore:

float operator ""E(const char*);    // error: reserved literal suffix (20.5.4.3.5, 5.13.8)
double operator""_Bq(long double);  // OK: does not use the reserved identifier _Bq (5.10)
double operator"" _Bq(long double); // uses the reserved identifier _Bq (5.10)

The only problematic thing thus seems to be the fact that the standard makes the whitespace between "" and the UDL name significant.