Why is `""s.size()` legal, but `5ms.count()` is not?

#include <chrono>
#include <string>

using namespace std::literals;

int main() {
    ""s.size();     // ok
    (""s).size();   // ok
    (5ms).count();  // ok
    5ms.count();    // error
}

The error message is:

error: unable to find numeric literal operator 'operator""ms.count'

See https://godbolt.org/z/qT6G1nbzM

Why is ""s.size() legal, but 5ms.count() is not? What is the rationale behind the rules?

Per User defined literals on cppreference:

Due to maximal munch, user-defined integer and floating point literals ending in p, P (since C++17), e and E, when followed by the operators + or -, must be separated from the operator with whitespace or parentheses in the source

long double operator""_E(long double);
long double operator""_a(long double);
int operator""_p(unsigned long long);
 
auto x = 1.0_E+2.0;   // error
auto y = 1.0_a+2.0;   // OK
auto z = 1.0_E +2.0;  // OK
auto q = (1.0_E)+2.0; // OK
auto w = 1_p+2;       // error
auto u = 1_p +2;      // OK

Same applies to dot operator following an integer or floating-point user-defined literal:

#include <chrono>

using namespace std::literals;
 
auto a = 4s.count();   // Error
auto b = 4s .count();  // OK
auto c = (4s).count(); // OK

Since 5ms is an integer user-defined literal, you then need (5ms).count() or 5ms .count().