When is narrowing conversion not implementation defined?

Question

C++ / C#

Hello everyone

Do I understand the rules of narrowing conversion correctly:

1. Wide type -> Narrow type. The resulting type fits the resulting value. For example, int32_t = uint64_t, value on the right <= maxValue of the value on the left. OK

2. Wide type -> Narrow type. Final type Does not fit the resulting value. For example, int32_t = uint64_t, value on the right > maxValue of the value on the left. Implementation defined