[mjevans]

The difference is that Go expects your majority use case to be copy or concatenate. If you're taking a string sequence value you're normally not going to change it, or you're going to combine it together with something else. If you have valid UTF-8 input, you should get output that is valid, but might not be 'normalized' to a single form. IF you care about normalizing you can decide when to do that (usually in output construction).

If you need to make a decision based on the content of a string, then you often need to make a normalized (the same way for both) copy the inputs.

Most importantly, if you feed in garbage, you get out the SAME garbage. The real world, and historical data, are messy. Trying to be smart can often lead to the most disastrous consequences. Being conservative and tolerant allows for intentional planning to handle the conversion at the source, if and when desired.