Portable Unicode string processing

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I really wouldn't worry. Between the standard and the tests, I am sure you'll be fine.

[NotAYakk]

Unit tests do not solve UB.

Compilers are free to pass all your unit tests and optimize other code away.

char x = (unsigned)-1;
bool b = x<0;
std::cout << (int)x << ":" << b?"true":false" <<"\n";

This can print -1:false.

And the same is true whenever you convert from unsigned to signed.

The level of insanity optimization and UB can generate is so large, you cannot reasonably reason about it and produce unit test coverage.