The question is clear.
I wonder why they even thought this would be handy, as clearly negative indices are unusable in the containers that would be used with them (see for example QList's docs).
I thought they wanted to allow that for some crazy form of indexing, but it seems unsupported?
It also generates a ton of (correct) compiler warnings about casting to and comparing of signed/unsigned types (on MSVC).
It just seems incompatible with the STL by design for some reason...
Because, realistically, you usually want to perform arithmetic on indices, which means that you might want to create temporaries that are negative. This is clearly painful when the underlying indexing type is unsigned.
The only appropriate time to use unsigned numbers is with modulus arithmetic. Using "unsgined" as some kind of contract specifier "a number in the range [0..." is just clumsy, and too coarse to be useful.
Consider: What type should I use to represent the idea that the number should be a positive integer between 1 and 10? Why is 0...2^x a more special range?