Why does ‘auto’ not respect the unary minus operator?

The actual issue here is that use of unary minus operator, just like the rest of built-in arithmetic operators, is a subject to integral promotions. So surprisingly the result of applying unary minus to size_t will be still size_t and there is no need to blame auto.

Counter-example. In this case due to integral promotions type of x will be int so output will be -1:

unsigned short a{1};
auto x{-a};
cout << x << endl;