Why does this compile?
std::uniform_int_distribution<> dis(0, array.size() - 1);
but this fails class template argument deduction?
std::uniform_int_distribution dis(0, array.size() - 1);
What semantic difference am I making by omitting the angle brackets?
template< class IntType = int >
class uniform_int_distribution;
The class only has one template parameter and it is int by default.
In std::uniform_int_distribution<> dis(0, array.size() - 1); you are not using class template argument deduction but tell it to use the default so IntType is int.
In std::uniform_int_distribution dis(0, array.size() - 1); you actually do rely on class template argument deduction - but since it fails, it means that 0 and array.size() are of different types. If array is any of any of the container types in the standard library, its type is std::size_t but 0 is int. IntType can therefore not be deduced.
The most probable solution to get the deduction working:
std::uniform_int_distribution dis(std::size_t{0}, array.size() - 1);
A foolproof solution would check what type array.size() - 1 has and initialize the first parameter with that type:
std::uniform_int_distribution dis(decltype(array.size() - 1){}, array.size() - 1);
