What is the following list of behind the scenes inside the range-based for loop?

What is the temporary {1,2,3,4,5} in the for loop behind the scenes?

a std::initializer_list<int>

The object here is an instance of std::initializer_list<int>. From the reference (emphasis mine):

A std::initializer_list object is automatically constructed when:

a braced-init-list is used to list-initialize an object, where the corresponding constructor accepts an std::initializer_list parameter

a braced-init-list is used as the right operand of assignment or as a function call argument, and the corresponding assignment operator/function accepts an std::initializer_list parameter

a braced-init-list is bound to auto, including in a ranged for loop