c++ - STL size_type(-1) vs size_t(-1) -

this question has answer here:

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in c++ stl implementation, see code block:

size_type max_size() const {     return size_type(-1); } 

i understand works when size_type size_t. purpose of adding indirect level here? i.e. why not use size_t(-1) directly in max_size()?

is there specification says (size_type(-1)) should evaluate maximum object size on platform)...