Why did the range based 'for' loop specification change in C++17?

Using

auto __begin = begin_expr, __end = end_expr;

requires both begin_expr and end_expr to return the same type. This means you cannot have a sentinel iterator type that is different from the beginning type. Using

auto __begin = begin_expr ;
auto __end = end_expr ;

fixes that issue while proving full backwards compatibility with C++14.

It is explained later in the "notes":

As of C++17, the types of the begin_expr and the end_expr do not have to be the same ...

and you can't have that with:

auto __begin = begin_expr, __end = end_expr;