For example, the assignment operators for std::slice_array:
void operator=(const valarray<T>&) const; //#1
void operator=(const T&) const; //#2
const slice_array& operator=(const slice_array&) const; //#3
#1 and #2 return void, but #3 returns const slice_array&.
It prohibits some code, such as:
std::valarray<int> va{1, 2, 3, 4, 5, 6};
va[std::slice(3, 2, 2)] = va[std::slice(0, 2, 2)] = va[0];
Why?
While returning a reference from operator= is the common and reasonable way for implementations, keep in mind that valarray is a rarely used and an unsupported library. See C++ valarray vs. vector for more detailed answers on this topic.
From one of the answers:
ISTR that the main reason it wasn't removed from the standard is that nobody took the time to evaluate the issue thoroughly and write a proposal to remove it.
Even before going into debates about opinions, there are bugs in the current implementation. See assigning to gslice_array gives runtime error for instance.