Proposed resolution:

This wording is relative to N4964.

1. Modify [span.elem] as indicated:

   constexpr reference operator[](size_type idx) const;
   

   -1- Preconditions: idx < size() is true.

   -2- EffectsReturns: Equivalent to: return *(data() + idx);.

   -?- Throws: Nothing.

   constexpr reference front() const;
   

   -3- Preconditions: empty() is false.

   -4- EffectsReturns: Equivalent to: return *data();.

   -?- Throws: Nothing.

   constexpr reference back() const;
   

   -5- Preconditions: empty() is false.

   -6- EffectsReturns: Equivalent to: return *(data() + (size() - 1));.

   -?- Throws: Nothing.

   constexpr pointer data() const noexcept;
   

   -7- EffectsReturns: Equivalent to: return data_;.

In reviewing the wording for P2821 span.at(), it had been noticed that [span.elem] uses a lot of "Effects: Equivalent to return […];" which could be simply "Returns: […]".

For comparison, [string.view.access] uses "Returns: ..." instead, so I suggest that [span.elem] should be consistent with that.