label system of math directive is confusable

Problem

In reST world, usually, we have two ways to give a label to an arbitrary node which is generated by directive. The first way is hyperlink target. And another is :name: option.
For example, we can give a label to figures like following:

.. _figure1:
.. figure:: sphinx-logo.png

.. figure:: sphinx-logo.png
   :name: figure2

Both are working fine. Off course, we can refer it using :ref: role.

But math directive is not. It also supports :name: option (it is an alias of :label: option). But :ref: role can't refer it.

In addition, the behavior of equation number is not same between :name: option and hyperlink targets. With the option, the math_block has equation number automatically. On the other hand, putting hyperlink targets before math directive, the math_block does not have equation number.

Procedure to reproduce the problem

.. math:: E = mc^2
   :name: equation1

.. _equation2:
.. math:: E = mc^2

Error logs / results

With :name: option, it is rendered as following:

\begin{equation}\label{equation:index:equation1}
\begin{split}E = mc^2\end{split}
\end{equation}

With a hyperlink target, it is rendered:

\phantomsection\label{\detokenize{index:equation2}}\begin{equation*}
\begin{split}E = mc^2\end{split}
\end{equation*}

Expected results

Both case are converted to same result.

Environment info

• OS: Mac
• Python version: 3.6.5
• Sphinx version: 1.7.4

Reproduce the two math directive cases and compare their :ref: behavior and rendered LaTeX output. Start from the math directive and its :name: or 🏷️ handling, then trace how hyperlink targets are processed. Done means both forms produce equivalent references and equation-number behavior.