Use "align" environment to wrap math equations instead of "gather"

Using align brings the benefit when multiple math equations are in one
math directive, alignment is provided. We also use "aligned" to wrap the
equations since in this way all the equations will be given only one
label/tag, while previously only the first equation is labeled.

sphinx/ext/mathbase.py

@@ -34,16 +34,19 @@ def wrap_displaymath(math, label, numbering):
     for i, part in enumerate(parts):
         if not part.strip():
             continue
-        if label is not None and i == 0:
-            ret.append('\\begin{split}%s\\end{split}' % part +
-                       (label and '\\label{'+label+'}' or ''))
-        else:
-            ret.append(r'\begin{split}%s\end{split}' % part)
-            if not numbering:
-                ret.append(r'\notag')
+        ret.append(r'\begin{split}%s\end{split}' % part)
     if not ret:
         return ''
-    return '\\begin{gather}\n' + '\\\\'.join(ret) + '\n\\end{gather}'
+    if label is not None or numbering:
+        env_begin = r'\begin{align}'
+        if label is not None:
+            env_begin += r'\label{%s}' % label
+        env_end = r'\end{align}'
+    else:
+        env_begin = r'\begin{align*}'
+        env_end = r'\end{align*}'
+    return ('%s\\begin{aligned}\n%s\\end{aligned}%s') % (
+        env_begin, '\\\\\n'.join(ret), env_end)
 
 
 def math_role(role, rawtext, text, lineno, inliner, options={}, content=[]):

tests/roots/test-ext-math/conf.py

@@ -1,3 +1,6 @@
 # -*- coding: utf-8 -*-
 
 master_doc = 'index'
+
+latex_documents = [
+    (master_doc, 'test.tex', 'Math Extension Testing', 'Sphinx', 'report')]

tests/roots/test-ext-math/index.rst

@@ -8,3 +8,11 @@ Inline :math:`E=mc^2`
 Second math
 
 .. math:: e^{i\pi}+1=0
+
+Multi math equations
+
+.. math::
+
+   S &= \pi r^2
+
+   V &= \frac{4}{3} \pi r^3

tests/test_ext_math.py

@@ -53,3 +53,10 @@ def test_math_number_all(app, status, warning):
     html = (r'<div class="math">\s*'
             r'<span class="eqno">\(1\)</span>\\\[a\^2\+b\^2=c\^2\\\]</div>')
     assert re.search(html, content, re.S)
+
+@with_app('latex', testroot='ext-math',
+          confoverrides={'extensions': ['sphinx.ext.mathjax']})
+def test_math_number_all(app, status, warning):
+    app.builder.build_all()
+
+    content = (app.outdir / 'test.tex').text()