Do not put option part after = to Index.

Fixes sphinx-doc#9981.

sphinx/domains/std.py

@@ -244,6 +244,8 @@ def add_target_and_index(self, firstname: str, sig: str, signode: desc_signature
         else:
             descr = _('command line option')
         for option in sig.split(', '):
+            # remove trailing part followed by equal symbol
+            option = option.split('=')[0]
             entry = '; '.join([descr, option])
             self.indexnode['entries'].append(('pair', entry, signode['ids'][0], '', None))
 

tests/test_domain_std.py

@@ -341,6 +341,18 @@ def test_cmdoption_for_None(app):
     assert domain.progoptions[(None, '-l')] == ('index', 'cmdoption-l')
 
 
+def test_option_with_equal_sign(app):
+    text = (".. option:: -foo=bar|baz\n"
+            "\n"
+            ".. option:: -l=TYPE\n")
+    domain = app.env.get_domain('std')
+    doctree = restructuredtext.parse(app, text)
+    assert_node(doctree[0], addnodes.index,
+                entries=[('pair', 'command line option; -foo', 'cmdoption-foo', '', None)])
+    assert_node(doctree[2], addnodes.index,
+                entries=[('pair', 'command line option; -l', 'cmdoption-l', '', None)])
+
+
 def test_multiple_cmdoptions(app):
     text = (".. program:: cmd\n"
             "\n"