Allow case-insensitive match of label-refs through intersphinx

Look up `:std:label:` object types case-insensitively in an intersphinx
inventory. This is needed because `ref` and `numref` references (which
both resolve to `:std:label:`) are case insensitive. They lowercase
their target under the assumption that the target name in any inventory
file is also lowercased.

That assumption holds for labels in inventory files generated by Sphinx
from `.rst` files, since it generates lowercase labels. However, other
documentation generators (such as the Julia Documenter.jl, which
also writes `objects.inv` inventory files as of version 1.3) use
mixed-case labels for section title.

With the change in this commit, if the lowercase label cannot be found
in the inventory, also try a case-insensitive match.

See sphinx-doc#12008

sphinx/ext/intersphinx.py

@@ -334,8 +334,10 @@ def _resolve_reference_in_domain_by_target(
         if target in inventory[objtype]:
             # Case sensitive match, use it
             data = inventory[objtype][target]
-        elif objtype == 'std:term':
-            # Check for potential case insensitive matches for terms only
+        elif objtype in ['std:label', 'std:term']:
+            # Some types require case insensitive matches:
+            # * 'term': https://github.com/sphinx-doc/sphinx/issues/9291
+            # * 'label': https://github.com/sphinx-doc/sphinx/issues/12008
             target_lower = target.lower()
             insensitive_matches = list(filter(lambda k: k.lower() == target_lower,
                                               inventory[objtype].keys()))

tests/test_extensions/test_ext_intersphinx.py

@@ -236,6 +236,16 @@ def test_missing_reference_stddomain(tmp_path, app, status, warning):
     rn = missing_reference(app, app.env, node, contnode)
     assert rn.astext() == 'A TERM'
 
+    # label reference (normal)
+    node, contnode = fake_node('std', 'ref', 'The-Julia-Domain', 'The-Julia-Domain')
+    rn = missing_reference(app, app.env, node, contnode)
+    assert rn.astext() == 'The Julia Domain'
+
+    # label reference (case insensitive)
+    node, contnode = fake_node('std', 'ref', 'the-julia-domain', 'the-julia-domain')
+    rn = missing_reference(app, app.env, node, contnode)
+    assert rn.astext() == 'The Julia Domain'
+
 
 @pytest.mark.sphinx('html', testroot='ext-intersphinx-cppdomain')
 def test_missing_reference_cppdomain(tmp_path, app, status, warning):

tests/test_util/test_util_inventory.py

@@ -41,6 +41,7 @@
 foo.bar.baz js:method 1 index.html#foo.bar.baz -
 foo.bar.qux js:data 1 index.html#foo.bar.qux -
 a term including:colon std:term -1 glossary.html#term-a-term-including-colon -
+The-Julia-Domain std:label -1 write_inventory/#$ The Julia Domain
 ''')
 
 inventory_v2_not_having_version = b'''\