Raise `ConfigError on invalid html_sidebars` values

CHANGES.rst

@@ -39,6 +39,8 @@ Incompatible changes
 * Removed :py:func:`!sphinx.util.typing.stringify`
   (use :py:func:`!sphinx.util.typing.stringify_annotation`).
   Patch by Adam Turner.
+* #12593: Raise an error for invalid :confval:`html_sidebars` values.
+  Patch by Adam Turner.
 
 Deprecated
 ----------

sphinx/builders/html/__init__.py

@@ -982,10 +982,7 @@ class StandaloneHTMLBuilder(Builder):
                 matched = pattern
                 sidebars = pat_sidebars
 
-        # See error_on_html_sidebars_string_values.
-        # Replace with simple list coercion in Sphinx 8.0
-        # xref: RemovedInSphinx80Warning
-        ctx['sidebars'] = sidebars
+        ctx['sidebars'] = list(sidebars)
 
     # --------- these are overwritten by the serialization builder
 
@@ -1300,10 +1297,7 @@ def error_on_html_sidebars_string_values(app: Sphinx, config: Config) -> None:
              "Change to `html_sidebars = %r`.")
     bad_patterns = ', '.join(map(repr, errors))
     fixed = config.html_sidebars | errors
-    logger.error(msg, bad_patterns, fixed)
-    # Enable hard error in next major version.
-    # xref: RemovedInSphinx80Warning
-    # raise ConfigError(msg % (bad_patterns, fixed))
+    raise ConfigError(msg % (bad_patterns, fixed))
 
 
 def error_on_html_4(_app: Sphinx, config: Config) -> None:

tests/test_builders/test_build_html.py

@@ -1,6 +1,5 @@
 """Test the HTML builder and check output against XPath."""
 
-import contextlib
 import os
 import posixpath
 import re
@@ -9,7 +8,7 @@ import pytest
 
 from sphinx.builders.html import validate_html_extra_path, validate_html_static_path
 from sphinx.deprecation import RemovedInSphinx80Warning
-from sphinx.errors import ConfigError, ThemeError
+from sphinx.errors import ConfigError
 from sphinx.util.console import strip_colors
 from sphinx.util.inventory import InventoryFile
 
@@ -19,27 +18,17 @@ from tests.test_builders.xpath_util import check_xpath
 
 def test_html_sidebars_error(make_app, tmp_path):
     (tmp_path / 'conf.py').touch()
-    (tmp_path / 'index.rst').touch()
-    app = make_app(
-        buildername='html',
-        srcdir=tmp_path,
-        confoverrides={'html_sidebars': {'index': 'searchbox.html'}},
-    )
-
-    # Test that the error is logged
-    warnings = app.warning.getvalue()
-    assert ("ERROR: Values in 'html_sidebars' must be a list of strings. "
-            "At least one pattern has a string value: 'index'. "
-            "Change to `html_sidebars = {'index': ['searchbox.html']}`.") in warnings
-
-    # But that the value is unchanged.
-    # (Remove this bit of the test in Sphinx 8)
-    def _html_context_hook(app, pagename, templatename, context, doctree):
-        assert context["sidebars"] == 'searchbox.html'
-    app.connect('html-page-context', _html_context_hook)
-    with contextlib.suppress(ThemeError):
-        # ignore template rendering issues (ThemeError).
-        app.build()
+    with pytest.raises(
+        ConfigError,
+        match="Values in 'html_sidebars' must be a list of strings. "
+              "At least one pattern has a string value: 'index'. "
+              r"Change to `html_sidebars = \{'index': \['searchbox.html'\]\}`.",
+    ):
+        make_app(
+            buildername='html',
+            srcdir=tmp_path,
+            confoverrides={'html_sidebars': {'index': 'searchbox.html'}},
+        )
 
 
 def test_html4_error(make_app, tmp_path):