Revert removing pre-v3 syntax in the C domain

sphinx/domains/c.py

@@ -12,6 +12,7 @@
 from sphinx.addnodes import pending_xref
 from sphinx.application import Sphinx
 from sphinx.builders import Builder
+from sphinx.deprecation import RemovedInSphinx70Warning
 from sphinx.directives import ObjectDescription
 from sphinx.domains import Domain, ObjType
 from sphinx.environment import BuildEnvironment
@@ -3252,8 +3253,27 @@ def handle_signature(self, sig: str, signode: TextElement) -> ASTDeclaration:
 
         parser = DefinitionParser(sig, location=signode, config=self.env.config)
         try:
-            ast = self.parse_definition(parser)
-            parser.assert_end()
+            try:
+                ast = self.parse_definition(parser)
+                parser.assert_end()
+            except DefinitionError as eOrig:
+                if not self.env.config['c_allow_pre_v3']:
+                    raise
+                if self.objtype != 'type':
+                    raise
+                try:
+                    ast = self.parse_pre_v3_type_definition(parser)
+                    parser.assert_end()
+                except DefinitionError:
+                    raise eOrig
+                self.object_type = ast.objectType  # type: ignore
+                if self.env.config['c_warn_on_allowed_pre_v3']:
+                    msg = "{}: Pre-v3 C type directive '.. c:type:: {}' converted to " \
+                          "'.. c:{}:: {}'." \
+                          "\nThe original parsing error was:\n{}"
+                    msg = msg.format(RemovedInSphinx70Warning.__name__,
+                                     sig, ast.objectType, ast, eOrig)
+                    logger.warning(msg, location=signode)
         except DefinitionError as e:
             logger.warning(e, location=signode)
             # It is easier to assume some phony name than handling the error in
@@ -3655,6 +3675,39 @@ def process_link(self, env: BuildEnvironment, refnode: Element,
                     title = title[dot + 1:]
         return title, target
 
+    def run(self) -> Tuple[List[Node], List[system_message]]:
+        if not self.env.config['c_allow_pre_v3']:
+            return super().run()
+
+        text = self.text.replace('\n', ' ')
+        parser = DefinitionParser(text, location=self.get_location(),
+                                  config=self.env.config)
+        try:
+            parser.parse_xref_object()
+            # it succeeded, so let it through
+            return super().run()
+        except DefinitionError as eOrig:
+            # try as if it was an c:expr
+            parser.pos = 0
+            try:
+                ast = parser.parse_expression()
+            except DefinitionError:
+                # that didn't go well, just default back
+                return super().run()
+            classes = ['xref', 'c', 'c-texpr']
+            parentSymbol = self.env.temp_data.get('cpp:parent_symbol', None)
+            if parentSymbol is None:
+                parentSymbol = self.env.domaindata['c']['root_symbol']
+            signode = nodes.inline(classes=classes)
+            ast.describe_signature(signode, 'markType', self.env, parentSymbol)
+
+            if self.env.config['c_warn_on_allowed_pre_v3']:
+                msg = "{}: Pre-v3 C type role ':c:type:`{}`' converted to ':c:expr:`{}`'."
+                msg += "\nThe original parsing error was:\n{}"
+                msg = msg.format(RemovedInSphinx70Warning.__name__, text, text, eOrig)
+                logger.warning(msg, location=self.get_location())
+            return [signode], []
+
 
 class CExprRole(SphinxRole):
     def __init__(self, asCode: bool) -> None:
@@ -3864,6 +3917,9 @@ def setup(app: Sphinx) -> Dict[str, Any]:
     app.add_config_value("c_extra_keywords", _macroKeywords, 'env')
     app.add_post_transform(AliasTransform)
 
+    app.add_config_value("c_allow_pre_v3", False, 'env')
+    app.add_config_value("c_warn_on_allowed_pre_v3", True, 'env')
+
     return {
         'version': 'builtin',
         'env_version': 2,