attribs: Don't canonicalize lookup_scoped_attribute_spec argument [PR113674]

The C and C++ FEs when parsing attributes already canonicalize them
(i.e. if they start with __ and end with __ substrings, we remove those).
lookup_attribute already verifies in gcc_assert that the first character
of name is not an underscore, and even lookup_scoped_attribute_spec doesn't
attempt to canonicalize the namespace it is passed.  But for some historic
reason it was canonicalizing the name argument, which misbehaves when
an attribute starts with ____ and ends with ____.
I believe it is just wrong to try to canonicalize
lookup_scope_attribute_spec name attribute, it should have been
canonicalized already, in other spots where it is called it is already
canonicalized before.

2024-02-12  Jakub Jelinek  <jakub@redhat.com>

	PR c++/113674
	* attribs.cc (extract_attribute_substring): Remove.
	(lookup_scoped_attribute_spec): Don't call it.

	* c-c++-common/Wattributes-3.c: New test.

gcc/attribs.cc

@@ -116,15 +116,6 @@ get_gnu_namespace ()
   return gnu_namespace_cache;
 }
 
-/* Return base name of the attribute.  Ie '__attr__' is turned into 'attr'.
-   To avoid need for copying, we simply return length of the string.  */
-
-static void
-extract_attribute_substring (struct substring *str)
-{
-  canonicalize_attr_name (str->str, str->length);
-}
-
 /* Insert SPECS into its namespace.  IGNORED_P is true iff all unknown
    attributes in this namespace should be ignored for the purposes of
    -Wattributes.  The function returns the namespace into which the
@@ -398,7 +389,6 @@ lookup_scoped_attribute_spec (const_tree ns, const_tree name)
 
   attr.str = IDENTIFIER_POINTER (name);
   attr.length = IDENTIFIER_LENGTH (name);
-  extract_attribute_substring (&attr);
   return attrs->attribute_hash->find_with_hash (&attr,
 						substring_hash (attr.str,
 							       	attr.length));

gcc/testsuite/c-c++-common/Wattributes-3.c

@@ -0,0 +1,13 @@
+/* PR c++/113674 */
+/* { dg-do compile { target { c || c++11 } } } */
+/* { dg-options "" } */
+
+[[____noreturn____]] int foo (int i)		/* { dg-warning "'__noreturn__' attribute (directive )?ignored" } */
+{
+  return i;
+}
+
+[[____maybe_unused____]] int bar (int i)	/* { dg-warning "'__maybe_unused__' attribute (directive )?ignored" } */
+{
+  return i;
+}