c: Add support for [[__extension__ ...]]

[[]] attributes are a recent addition to C, but as a GNU extension,
GCC allows them to be used in C11 and earlier.  Normally this use
would trigger a pedwarn (for -pedantic, -Wc11-c2x-compat, etc.).

This patch allows the pedwarn to be suppressed by starting the
attribute-list with __extension__.

Also, :: is not a single lexing token prior to C2X, so it wasn't
possible to use scoped attributes in C11, even as a GNU extension.
The patch allows two colons to be used in place of :: when
__extension__ is used.  No attempt is made to check whether the
two colons are immediately adjacent.

gcc/
	* doc/extend.texi: Document the C [[__extension__ ...]] construct.

gcc/c/
	* c-parser.cc (c_parser_std_attribute): Conditionally allow
	two colons to be used in place of ::.
	(c_parser_std_attribute_list): New function, split out from...
	(c_parser_std_attribute_specifier): ...here.  Allow the attribute-list
	to start with __extension__.  When it does, also allow two colons
	to be used in place of ::.

gcc/testsuite/
	* gcc.dg/c2x-attr-syntax-6.c: New test.
	* gcc.dg/c2x-attr-syntax-7.c: Likewise.

0 files changed, 0 insertions, 0 deletions