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

  Joseph Myers <joseph@codesourcery.com> writes:
> On Wed, 16 Aug 2023, Richard Sandiford via Gcc-patches wrote:
>
>> Would it be OK to add support for:
>> 
>>   [[__extension__ ...]]
>> 
>> to suppress the pedwarn about using [[]] prior to C2X?  Then we can
>
> That seems like a plausible feature to add.

Thanks.  Of course, once I actually tried it, I hit a snag:
:: isn't a single lexing token prior to C2X, and so something like:

  [[__extension__ arm::streaming]]

would not be interpreted as a scoped attribute in C11.  The patch
gets around that by allowing two colons in place of :: when
__extension__ is used.  I realise that's pushing the bounds of
acceptability though...

I wondered about trying to require the two colons to be immediately
adjacent.  But:

(a) There didn't appear to be an existing API to check that, which seemed
    like a red flag.  The closest I could find was get_source_text_between.

    Similarly to that, it would in principle be possible to compare
    two expanded locations.  But...

(b) I had a vague impression that locations were allowed to drop column
    information for very large inputs (maybe I'm wrong).

(c) It wouldn't cope with token pasting.

So in the end I just used a simple two-token test, like for [[ and ]].

Bootstrapped & regression-tested on aarch64-linux-gnu.

Richard

----

[[]] 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.
---
 gcc/c/c-parser.cc                        | 68 ++++++++++++++++++------
 gcc/doc/extend.texi                      | 27 ++++++++--
 gcc/testsuite/gcc.dg/c2x-attr-syntax-6.c | 50 +++++++++++++++++
 gcc/testsuite/gcc.dg/c2x-attr-syntax-7.c | 48 +++++++++++++++++
 4 files changed, 173 insertions(+), 20 deletions(-)
 create mode 100644 gcc/testsuite/gcc.dg/c2x-attr-syntax-6.c
 create mode 100644 gcc/testsuite/gcc.dg/c2x-attr-syntax-7.c
  

> Am 17.08.2023 um 13:25 schrieb Richard Sandiford via Gcc-patches <gcc-patches@gcc.gnu.org>:
> 
> Joseph Myers <joseph@codesourcery.com> writes:
>>> On Wed, 16 Aug 2023, Richard Sandiford via Gcc-patches wrote:
>>> 
>>> Would it be OK to add support for:
>>> 
>>>  [[__extension__ ...]]
>>> 
>>> to suppress the pedwarn about using [[]] prior to C2X?  Then we can
>> 
>> That seems like a plausible feature to add.
> 
> Thanks.  Of course, once I actually tried it, I hit a snag:
> :: isn't a single lexing token prior to C2X, and so something like:
> 
>  [[__extension__ arm::streaming]]
> 
> would not be interpreted as a scoped attribute in C11.  The patch
> gets around that by allowing two colons in place of :: when
> __extension__ is used.  I realise that's pushing the bounds of
> acceptability though...
> 
> I wondered about trying to require the two colons to be immediately
> adjacent.  But:
> 
> (a) There didn't appear to be an existing API to check that, which seemed
>    like a red flag.  The closest I could find was get_source_text_between.

IStR a cop Toben has ->prev_white or so

>    Similarly to that, it would in principle be possible to compare
>    two expanded locations.  But...
> 
> (b) I had a vague impression that locations were allowed to drop column
>    information for very large inputs (maybe I'm wrong).
> 
> (c) It wouldn't cope with token pasting.
> 
> So in the end I just used a simple two-token test, like for [[ and ]].
> Bootstrapped & regression-tested on aarch64-linux-gnu.
> 
> Richard
> 
> ----
> 
> [[]] 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.
> ---
> gcc/c/c-parser.cc                        | 68 ++++++++++++++++++------
> gcc/doc/extend.texi                      | 27 ++++++++--
> gcc/testsuite/gcc.dg/c2x-attr-syntax-6.c | 50 +++++++++++++++++
> gcc/testsuite/gcc.dg/c2x-attr-syntax-7.c | 48 +++++++++++++++++
> 4 files changed, 173 insertions(+), 20 deletions(-)
> create mode 100644 gcc/testsuite/gcc.dg/c2x-attr-syntax-6.c
> create mode 100644 gcc/testsuite/gcc.dg/c2x-attr-syntax-7.c
> 
> diff --git a/gcc/c/c-parser.cc b/gcc/c/c-parser.cc
> index 33fe7b115ff..82e56b28446 100644
> --- a/gcc/c/c-parser.cc
> +++ b/gcc/c/c-parser.cc
> @@ -5390,10 +5390,18 @@ c_parser_balanced_token_sequence (c_parser *parser)
>      ( balanced-token-sequence[opt] )
> 
>    Keywords are accepted as identifiers for this purpose.
> -*/
> +
> +   As an extension, we permit an attribute-specifier to be:
> +
> +     [ [ __extension__ attribute-list ] ]
> +
> +   Two colons are then accepted as a synonym for ::.  No attempt is made
> +   to check whether the colons are immediately adjacent.  LOOSE_SCOPE_P
> +   indicates whether this relaxation is in effect.  */
> 
> static tree
> -c_parser_std_attribute (c_parser *parser, bool for_tm)
> +c_parser_std_attribute (c_parser *parser, bool for_tm,
> +            bool loose_scope_p = false)
> {
>   c_token *token = c_parser_peek_token (parser);
>   tree ns, name, attribute;
> @@ -5406,9 +5414,18 @@ c_parser_std_attribute (c_parser *parser, bool for_tm)
>     }
>   name = canonicalize_attr_name (token->value);
>   c_parser_consume_token (parser);
> -  if (c_parser_next_token_is (parser, CPP_SCOPE))
> +  if (c_parser_next_token_is (parser, CPP_SCOPE)
> +      || (loose_scope_p
> +      && c_parser_next_token_is (parser, CPP_COLON)
> +      && c_parser_peek_token (parser)->type == CPP_COLON))
>     {
>       ns = name;
> +      if (c_parser_next_token_is (parser, CPP_COLON))
> +    {
> +      c_parser_consume_token (parser);
> +      if (!c_parser_next_token_is (parser, CPP_COLON))
> +        gcc_unreachable ();
> +    }
>       c_parser_consume_token (parser);
>       token = c_parser_peek_token (parser);
>       if (token->type != CPP_NAME && token->type != CPP_KEYWORD)
> @@ -5481,19 +5498,9 @@ c_parser_std_attribute (c_parser *parser, bool for_tm)
> }
> 
> static tree
> -c_parser_std_attribute_specifier (c_parser *parser, bool for_tm)
> +c_parser_std_attribute_list (c_parser *parser, bool for_tm,
> +                 bool loose_scope_p = false)
> {
> -  location_t loc = c_parser_peek_token (parser)->location;
> -  if (!c_parser_require (parser, CPP_OPEN_SQUARE, "expected %<[%>"))
> -    return NULL_TREE;
> -  if (!c_parser_require (parser, CPP_OPEN_SQUARE, "expected %<[%>"))
> -    {
> -      c_parser_skip_until_found (parser, CPP_CLOSE_SQUARE, "expected %<]%>");
> -      return NULL_TREE;
> -    }
> -  if (!for_tm)
> -    pedwarn_c11 (loc, OPT_Wpedantic,
> -         "ISO C does not support %<[[]]%> attributes before C2X");
>   tree attributes = NULL_TREE;
>   while (true)
>     {
> @@ -5505,7 +5512,7 @@ c_parser_std_attribute_specifier (c_parser *parser, bool for_tm)
>      c_parser_consume_token (parser);
>      continue;
>    }
> -      tree attribute = c_parser_std_attribute (parser, for_tm);
> +      tree attribute = c_parser_std_attribute (parser, for_tm, loose_scope_p);
>       if (attribute != error_mark_node)
>    {
>      TREE_CHAIN (attribute) = attributes;
> @@ -5514,6 +5521,35 @@ c_parser_std_attribute_specifier (c_parser *parser, bool for_tm)
>       if (c_parser_next_token_is_not (parser, CPP_COMMA))
>    break;
>     }
> +  return attributes;
> +}
> +
> +static tree
> +c_parser_std_attribute_specifier (c_parser *parser, bool for_tm)
> +{
> +  location_t loc = c_parser_peek_token (parser)->location;
> +  if (!c_parser_require (parser, CPP_OPEN_SQUARE, "expected %<[%>"))
> +    return NULL_TREE;
> +  if (!c_parser_require (parser, CPP_OPEN_SQUARE, "expected %<[%>"))
> +    {
> +      c_parser_skip_until_found (parser, CPP_CLOSE_SQUARE, "expected %<]%>");
> +      return NULL_TREE;
> +    }
> +  tree attributes;
> +  if (c_parser_next_token_is_keyword (parser, RID_EXTENSION))
> +    {
> +      auto ext = disable_extension_diagnostics ();
> +      c_parser_consume_token (parser);
> +      attributes = c_parser_std_attribute_list (parser, for_tm, true);
> +      restore_extension_diagnostics (ext);
> +    }
> +  else
> +    {
> +      if (!for_tm)
> +    pedwarn_c11 (loc, OPT_Wpedantic,
> +             "ISO C does not support %<[[]]%> attributes before C2X");
> +      attributes = c_parser_std_attribute_list (parser, for_tm);
> +    }
>   c_parser_skip_until_found (parser, CPP_CLOSE_SQUARE, "expected %<]%>");
>   c_parser_skip_until_found (parser, CPP_CLOSE_SQUARE, "expected %<]%>");
>   return nreverse (attributes);
> diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
> index 73a997276cb..d764e44cbce 100644
> --- a/gcc/doc/extend.texi
> +++ b/gcc/doc/extend.texi
> @@ -11914,10 +11914,29 @@ macros to replace them with the customary keywords.  It looks like this:
> @findex __extension__
> @opindex pedantic
> @option{-pedantic} and other options cause warnings for many GNU C extensions.
> -You can
> -prevent such warnings within one expression by writing
> -@code{__extension__} before the expression.  @code{__extension__} has no
> -effect aside from this.
> +You can suppress such warnings using the keyword @code{__extension__}.
> +Specifically:
> +
> +@itemize @bullet
> +@item
> +Writing @code{__extension__} before an expression prevents warnings
> +about extensions within that expression.
> +
> +@item
> +In C, writing:
> +
> +@smallexample
> +[[__extension__ @dots{}]]
> +@end smallexample
> +
> +suppresses warnings about using @samp{[[]]} attributes in C versions
> +that predate C2X@.  Since the scope token @samp{::} is not a single
> +lexing token in earlier versions of C, this construct also allows two colons
> +to be used in place of @code{::}.  GCC does not check whether the two
> +colons are immediately adjacent.
> +@end itemize
> +
> +@code{__extension__} has no effect aside from this.
> 
> @node Incomplete Enums
> @section Incomplete @code{enum} Types
> diff --git a/gcc/testsuite/gcc.dg/c2x-attr-syntax-6.c b/gcc/testsuite/gcc.dg/c2x-attr-syntax-6.c
> new file mode 100644
> index 00000000000..01eff1f32df
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/c2x-attr-syntax-6.c
> @@ -0,0 +1,50 @@
> +/* Test C2x attribute syntax: use of __extension__ in C11 mode.  */
> +/* { dg-do compile } */
> +/* { dg-options "-std=c11 -pedantic-errors" } */
> +
> +#define FOO ::
> +#define BAR :
> +
> +typedef int [[__extension__ gnu :: vector_size (4)]] g1;
> +typedef int [[__extension__ gnu : : vector_size (4)]] g2;
> +typedef int [[__extension__ gnu FOO vector_size (4)]] g3;
> +typedef int [[__extension__ gnu BAR BAR vector_size (4)]] g4;
> +typedef int [[__extension__ gnu :: vector_size (sizeof (void (*)(...)))]] g5;
> +typedef int [[__extension__]] g6;
> +typedef int [[__extension__,]] g7;
> +typedef int [[__extension__, ,,,, ,, ,]] g8;
> +[[__extension__ deprecated]] int g9 ();
> +[[__extension__ nodiscard]] int g10 ();
> +[[__extension__ noreturn]] void g11 ();
> +
> +int
> +cases (int x)
> +{
> +  switch (x)
> +    {
> +    case 1:
> +    case 2:
> +    case 4:
> +      x += 1;
> +      [[__extension__ fallthrough]];
> +    case 19:
> +    case 33:
> +      x *= 2;
> +      [[fallthrough]];  /* { dg-error {attributes before C2X} } */
> +    case 99:
> +      return x;
> +
> +    default:
> +      return 0;
> +    }
> +}
> +
> +typedef int [[__extension__ vector_size (4)]] b1; /* { dg-error {'vector_size' attribute ignored} } */
> +typedef int [[__extension__ __extension__]] b2; /* { dg-error {'extension' attribute ignored} } */
> +typedef int [[__extension__ unknown_attribute]] b3; /* { dg-error {'unknown_attribute' attribute ignored} } */
> +typedef int [[gnu::vector_size(4)]] b4; /* { dg-error {expected '\]' before ':'} } */
> +/* { dg-error {'gnu' attribute ignored} "" { target *-*-* } .-1 } */
> +/* { dg-error {attributes before C2X} "" { target *-*-* } .-2 } */
> +typedef int [[gnu : : vector_size(4)]] b5; /* { dg-error {expected '\]' before ':'} } */
> +/* { dg-error {'gnu' attribute ignored} "" { target *-*-* } .-1 } */
> +/* { dg-error {attributes before C2X} "" { target *-*-* } .-2 } */
> diff --git a/gcc/testsuite/gcc.dg/c2x-attr-syntax-7.c b/gcc/testsuite/gcc.dg/c2x-attr-syntax-7.c
> new file mode 100644
> index 00000000000..cfa9b00cfc9
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/c2x-attr-syntax-7.c
> @@ -0,0 +1,48 @@
> +/* Test C2x attribute syntax: use of __extension__ in C11 mode.  */
> +/* { dg-do compile } */
> +/* { dg-options "-std=c2x -pedantic-errors -Wc11-c2x-compat" } */
> +
> +#define FOO ::
> +#define BAR :
> +
> +typedef int [[__extension__ gnu :: vector_size (4)]] g1;
> +typedef int [[__extension__ gnu : : vector_size (4)]] g2;
> +typedef int [[__extension__ gnu FOO vector_size (4)]] g3;
> +typedef int [[__extension__ gnu BAR BAR vector_size (4)]] g4;
> +typedef int [[__extension__ gnu :: vector_size (sizeof (void (*)(...)))]] g5;
> +typedef int [[__extension__]] g6;
> +typedef int [[__extension__,]] g7;
> +typedef int [[__extension__, ,,,, ,, ,]] g8;
> +[[__extension__ deprecated]] int g9 ();
> +[[__extension__ nodiscard]] int g10 ();
> +[[__extension__ noreturn]] void g11 ();
> +
> +int
> +cases (int x)
> +{
> +  switch (x)
> +    {
> +    case 1:
> +    case 2:
> +    case 4:
> +      x += 1;
> +      [[__extension__ fallthrough]];
> +    case 19:
> +    case 33:
> +      x *= 2;
> +      [[fallthrough]];  /* { dg-warning {attributes before C2X} } */
> +    case 99:
> +      return x;
> +
> +    default:
> +      return 0;
> +    }
> +}
> +
> +typedef int [[__extension__ vector_size (4)]] b1; /* { dg-error {'vector_size' attribute ignored} } */
> +typedef int [[__extension__ __extension__]] b2; /* { dg-error {'extension' attribute ignored} } */
> +typedef int [[__extension__ unknown_attribute]] b3; /* { dg-error {'unknown_attribute' attribute ignored} } */
> +typedef int [[gnu::vector_size(4)]] b4; /* { dg-warning {attributes before C2X} } */
> +typedef int [[gnu : : vector_size(4)]] b5; /* { dg-error {expected '\]' before ':'} } */
> +/* { dg-error {'gnu' attribute ignored} "" { target *-*-* } .-1 } */
> +/* { dg-warning {attributes before C2X} "" { target *-*-* } .-2 } */
> -- 
> 2.25.1
>
  

Richard Biener <richard.guenther@gmail.com> writes:
>> Am 17.08.2023 um 13:25 schrieb Richard Sandiford via Gcc-patches <gcc-patches@gcc.gnu.org>:
>> 
>> Joseph Myers <joseph@codesourcery.com> writes:
>>>> On Wed, 16 Aug 2023, Richard Sandiford via Gcc-patches wrote:
>>>> 
>>>> Would it be OK to add support for:
>>>> 
>>>>  [[__extension__ ...]]
>>>> 
>>>> to suppress the pedwarn about using [[]] prior to C2X?  Then we can
>>> 
>>> That seems like a plausible feature to add.
>> 
>> Thanks.  Of course, once I actually tried it, I hit a snag:
>> :: isn't a single lexing token prior to C2X, and so something like:
>> 
>>  [[__extension__ arm::streaming]]
>> 
>> would not be interpreted as a scoped attribute in C11.  The patch
>> gets around that by allowing two colons in place of :: when
>> __extension__ is used.  I realise that's pushing the bounds of
>> acceptability though...
>> 
>> I wondered about trying to require the two colons to be immediately
>> adjacent.  But:
>> 
>> (a) There didn't appear to be an existing API to check that, which seemed
>>    like a red flag.  The closest I could find was get_source_text_between.
>
> IStR a cop Toben has ->prev_white or so

Ah, thanks.

  if (c_parser_next_token_is (parser, CPP_SCOPE)
      || (loose_scope_p
	  && c_parser_next_token_is (parser, CPP_COLON)
	  && c_parser_peek_2nd_token (parser)->type == CPP_COLON
	  && !(c_parser_peek_2nd_token (parser)->flags & PREV_WHITE)))

seems to work for (i.e. reject):

typedef int [[__extension__ gnu : : vector_size (4)]] g3;
typedef int [[__extension__ gnu :/**/: vector_size (4)]] g13;

but not:

#define BAR :
typedef int [[__extension__ gnu BAR BAR vector_size (4)]] g5;

#define JOIN(A, B) A/**/B
typedef int [[__extension__ gnu JOIN(:,:) vector_size (4)]] g14;

I now realise the patch was peeking at the wrong token.  Will fix,
and add more tests.

Richard
  

Richard Sandiford <richard.sandiford@arm.com> writes:
> Joseph Myers <joseph@codesourcery.com> writes:
>> On Wed, 16 Aug 2023, Richard Sandiford via Gcc-patches wrote:
>>
>>> Would it be OK to add support for:
>>> 
>>>   [[__extension__ ...]]
>>> 
>>> to suppress the pedwarn about using [[]] prior to C2X?  Then we can
>>
>> That seems like a plausible feature to add.
>
> Thanks.  Of course, once I actually tried it, I hit a snag:
> :: isn't a single lexing token prior to C2X, and so something like:
>
>   [[__extension__ arm::streaming]]
>
> would not be interpreted as a scoped attribute in C11.  The patch
> gets around that by allowing two colons in place of :: when
> __extension__ is used.  I realise that's pushing the bounds of
> acceptability though...
>
> I wondered about trying to require the two colons to be immediately
> adjacent.  But:
>
> (a) There didn't appear to be an existing API to check that, which seemed
>     like a red flag.  The closest I could find was get_source_text_between.
>
>     Similarly to that, it would in principle be possible to compare
>     two expanded locations.  But...
>
> (b) I had a vague impression that locations were allowed to drop column
>     information for very large inputs (maybe I'm wrong).
>
> (c) It wouldn't cope with token pasting.
>
> So in the end I just used a simple two-token test, like for [[ and ]].
>
> Bootstrapped & regression-tested on aarch64-linux-gnu.

Gah, as mentioned yesterday, the patch was peeking the wrong token.
I've fixed that, and added corresponding tests.  Sorry for missing
it first time.

Richard

-----

[[]] 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.
---
 gcc/c/c-parser.cc                        | 64 ++++++++++++++++++------
 gcc/doc/extend.texi                      | 27 ++++++++--
 gcc/testsuite/gcc.dg/c2x-attr-syntax-6.c | 62 +++++++++++++++++++++++
 gcc/testsuite/gcc.dg/c2x-attr-syntax-7.c | 60 ++++++++++++++++++++++
 4 files changed, 193 insertions(+), 20 deletions(-)
 create mode 100644 gcc/testsuite/gcc.dg/c2x-attr-syntax-6.c
 create mode 100644 gcc/testsuite/gcc.dg/c2x-attr-syntax-7.c

diff --git a/gcc/c/c-parser.cc b/gcc/c/c-parser.cc
index 33fe7b115ff..ca60c51ddb2 100644
--- a/gcc/c/c-parser.cc
+++ b/gcc/c/c-parser.cc
@@ -5390,10 +5390,18 @@ c_parser_balanced_token_sequence (c_parser *parser)
      ( balanced-token-sequence[opt] )
 
    Keywords are accepted as identifiers for this purpose.
-*/
+
+   As an extension, we permit an attribute-specifier to be:
+
+     [ [ __extension__ attribute-list ] ]
+
+   Two colons are then accepted as a synonym for ::.  No attempt is made
+   to check whether the colons are immediately adjacent.  LOOSE_SCOPE_P
+   indicates whether this relaxation is in effect.  */
 
 static tree
-c_parser_std_attribute (c_parser *parser, bool for_tm)
+c_parser_std_attribute (c_parser *parser, bool for_tm,
+			bool loose_scope_p = false)
 {
   c_token *token = c_parser_peek_token (parser);
   tree ns, name, attribute;
@@ -5406,9 +5414,14 @@ c_parser_std_attribute (c_parser *parser, bool for_tm)
     }
   name = canonicalize_attr_name (token->value);
   c_parser_consume_token (parser);
-  if (c_parser_next_token_is (parser, CPP_SCOPE))
+  if (c_parser_next_token_is (parser, CPP_SCOPE)
+      || (loose_scope_p
+	  && c_parser_next_token_is (parser, CPP_COLON)
+	  && c_parser_peek_2nd_token (parser)->type == CPP_COLON))
     {
       ns = name;
+      if (c_parser_next_token_is (parser, CPP_COLON))
+	c_parser_consume_token (parser);
       c_parser_consume_token (parser);
       token = c_parser_peek_token (parser);
       if (token->type != CPP_NAME && token->type != CPP_KEYWORD)
@@ -5481,19 +5494,9 @@ c_parser_std_attribute (c_parser *parser, bool for_tm)
 }
 
 static tree
-c_parser_std_attribute_specifier (c_parser *parser, bool for_tm)
+c_parser_std_attribute_list (c_parser *parser, bool for_tm,
+			     bool loose_scope_p = false)
 {
-  location_t loc = c_parser_peek_token (parser)->location;
-  if (!c_parser_require (parser, CPP_OPEN_SQUARE, "expected %<[%>"))
-    return NULL_TREE;
-  if (!c_parser_require (parser, CPP_OPEN_SQUARE, "expected %<[%>"))
-    {
-      c_parser_skip_until_found (parser, CPP_CLOSE_SQUARE, "expected %<]%>");
-      return NULL_TREE;
-    }
-  if (!for_tm)
-    pedwarn_c11 (loc, OPT_Wpedantic,
-		 "ISO C does not support %<[[]]%> attributes before C2X");
   tree attributes = NULL_TREE;
   while (true)
     {
@@ -5505,7 +5508,7 @@ c_parser_std_attribute_specifier (c_parser *parser, bool for_tm)
 	  c_parser_consume_token (parser);
 	  continue;
 	}
-      tree attribute = c_parser_std_attribute (parser, for_tm);
+      tree attribute = c_parser_std_attribute (parser, for_tm, loose_scope_p);
       if (attribute != error_mark_node)
 	{
 	  TREE_CHAIN (attribute) = attributes;
@@ -5514,6 +5517,35 @@ c_parser_std_attribute_specifier (c_parser *parser, bool for_tm)
       if (c_parser_next_token_is_not (parser, CPP_COMMA))
 	break;
     }
+  return attributes;
+}
+
+static tree
+c_parser_std_attribute_specifier (c_parser *parser, bool for_tm)
+{
+  location_t loc = c_parser_peek_token (parser)->location;
+  if (!c_parser_require (parser, CPP_OPEN_SQUARE, "expected %<[%>"))
+    return NULL_TREE;
+  if (!c_parser_require (parser, CPP_OPEN_SQUARE, "expected %<[%>"))
+    {
+      c_parser_skip_until_found (parser, CPP_CLOSE_SQUARE, "expected %<]%>");
+      return NULL_TREE;
+    }
+  tree attributes;
+  if (c_parser_next_token_is_keyword (parser, RID_EXTENSION))
+    {
+      auto ext = disable_extension_diagnostics ();
+      c_parser_consume_token (parser);
+      attributes = c_parser_std_attribute_list (parser, for_tm, true);
+      restore_extension_diagnostics (ext);
+    }
+  else
+    {
+      if (!for_tm)
+	pedwarn_c11 (loc, OPT_Wpedantic,
+		     "ISO C does not support %<[[]]%> attributes before C2X");
+      attributes = c_parser_std_attribute_list (parser, for_tm);
+    }
   c_parser_skip_until_found (parser, CPP_CLOSE_SQUARE, "expected %<]%>");
   c_parser_skip_until_found (parser, CPP_CLOSE_SQUARE, "expected %<]%>");
   return nreverse (attributes);
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index f657032cbef..d9a907cb1cb 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -11938,10 +11938,29 @@ macros to replace them with the customary keywords.  It looks like this:
 @findex __extension__
 @opindex pedantic
 @option{-pedantic} and other options cause warnings for many GNU C extensions.
-You can
-prevent such warnings within one expression by writing
-@code{__extension__} before the expression.  @code{__extension__} has no
-effect aside from this.
+You can suppress such warnings using the keyword @code{__extension__}.
+Specifically:
+
+@itemize @bullet
+@item
+Writing @code{__extension__} before an expression prevents warnings
+about extensions within that expression.
+
+@item
+In C, writing:
+
+@smallexample
+[[__extension__ @dots{}]]
+@end smallexample
+
+suppresses warnings about using @samp{[[]]} attributes in C versions
+that predate C2X@.  Since the scope token @samp{::} is not a single
+lexing token in earlier versions of C, this construct also allows two colons
+to be used in place of @code{::}.  GCC does not check whether the two
+colons are immediately adjacent.
+@end itemize
+
+@code{__extension__} has no effect aside from this.
 
 @node Incomplete Enums
 @section Incomplete @code{enum} Types
diff --git a/gcc/testsuite/gcc.dg/c2x-attr-syntax-6.c b/gcc/testsuite/gcc.dg/c2x-attr-syntax-6.c
new file mode 100644
index 00000000000..9e5f65ce469
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c2x-attr-syntax-6.c
@@ -0,0 +1,62 @@
+/* Test C2x attribute syntax: use of __extension__ in C11 mode.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c11 -pedantic-errors" } */
+
+#define FOO ::
+#define BAR :
+#define JOIN(A, B) A/**/B
+#define JOIN2(A, B) A##B
+
+typedef int [[__extension__ gnu::vector_size (4)]] g1;
+typedef int [[__extension__ gnu :: vector_size (4)]] g2;
+typedef int [[__extension__ gnu : : vector_size (4)]] g3;
+typedef int [[__extension__ gnu: :vector_size (4)]] g4;
+typedef int [[__extension__ gnu FOO vector_size (4)]] g5;
+typedef int [[__extension__ gnu BAR BAR vector_size (4)]] g6;
+typedef int [[__extension__ gnu :/**/: vector_size (4)]] g7;
+typedef int [[__extension__ gnu JOIN(:,:) vector_size (4)]] g8;
+typedef int [[__extension__ gnu :: vector_size (sizeof (void (*)(...)))]] g10;
+typedef int [[__extension__]] g11;
+typedef int [[__extension__,]] g12;
+typedef int [[__extension__, ,,,, ,, ,]] g13;
+[[__extension__ deprecated]] int g14 ();
+[[__extension__ nodiscard]] int g15 ();
+[[__extension__ noreturn]] void g16 ();
+
+int
+cases (int x)
+{
+  switch (x)
+    {
+    case 1:
+    case 2:
+    case 4:
+      x += 1;
+      [[__extension__ fallthrough]];
+    case 19:
+    case 33:
+      x *= 2;
+      [[fallthrough]];  /* { dg-error {attributes before C2X} } */
+    case 99:
+      return x;
+
+    default:
+      return 0;
+    }
+}
+
+typedef int [[__extension__ vector_size (4)]] b1; /* { dg-error {'vector_size' attribute ignored} } */
+typedef int [[__extension__ __extension__]] b2; /* { dg-error {'extension' attribute ignored} } */
+typedef int [[__extension__ unknown_attribute]] b3; /* { dg-error {'unknown_attribute' attribute ignored} } */
+typedef int [[__extension__ gnu:vector_size(4)]] b4; /* { dg-error {expected '\]' before ':'} } */
+/* { dg-error {'gnu' attribute ignored} "" { target *-*-* } .-1 } */
+typedef int [[__extension__ gnu JOIN2(:,:) vector_size (4)]] b5; /* { dg-error {pasting ":" and ":" does not give a valid preprocessing token} } */
+typedef int [[gnu::vector_size(4)]] b6; /* { dg-error {expected '\]' before ':'} } */
+/* { dg-error {'gnu' attribute ignored} "" { target *-*-* } .-1 } */
+/* { dg-error {attributes before C2X} "" { target *-*-* } .-2 } */
+typedef int [[gnu : : vector_size(4)]] b7; /* { dg-error {expected '\]' before ':'} } */
+/* { dg-error {'gnu' attribute ignored} "" { target *-*-* } .-1 } */
+/* { dg-error {attributes before C2X} "" { target *-*-* } .-2 } */
+typedef int [[gnu : vector_size(4)]] b8; /* { dg-error {expected '\]' before ':'} } */
+/* { dg-error {'gnu' attribute ignored} "" { target *-*-* } .-1 } */
+/* { dg-error {attributes before C2X} "" { target *-*-* } .-2 } */
diff --git a/gcc/testsuite/gcc.dg/c2x-attr-syntax-7.c b/gcc/testsuite/gcc.dg/c2x-attr-syntax-7.c
new file mode 100644
index 00000000000..702f733b171
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c2x-attr-syntax-7.c
@@ -0,0 +1,60 @@
+/* Test C2x attribute syntax: use of __extension__ in C11 mode.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c2x -pedantic-errors -Wc11-c2x-compat" } */
+
+#define FOO ::
+#define BAR :
+#define JOIN(A, B) A/**/B
+#define JOIN2(A, B) A##B
+
+typedef int [[__extension__ gnu::vector_size (4)]] g1;
+typedef int [[__extension__ gnu :: vector_size (4)]] g2;
+typedef int [[__extension__ gnu : : vector_size (4)]] g3;
+typedef int [[__extension__ gnu: :vector_size (4)]] g4;
+typedef int [[__extension__ gnu FOO vector_size (4)]] g5;
+typedef int [[__extension__ gnu BAR BAR vector_size (4)]] g6;
+typedef int [[__extension__ gnu :/**/: vector_size (4)]] g7;
+typedef int [[__extension__ gnu JOIN(:,:) vector_size (4)]] g8;
+typedef int [[__extension__ gnu :: vector_size (sizeof (void (*)(...)))]] g10;
+typedef int [[__extension__]] g11;
+typedef int [[__extension__,]] g12;
+typedef int [[__extension__, ,,,, ,, ,]] g13;
+[[__extension__ deprecated]] int g14 ();
+[[__extension__ nodiscard]] int g15 ();
+[[__extension__ noreturn]] void g16 ();
+
+int
+cases (int x)
+{
+  switch (x)
+    {
+    case 1:
+    case 2:
+    case 4:
+      x += 1;
+      [[__extension__ fallthrough]];
+    case 19:
+    case 33:
+      x *= 2;
+      [[fallthrough]];  /* { dg-warning {attributes before C2X} } */
+    case 99:
+      return x;
+
+    default:
+      return 0;
+    }
+}
+
+typedef int [[__extension__ vector_size (4)]] b1; /* { dg-error {'vector_size' attribute ignored} } */
+typedef int [[__extension__ __extension__]] b2; /* { dg-error {'extension' attribute ignored} } */
+typedef int [[__extension__ unknown_attribute]] b3; /* { dg-error {'unknown_attribute' attribute ignored} } */
+typedef int [[__extension__ gnu:vector_size(4)]] b4; /* { dg-error {expected '\]' before ':'} } */
+/* { dg-error {'gnu' attribute ignored} "" { target *-*-* } .-1 } */
+typedef int [[__extension__ gnu JOIN2(:,:) vector_size (4)]] b5;
+typedef int [[gnu::vector_size(4)]] b6; /* { dg-warning {attributes before C2X} } */
+typedef int [[gnu : : vector_size(4)]] b7; /* { dg-error {expected '\]' before ':'} } */
+/* { dg-error {'gnu' attribute ignored} "" { target *-*-* } .-1 } */
+/* { dg-warning {attributes before C2X} "" { target *-*-* } .-2 } */
+typedef int [[gnu : vector_size(4)]] b8; /* { dg-error {expected '\]' before ':'} } */
+/* { dg-error {'gnu' attribute ignored} "" { target *-*-* } .-1 } */
+/* { dg-warning {attributes before C2X} "" { target *-*-* } .-2 } */
  

On Fri, 18 Aug 2023, Richard Sandiford via Gcc-patches wrote:

> [[]] 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.

OK.