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.

4 files changed, 193 insertions, 20 deletions

diff --git a/gcc/c/c-parser.cc b/gcc/c/c-parser.cc
index 4a4820d..2f3afb2 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 acd5317..2632867 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -11925,10 +11925,29 @@ 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 0000000..9e5f65c
--- /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 0000000..702f733
--- /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 } */