c++: Allow mixing GNU/std-style attributes [PR69585]

cp_parser_attributes_opt doesn't accept GNU attributes followed by
[[]] attributes and vice versa; only a sequence of attributes of the
same kind.  That causes grief for code like:

  struct __attribute__ ((may_alias)) alignas (2) struct S { };

or

  #define EXPORT __attribute__((visibility("default")))
  struct [[nodiscard]] EXPORT F { };

It doesn't seem to a documented restriction, so this patch fixes the
problem.

However, the patch does not touch the C FE.  The C FE doesn't have
a counterpart to C++'s cp_parser_attributes_opt -- it only has
c_parser_transaction_attributes (which parses both GNU and [[]]
attributes), but that's TM-specific.  The C FE seems to use either
c_parser_gnu_attributes or c_parser_std_attribute_specifier_sequence.
As a consequence, this works:

  [[maybe_unused]] __attribute__((deprecated)) void f2 ();

but this doesn't:

  __attribute__((deprecated)) [[maybe_unused]] void f1 ();

I'm not sure what, if anything, should be done about this.

	PR c++/102399
	PR c++/69585

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_attributes_opt): Accept GNU attributes
	followed by [[]] attributes and vice versa.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/attrib65.C: New test.
	* g++.dg/ext/attrib66.C: New test.
	* g++.dg/ext/attrib67.C: New test.

4 files changed, 72 insertions, 3 deletions

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 3fc7344..535bf7e 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -28727,9 +28727,17 @@ cp_nth_tokens_can_be_attribute_p (cp_parser *parser, size_t n)
 static tree
 cp_parser_attributes_opt (cp_parser *parser)
 {
-  if (cp_next_tokens_can_be_gnu_attribute_p (parser))
-    return cp_parser_gnu_attributes_opt (parser);
-  return cp_parser_std_attribute_spec_seq (parser);
+  tree attrs = NULL_TREE;
+  while (true)
+    {
+      if (cp_next_tokens_can_be_gnu_attribute_p (parser))
+	attrs = attr_chainon (attrs, cp_parser_gnu_attributes_opt (parser));
+      else if (cp_next_tokens_can_be_std_attribute_p (parser))
+	attrs = attr_chainon (attrs, cp_parser_std_attribute_spec_seq (parser));
+      else
+	break;
+    }
+  return attrs;
 }
 
 /* Parse an (optional) series of attributes.
diff --git a/gcc/testsuite/g++.dg/ext/attrib65.C b/gcc/testsuite/g++.dg/ext/attrib65.C
new file mode 100644
index 0000000..0af1387
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/attrib65.C
@@ -0,0 +1,7 @@
+// PR c++/102399
+// { dg-do compile { target c++11 } }
+// Test mixing the GNU and standard forms of attributes.
+
+#define EXPORT __attribute__((visibility("default")))
+
+struct [[nodiscard]] EXPORT Foo { Foo(); };
diff --git a/gcc/testsuite/g++.dg/ext/attrib66.C b/gcc/testsuite/g++.dg/ext/attrib66.C
new file mode 100644
index 0000000..102ed70
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/attrib66.C
@@ -0,0 +1,27 @@
+// PR c++/69585
+// { dg-do compile { target c++11 } }
+
+struct __attribute__ ((aligned (2))) __attribute__ ((may_alias))
+S1 { };
+
+struct __attribute__ ((aligned (2))) [[gnu::may_alias]]
+S2 { };
+
+struct alignas (2) __attribute__ ((may_alias))
+S3 { };
+
+struct alignas (2) [[gnu::may_alias]]
+S4 { };
+
+
+struct __attribute__ ((may_alias)) __attribute__ ((aligned (2)))
+S1_2 { };
+
+struct [[gnu::may_alias]] __attribute__ ((aligned (2)))
+S2_2 { };
+
+struct __attribute__ ((may_alias)) alignas (2)
+S3_2 { };
+
+struct [[gnu::may_alias]] alignas (2)
+S4_2 { };
diff --git a/gcc/testsuite/g++.dg/ext/attrib67.C b/gcc/testsuite/g++.dg/ext/attrib67.C
new file mode 100644
index 0000000..a510766
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/attrib67.C
@@ -0,0 +1,27 @@
+// PR c++/69585
+// { dg-do compile { target c++11 } }
+// Test mixing the GNU and standard forms of attributes.
+
+__attribute__((deprecated)) [[maybe_unused]] void f1 ();
+[[maybe_unused]] __attribute__((deprecated)) void f2 ();
+[[maybe_unused]] __attribute__((deprecated)) [[nodiscard]] int f3 ();
+__attribute__((unused)) [[nodiscard]] __attribute__((deprecated)) int f4 ();
+
+struct [[maybe_unused]] __attribute__((aligned)) S1 { double d; };
+struct __attribute__((aligned)) [[maybe_unused]] S2 { double d; };
+
+enum E {
+  X [[maybe_unused]] __attribute__((unavailable)),
+  Y  __attribute__((unavailable)) [[maybe_unused]],
+};
+
+void
+g ([[maybe_unused]] __attribute__((unavailable)) int i1,
+   __attribute__((unavailable)) [[maybe_unused]] int i2)
+{
+  [[maybe_unused]] __attribute__((aligned)) int i3;
+  __attribute__((aligned)) [[maybe_unused]] int i4;
+
+[[maybe_unused]]
+lab:  __attribute__((cold));
+}