Explicitly document that the "counted_by" attribute is only supported in C.

The "counted_by" attribute currently is only supported in C, mention this
explicitly in documentation and also issue warnings when see "counted_by"
attribute in C++ with -Wattributes.

gcc/c-family/ChangeLog:

	* c-attribs.cc (handle_counted_by_attribute): Is ignored and issues
	warning with -Wattributes in C++ for now.

gcc/ChangeLog:

	* doc/extend.texi: Explicitly mentions counted_by is available
	only in C for now.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/flex-array-counted-by.C: New test.
	* g++.dg/ext/flex-array-counted-by-2.C: New test.

4 files changed, 37 insertions, 1 deletions

diff --git a/gcc/c-family/c-attribs.cc b/gcc/c-family/c-attribs.cc
index cf27cd6..7930351 100644
--- a/gcc/c-family/c-attribs.cc
+++ b/gcc/c-family/c-attribs.cc
@@ -2867,8 +2867,16 @@ handle_counted_by_attribute (tree *node, tree name,
   tree argval = TREE_VALUE (args);
   tree old_counted_by = lookup_attribute ("counted_by", DECL_ATTRIBUTES (decl));
 
+  /* This attribute is not supported in C++.  */
+  if (c_dialect_cxx ())
+    {
+      warning_at (DECL_SOURCE_LOCATION (decl), OPT_Wattributes,
+		  "%qE attribute is not supported for C++ for now, ignored",
+		  name);
+      *no_add_attrs = true;
+    }
   /* This attribute only applies to field decls of a structure.  */
-  if (TREE_CODE (decl) != FIELD_DECL)
+  else if (TREE_CODE (decl) != FIELD_DECL)
     {
       error_at (DECL_SOURCE_LOCATION (decl),
 		"%qE attribute is not allowed for a non-field"
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index 5845bce..ebfa677 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -7926,6 +7926,10 @@ The @code{counted_by} attribute may be attached to the C99 flexible array
 member of a structure.  It indicates that the number of the elements of the
 array is given by the field "@var{count}" in the same structure as the
 flexible array member.
+
+This attribute is available only in C for now.
+In C++ this attribute is ignored.
+
 GCC may use this information to improve detection of object size information
 for such structures and provide better results in compile-time diagnostics
 and runtime features like the array bound sanitizer and
diff --git a/gcc/testsuite/g++.dg/ext/flex-array-counted-by-2.C b/gcc/testsuite/g++.dg/ext/flex-array-counted-by-2.C
new file mode 100644
index 0000000..6ac2b50
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/flex-array-counted-by-2.C
@@ -0,0 +1,13 @@
+/* Testing the fact that the attribute counted_by is not supported in C++.  */
+/* { dg-do compile { target c++11 } } */
+/* { dg-options "-Wattributes" } */
+
+struct trailing {
+  int count;
+  int field [[gnu::counted_by (count)]] []; /* { dg-warning "attribute is not supported for C\\+\\+ for now, ignored" } */
+};
+
+struct trailing1 {
+  int count1;
+  [[gnu::counted_by (count)]] int field []; /* { dg-warning "attribute is not supported for C\\+\\+ for now, ignored" } */
+};
diff --git a/gcc/testsuite/g++.dg/ext/flex-array-counted-by.C b/gcc/testsuite/g++.dg/ext/flex-array-counted-by.C
new file mode 100644
index 0000000..8bc79d4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/flex-array-counted-by.C
@@ -0,0 +1,11 @@
+/* Testing the fact that the attribute counted_by is not supported in C++.  */
+/* { dg-do compile } */
+/* { dg-options "-Wattributes" } */
+
+int size;
+int x __attribute ((counted_by (size))); /* { dg-warning "attribute is not supported for C\\+\\+ for now, ignored" } */
+
+struct trailing {
+  int count;
+  int field[] __attribute ((counted_by (count))); /* { dg-warning "attribute is not supported for C\\+\\+ for now, ignored" } */
+};