c++: correct __has_attribute(init_priority)

Currently __has_attribute(init_priority) always returns true, even on
targets that don't actually support init priorities, and when using the
attribute on such targets we just get a hard error about them being
unsupported.  This makes it impossible to conditionally use the attribute
by querying __has_attribute.

This patch fixes this by including init_priority in the attribute table
only if the target supports init priorities.  Thus on such targets
__has_attribute(init_priority) will now return false and we'll treat it
as just another unrecognized attribute (e.g. using it gives a -Wattribute
warning instead of a hard error).

gcc/cp/ChangeLog:

	* tree.cc (cxx_attribute_table): Include init_priority entry
	only if SUPPORTS_INIT_PRIORITY.
	(handle_init_priority_attribute): Add ATTRIBUTE_UNUSED.  Assert
	SUPPORTS_INIT_PRIORITY is true.

gcc/testsuite/ChangeLog:

	* g++.dg/special/initpri3.C: New test.

2 files changed, 17 insertions, 13 deletions

diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
index 45348c5..c30bbeb 100644
--- a/gcc/cp/tree.cc
+++ b/gcc/cp/tree.cc
@@ -5010,8 +5010,10 @@ const struct attribute_spec cxx_attribute_table[] =
 {
   /* { name, min_len, max_len, decl_req, type_req, fn_type_req,
        affects_type_identity, handler, exclude } */
+#if SUPPORTS_INIT_PRIORITY
   { "init_priority",  1, 1, true,  false, false, false,
     handle_init_priority_attribute, NULL },
+#endif
   { "abi_tag", 1, -1, false, false, false, true,
     handle_abi_tag_attribute, NULL },
   { NULL, 0, 0, false, false, false, false, NULL, NULL }
@@ -5039,7 +5041,7 @@ const struct attribute_spec std_attribute_table[] =
 
 /* Handle an "init_priority" attribute; arguments as in
    struct attribute_spec.handler.  */
-static tree
+ATTRIBUTE_UNUSED static tree
 handle_init_priority_attribute (tree* node,
 				tree name,
 				tree args,
@@ -5103,18 +5105,10 @@ handle_init_priority_attribute (tree* node,
 	 pri);
     }
 
-  if (SUPPORTS_INIT_PRIORITY)
-    {
-      SET_DECL_INIT_PRIORITY (decl, pri);
-      DECL_HAS_INIT_PRIORITY_P (decl) = 1;
-      return NULL_TREE;
-    }
-  else
-    {
-      error ("%qE attribute is not supported on this platform", name);
-      *no_add_attrs = true;
-      return NULL_TREE;
-    }
+  gcc_assert (SUPPORTS_INIT_PRIORITY);
+  SET_DECL_INIT_PRIORITY (decl, pri);
+  DECL_HAS_INIT_PRIORITY_P (decl) = 1;
+  return NULL_TREE;
 }
 
 /* DECL is being redeclared; the old declaration had the abi tags in OLD,
diff --git a/gcc/testsuite/g++.dg/special/initpri3.C b/gcc/testsuite/g++.dg/special/initpri3.C
new file mode 100644
index 0000000..3d745a42
--- /dev/null
+++ b/gcc/testsuite/g++.dg/special/initpri3.C
@@ -0,0 +1,10 @@
+// Verify __has_attribute(init_priority) is false on targets that
+// don't support init priorities, and is treated as an unrecognized
+// attribute in that case.
+
+#if !__has_attribute(init_priority)
+#error unsupported // { dg-error "" "" { target { ! init_priority } } }
+#endif
+
+struct A { A(); } a __attribute__((init_priority(500)));
+// { dg-warning "attribute directive ignored" "" { target { ! init_priority } } .-1 }