c, c++: -Wswitch warning on [[maybe_unused]] enumerator [PR105497]

This PR complains that we emit the "enumeration value not handled in
switch" warning even though the enumerator was marked with the
[[maybe_unused]] attribute.

I couldn't just check TREE_USED, because the enumerator could have been
used earlier in the function, which doesn't play well with the
c_do_switch_warnings warning.  Instead, I had to check the attributes on
the CONST_DECL.  This is easy since the TYPE_VALUES of an enum type are
now consistent between C and C++, both of which store the CONST_DECL in
its TREE_VALUE.

	PR c++/105497

gcc/c-family/ChangeLog:

	* c-warn.cc (c_do_switch_warnings): Don't warn about unhandled
	enumerator when it was marked with attribute unused.

gcc/testsuite/ChangeLog:

	* c-c++-common/Wswitch-1.c: New test.
	* g++.dg/warn/Wswitch-4.C: New test.

3 files changed, 90 insertions, 2 deletions

diff --git a/gcc/c-family/c-warn.cc b/gcc/c-family/c-warn.cc
index cae8929..ea7335f 100644
--- a/gcc/c-family/c-warn.cc
+++ b/gcc/c-family/c-warn.cc
@@ -1738,8 +1738,8 @@ c_do_switch_warnings (splay_tree cases, location_t switch_location,
   for (chain = TYPE_VALUES (type); chain; chain = TREE_CHAIN (chain))
     {
       tree value = TREE_VALUE (chain);
-      if (TREE_CODE (value) == CONST_DECL)
-	value = DECL_INITIAL (value);
+      tree attrs = DECL_ATTRIBUTES (value);
+      value = DECL_INITIAL (value);
       node = splay_tree_lookup (cases, (splay_tree_key) value);
       if (node)
 	{
@@ -1769,6 +1769,13 @@ c_do_switch_warnings (splay_tree cases, location_t switch_location,
       /* We've now determined that this enumerated literal isn't
 	 handled by the case labels of the switch statement.  */
 
+      /* Don't warn if the enumerator was marked as unused.  We can't use
+	 TREE_USED here: it could have been set on the enumerator if the
+	 enumerator was used earlier.  */
+      if (lookup_attribute ("unused", attrs)
+	  || lookup_attribute ("maybe_unused", attrs))
+	continue;
+
       /* If the switch expression is a constant, we only really care
 	 about whether that constant is handled by the switch.  */
       if (cond && tree_int_cst_compare (cond, value))
diff --git a/gcc/testsuite/c-c++-common/Wswitch-1.c b/gcc/testsuite/c-c++-common/Wswitch-1.c
new file mode 100644
index 0000000..de9ee03
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/Wswitch-1.c
@@ -0,0 +1,29 @@
+/* PR c++/105497 */
+/* { dg-options "-Wswitch" } */
+
+enum E {
+  A,
+  B,
+  C __attribute((unused)),
+  D
+};
+
+void
+g (enum E e)
+{
+  switch (e)
+    {
+    case A:
+    case B:
+    case D:
+      break;
+    }
+
+  switch (e) // { dg-warning "not handled in switch" }
+    {
+    case A:
+    case B:
+    case C:
+      break;
+    }
+}
diff --git a/gcc/testsuite/g++.dg/warn/Wswitch-4.C b/gcc/testsuite/g++.dg/warn/Wswitch-4.C
new file mode 100644
index 0000000..553a57d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wswitch-4.C
@@ -0,0 +1,52 @@
+// PR c++/105497
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wswitch" }
+
+enum class Button
+{
+    Left,
+    Right,
+    Middle,
+    NumberOfButtons [[maybe_unused]]
+};
+
+enum class Sound
+{
+  Bark,
+  Meow,
+  Hiss,
+  Moo __attribute((unused))
+};
+
+enum class Chordata
+{
+  Urochordata,
+  Cephalochordata,
+  Vertebrata
+};
+
+int main()
+{
+  Button b = Button::Left;
+  switch (b) { // { dg-bogus "not handled" }
+        case Button::Left:
+        case Button::Right:
+        case Button::Middle:
+            break;
+    }
+
+  Sound s = Sound::Bark;
+  switch (s) { // { dg-bogus "not handled" }
+    case Sound::Bark:
+    case Sound::Meow:
+    case Sound::Hiss:
+      break;
+  }
+
+  Chordata c = Chordata::Vertebrata;
+  switch (c) { // { dg-warning "not handled" }
+    case Chordata::Cephalochordata:
+    case Chordata::Vertebrata:
+      break;
+  }
+}