c++: #pragma doesn't disable -Wunused-label [PR113582]

The PR complains that

  void do_something(){
    #pragma GCC diagnostic push
    #pragma GCC diagnostic ignored "-Wunused-label"
    start:;
    #pragma GCC diagnostic pop
  } #1

doesn't work.  That's because we warn_for_unused_label only while we're
in finish_function, meaning we're at #1 where we're outside the #pragma
region.  We can use suppress_warning + warning_suppressed_p to fix this.

Note that I'm not using TREE_USED.  Propagating it in tsubst_stmt/LABEL_EXPR
from decl to label would mean that we don't warn in do_something2, but
I think we want the warning there: we're in a template and the goto is
a discarded statement.

	PR c++/113582

gcc/c-family/ChangeLog:

	* c-warn.cc (warn_for_unused_label): Don't warn if -Wunused-label has
	been suppressed for the label.

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_label_for_labeled_statement): suppress_warning
	if it's not enabled at input_location.
	* pt.cc (tsubst_stmt): Call copy_warning.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wunused-label-4.C: New test.

4 files changed, 42 insertions, 6 deletions

diff --git a/gcc/c-family/c-warn.cc b/gcc/c-family/c-warn.cc
index bff87be..5b2d680 100644
--- a/gcc/c-family/c-warn.cc
+++ b/gcc/c-family/c-warn.cc
@@ -2185,7 +2185,9 @@ warn_for_unused_label (tree label)
 {
   if (!TREE_USED (label))
     {
-      if (DECL_INITIAL (label))
+      if (warning_suppressed_p (label, OPT_Wunused_label))
+	/* Don't warn.  */;
+      else if (DECL_INITIAL (label))
 	warning (OPT_Wunused_label, "label %q+D defined but not used", label);
       else
 	warning (OPT_Wunused_label, "label %q+D declared but not defined", label);
diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index c419120..7306ce9 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -13108,7 +13108,11 @@ cp_parser_label_for_labeled_statement (cp_parser* parser, tree attributes)
       /* Anything else must be an ordinary label.  */
       label = finish_label_stmt (cp_parser_identifier (parser));
       if (label && TREE_CODE (label) == LABEL_DECL)
-	FALLTHROUGH_LABEL_P (label) = fallthrough_p;
+	{
+	  FALLTHROUGH_LABEL_P (label) = fallthrough_p;
+	  if (!warning_enabled_at (input_location, OPT_Wunused_label))
+	    suppress_warning (label, OPT_Wunused_label);
+	}
       break;
     }
 
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 1816bfd..8787eab 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -18837,11 +18837,12 @@ tsubst_stmt (tree t, tree args, tsubst_flags_t complain, tree in_decl)
     case LABEL_EXPR:
       {
 	tree decl = LABEL_EXPR_LABEL (t);
-	tree label;
-
-	label = finish_label_stmt (DECL_NAME (decl));
+	tree label = finish_label_stmt (DECL_NAME (decl));
 	if (TREE_CODE (label) == LABEL_DECL)
-	  FALLTHROUGH_LABEL_P (label) = FALLTHROUGH_LABEL_P (decl);
+	  {
+	    FALLTHROUGH_LABEL_P (label) = FALLTHROUGH_LABEL_P (decl);
+	    copy_warning (label, decl);
+	  }
 	if (DECL_ATTRIBUTES (decl) != NULL_TREE)
 	  cplus_decl_attributes (&label, DECL_ATTRIBUTES (decl), 0);
       }
diff --git a/gcc/testsuite/g++.dg/warn/Wunused-label-4.C b/gcc/testsuite/g++.dg/warn/Wunused-label-4.C
new file mode 100644
index 0000000..d194f04
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wunused-label-4.C
@@ -0,0 +1,29 @@
+// PR c++/113582
+// { dg-do compile { target c++17 } }
+// { dg-options "-Wunused-label" }
+
+template<bool B> void
+do_something ()
+{
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wunused-label"
+start:
+  if constexpr(B)
+    goto start;
+#pragma GCC diagnostic pop
+}
+
+template<bool B> void
+do_something2 ()
+{
+start: // { dg-warning "defined but not used" }
+  if constexpr(B)
+    goto start;
+}
+
+void
+g ()
+{
+  do_something<0>();
+  do_something2<0>();
+}