c++: Fix spurious fallthrough warning on break

The C++ frontend generates a break that results in the fallthrough
warning misfiring in nested switch blocks where cases in the inner
switch block return, rendering the break pointless.  The fallthrough
detection in finish_break_stmt does not work either because the
condition is encoded as an IF_STMT and not a COND_EXPR.

Fix this by adding a condition for IF_STMT in the
langhooks.block_may_fallthru for C++.  Fix tested on x86_64.

gcc/cp
        * cp-objcp-common.c (cxx_block_may_fallthru): Add case for
	        IF_STMT.

gcc/testsuite
        * g++.dg/nested-switch.C: New test case.

From-SVN: r257843

gcc/cp/ChangeLog

+2018-02-20  Siddhesh Poyarekar  <siddhesh@sourceware.org>
+
+	* cp-objcp-common.c (cxx_block_may_fallthru): Add case for
+	IF_STMT.
+
 2018-02-20  Paolo Carlini  <paolo.carlini@oracle.com>
 
 	PR c++/84446

gcc/cp/cp-objcp-common.c

@@ -349,6 +349,11 @@ cxx_block_may_fallthru (const_tree stmt)
     case THROW_EXPR:
       return false;
 
+    case IF_STMT:
+      if (block_may_fallthru (THEN_CLAUSE (stmt)))
+	return true;
+      return block_may_fallthru (ELSE_CLAUSE (stmt));
+
     case SWITCH_STMT:
       return (!SWITCH_STMT_ALL_CASES_P (stmt)
 	      || !SWITCH_STMT_NO_BREAK_P (stmt)

gcc/testsuite/ChangeLog

+2018-02-20  Siddhesh Poyarekar  <siddhesh@sourceware.org>
+
+	* g++.dg/warn/Wimplicit-fallthrough-3.C: New test case.
+
 2018-02-20  Martin Liska  <mliska@suse.cz>
 
 	PR c/84310

gcc/testsuite/g++.dg/warn/Wimplicit-fallthrough-3.C

+// Verify that there are no spurious warnings in nested switch statements due
+// to the unnecessary break in the inner switch block.
+// { dg-do compile }
+// { dg-options "-Wimplicit-fallthrough" } */
+
+int
+foo (int c1, int c2, int c3)
+{
+  switch (c2)
+    {
+    case 0:   
+      switch (c3)	// { dg-bogus "may fall through" }
+	{
+	case 0:
+	  if (c1)
+	    return 1;
+	  else
+	    return 2;
+	  break;
+
+	default:
+	  return 3;
+	}
+
+    case 1: 
+      return 4;
+    default:
+      return 5;
+      break;
+    }
+}