Re: [PATCH] c++: Fix spurious fallthrough warning on break

[Jakub Jelinek <jakub at redhat dot com>]

On Mon, Feb 19, 2018 at 09:59:00PM +0530, Siddhesh Poyarekar wrote:
> 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.  Full
> testsuite run is in progress.
> 
> Siddhesh
> 
> 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.

C++ testcase shouldn't be put directly into g++.dg/ (though yes, several
have been), but into a subdirectory.  In this case, because it is a (bogus)
warning, it should best go into g++.dg/warn/, and perhaps best named as
g++.dg/warn/Wimplicit-fallthrough-3.C .

> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/nested-switch.C
> @@ -0,0 +1,30 @@
> +// 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 "-Werror=implicit-fallthrough" } */

I'd go just for -Wimplicit-fallthrough.

> +
> +int
> +foo (int c1, int c2, int c3)
> +{
> +  switch (c2)
> +    {
> +    case 0:   
> +      switch (c3) {

And turn the above line into
      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;
> +    }
> +}

Ok for trunk with those nits, thanks.

	Jakub