C++20 [[likely]]/[[unlikely]] case statements erroneously fail to compile in the presence of [[fallthrough]]

[imallett]

Bugzilla Link	49454
Resolution	DUPLICATE
Resolved on	Oct 11, 2021 09:06
Version	unspecified
OS	All
CC	@AaronBallman,@mordante,@nico

Extended Description

See the following complete example:

//Compile with "-std=c++20"
void f(int i) {
	switch (i) {
		[[likely]] case 1:
			[[fallthrough]];
		case 2:
			break;
	}
}

This should compile (and does on every other compiler), but doesn't on Clang. Clang (tested 13.0.0) produces:

<source>:5:13: error: fallthrough annotation does not directly precede switch label
            [[fallthrough]];
            ^

Removing either [[likely]] or [[fallthrough]]; is sufficient to cause the compile to succeed.

[AaronBallman]

I can confirm that the issue occurs. From some early debugging, it looks like markFallthroughVisited() is never being called, likely because we're not peeking through attributed statements somewhere when comparing against case and label statements.

[nico]

My current WIP patch for bug 52103 happens to fix this too, so let's dupe it over.

*** This bug has been marked as a duplicate of bug llvm/llvm-bugzilla-archive#52103 ***

[nico]

mentioned in issue llvm/llvm-bugzilla-archive#52103