[SR-1554] //*/ does not close multi-line comment

[jckarter]

Previous ID	SR-1554
Radar	rdar://problem/26327545
Original Reporter	@jckarter
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, Parser, StarterBug
Assignee	None
Priority	Medium

md5: 2416cb9a05334e4c8d080874b91f289e

Issue Description:

//*/ should close a /* comment, but does not:

/*
//*/
print("wat")

[swift-ci]

Comment by Geoffrey Wiseman (JIRA)

I took a quick look at `Lexer.cpp`. I think I can see the problem.

Assuming you allow nested slash-star comments, which the lexer seems to allow, I am not sure how you're supposed to decide whether or not "//*/" is meant to open or close a comment. In the absence of any other information, I think it should probably do what it's already doing: assuming that the first valid interpretation – that of opening a comment – is the right one.

Did you have another heuristic in mind for how it should decide?

[jckarter]

geoffrey.wiseman (JIRA User) Good point, it's nonobvious what the right interpretation is here. Possibly, when lexing a nested /*, we could look ahead to see if there's a / immediately after the * and favor the */ interpretation.

[swift-ci]

Comment by Geoffrey Wiseman (JIRA)

In which case the lexer would prefer /* at the start and */ thereafter.

These would be legit:

/*/ comment /*/
/*/ comment /*/ code /*/ comment /*/

Granted, almost all of these scenarios are edge cases, and will come up infrequently enough that it's probably not THAT big a deal. Your suggestion doesn't seem complicated, and I can't think of a really ugly counter-example that seems to suggest the it's a problem. I'll give this a spin.

[belkadan]

Resetting assignee on all Starter Bugs not modified since 2018.