[cfe-dev] Marking declarations as invalid

[Nathan Wilson via cfe-dev]

On Thu, Nov 5, 2015 at 5:56 PM, Richard Smith <richard at metafoo.co.uk> wrote:

> On Thu, Nov 5, 2015 at 3:40 PM, Nathan Wilson <nwilson20 at gmail.com> wrote:
>
>> On Nov 5, 2015 5:18 PM, "Richard Smith" <richard at metafoo.co.uk> wrote:
>> >
>> > On Thu, Oct 29, 2015 at 8:04 PM, Nathan Wilson via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>> >>
>> >> Hi,
>> >>
>> >> Would someone mind explaining when (or when not) to mark a
>> declaration, i.e. FunctionDecl or VarDecl, an invalid declaration?
>> >>
>> >> Then, if the declaration is marked invalid, at what do we look at/use
>> that information?
>> >
>> > Here are some guidelines:
>> >
>> >   * You must not mark a declaration as invalid unless you have emitted
>> an error.
>> >   * If you emitted an error with a fix-it hint, you should not mark the
>> declaration as invalid (and you should recover as if the fix-it hint were
>> applied). We would like this to be a hard rule, but there are existing
>> cases where we don't follow this.
>> >   * You should mark a declaration as invalid if you think it's
>> sufficiently far from the user's intent that follow-on diagnostics should
>> be suppressed.
>> >   * You must mark a declaration as invalid if it does not correspond to
>> anything like a valid declaration (if it would be unreasonable to expect
>> downstream code to do the right thing with it).
>>
>> Thanks for this Richard. That helps a lot.
>>
>> So, I could (should?) have used a fix-it hint in the example above and
>> not mark the declaration invalid, correct? (Aside from your third bullet.)
>>
> Ah, well... you should only put a fix-it hint on an error when you are
> extremely confident you know what the user did wrong. People find it very
> frustrating when we incorrectly guess what they did wrong, and then produce
> follow-on diagnostics about our bad guess.
>

Ahh, yeah, that makes sense. Understandable as well.

> Would you be able to give an example of the fourth bullet and/or point to
>> where the downstream code would do the right thing?
>>
> A variable with type 'void', or a redeclaration of a variable with a
> different type. You can think about it this way: when you're writing code
> that looks at an AST node, it's reasonable for you to assume that the AST
> node corresponds to some part of the underlying language (unless it's
> marked as invalid), so you don't need to re-check properties that the
> language itself guarantees (such as, all declarations of a function have a
> type that is in fact a function type). And this puts a requirement on code
> that creates AST nodes, to mark them as invalid if it cannot satisfy that
> assumption.
>

I see. Thanks for the explanation (and follows from the first three bullets
you gave).
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