[cfe-dev] Suppress secondary diagnostics for typo correction

[Richard Smith via cfe-dev]

On Fri, 30 Oct 2020 at 10:15, David Blaikie <dblaikie at gmail.com> wrote:

> A major goal of Clang's diagnostic experience is that if a fixit is
> suggested (such as a typo correction) then subsequent errors are exactly
> as-if the fixit had been applied. It sounds like your suggestion would go
> counter to that?
>
> I think there's probably a good argument to be made that not all typo
> corrections are high-confidence enough to merit a fixit on the error itself
> - if the fixit is on a note instead, the above requirement of recovery
> isn't applicable (so that's where we can put, say "did you mean if (a ==
> b)" as well as "did you mean if ((a = b))" fixits on alternative notes on
> the general -Wparentheses warning) - so perhaps having some level of typo
> correction confidence would be useful to determine which kind of recovery
> we should do - full recovery as if the user wrote the code (with a fixit
> hint attached to the error itself) or "well, we're not sure but here's out
> best guess" where an invalid expr is created and the fixit hint is attached
> to a note with some wording that's a bit more vague/conveys the increased
> uncertainty compared to the former case.
>

Right. It's an explicit goal to recover as if the typo-correction is
applied, in the case where we're confident that it's right. Currently we
get that confidence by checking the enclosing context in which the typo
appears is valid once the correction is applied. But that's imperfect in
various ways -- one of them is that the context we check is a little too
narrow sometimes; another (the issue in this case) is that making the
enclosing context be valid is not really sufficient to know that the typo
correction actually makes sense.

Perhaps we could add some further heuristics to determine whether the
result of typo correction seems reasonable before deciding we're confident
it's correct; I could imagine, for example, annotating warnings with a
"causes typo correction to be considered 'bad'" flag, in much the same way
as we have a "causes SFINAE failure" flag, and using that to validate
corrections -- that is, reject typo corrections not only if they would make
the code invalid, but also if they would produce a warning that suggests
the code is unlikely to be what the user intended. (In this case I think
the warning is actually produced after we've finished correcting the typo,
though that's probably not all that hard to fix.)

On Fri, Oct 30, 2020 at 7:34 AM Haojian Wu via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> Hello folks,
>>
>> Given the following case:
>>
>> void free();
>> void test() {
>>    if (!force) {} // diagnostic 1:  use of undeclared identifier 'force';
>> did you mean 'free'?
>>                      // diagnostic 2:  warning: address of function
>> 'free' will always evaluate to 'true'
>> }
>>
>> The secondary diagnostic seems to be bogus, and it doesn't reflect the
>> written source code, which can easily cause confusions. My idea is to use a
>> dependent RecoveryExpr (which wraps the typo-correct AST node) to suppress
>> all secondary diagnostics.
>>
>> I have a prototype at https://reviews.llvm.org/D90459. I see some
>> improvements, but there are some regressions as well:
>>
>> Improvements
>> - the resulting AST look better because the error is visible in the AST
>> (with RecoveryExpr node)
>> - we emit more typo corrections for more cases, see [1]
>> <https://reviews.llvm.org/differential/changeset/?ref=2240247>, [2]
>> <https://reviews.llvm.org/differential/changeset/?ref=2240248>
>>
>> Regressions
>> - recursive/nested typo corrections, e.g. `TypoX.TypoY;`, we emit just 1
>> typo-correction while the old behavior emits two, see [1]
>> <https://reviews.llvm.org/differential/changeset/?ref=2240254>
>> - ambiguous typos, when there are ambiguous typo candidates (they have
>> the same edit distance), the old one seems to perform better in some cases,
>> see [1] <https://reviews.llvm.org/differential/changeset/?ref=2240246>
>> - other misc regressions, I think we could fix them
>>
>> The secondary diagnostics are not wrong from the AST perspective, but
>> they seem to be unnecessary. In clangd, we'd like to suppress all secondary
>> diagnostics, but I'm not sure this is a right choice for clang.
>>
>
> That would seem unfortunate to me - clang works pretty hard on diagnostic
> recovery so users can see/act on multiple diagnostics in one pass. Though I
> realize that model is a bit different if you're dealing with an editor
> that's recompiling after every textual change - is that always the case for
> clangd? I think it might still be useful to see more than one error in an
> IDE/editor's error list, and certainly if I were dealing with some code
> that's slow to compile or an editor that chooses to do less fine-grained
> recompiles.
>
>
>>
>> What do people think?
>>
>>
>> Thanks,
>> Haojian
>>
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>
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