[llvm-dev] RFC: Modernizing our use of auto

Stephen Kelly via llvm-dev llvm-dev at lists.llvm.org
Sun Feb 3 10:30:51 PST 2019

On 03/02/2019 17:59, Mehdi AMINI wrote:
> On Sun, Feb 3, 2019 at 6:50 AM Stephen Kelly via llvm-dev 
> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>     On 31/12/2018 04:54, Chris Lattner via llvm-dev wrote:
>     >> Do those uses conform to the guide? If they don't, then should
>     the guide be updated? Are the types there 'obvious’?
>     >
>     > If/when we revise the policy, then it would make sense for
>     non-conforming uses of auto to be changed. However, I don’t think
>     that actually making a widespread change would be high priority...
>     >
>     >> How did all of those uses get into the codebase? Does it
>     indicate that the guide is not followed, or does it indicate that
>     the guide is too subjective, or that maybe the 'the type must be
>     obvios' guide does not reflect the 'reality at the coalface'
>     anymore? Should those uses of auto be changed?
>     >
>     > My understanding is that there has been no widely understood or
>     accepted policy, so different coders and reviewers are doing
>     different things.
>     One of the things which has no consensus here is whether 'auto' may be
>     used in lambdas (using c++-14). 
> Under the current guidelines, my understanding is that nothing 
> prevents to use auto in lambda in order to "make the code more 
> readable" when "the type is already obvious from the context" or "when 
> the type would have been abstracted away anyways, often behind a 
> container’s typedef such as std::vector<T>::iterator".

Some people seem to have objections to use of auto with range-for loops too.

There doesn't seem to be consensus on what is 'readable'. Some people claim
strongly that 'the type must be obvious. There even seems to be 
consensus on
that phrase, though it doesn't seem to actually apply - We have lots of 
uses of
auto with AST Matchers for example, and the type of the matcher is not 
in that code. We have lots of similar 'the type is not "obvious"' code 
using auto.

And yet,

  if (const auto *TSI = D->getTypeSourceInfo())

draws review comments that it must be changed.

> We don't need to update the guideline on auto to be able to use auto 
> in lambda as soon as c++14 is available.

That seems to depend on the reviewer, which means the code and the 
reviews will be inconsistent.

>     This feature was celebrated as a big
>     feature which gets unlocked by migrating to toolchains which provide
>     that feature:
>     https://groups.google.com/forum/#!msg/llvm-dev/0VkIuhn10nE/QZ5FwYEmHAAJ
>     So, does this need a guideline update?
>     Is there consistency in celbrating that but writing 'remove all use of
>     auto from this file' in reviews?
>     If there's no consensus and no consistency, what does that mean
>     for the
>     code?
>     Is
>        if (const auto *TSI = D->getTypeSourceInfo())
>     ok?
>     Some reviewers say 'no'. What is the consensus and how is that
>     expressed
>     in the guidelines?
>     Does anyone have any interest in making the guidelines more clear on
>     this?
>     I have made several proposals, and at least Chris agreed that
>     something
>     should be improved, but then he left the discussion.
>     Does anyone else think that something can be improved? Is anyone
>     willing
>     to read and comment on my proposal and get a change to the guidelines
>     committed?
> I think that multiple people read your proposal and gave feedback on 
> Phabricator that mandates a revision (for instance for-range loop). 
> Also in this topic I believe some feedback was given that rewording in 
> order to remove ambiguity is always good, as long as the "spirit" of 
> the current rule as it is written is preserved.
> So my take on the subject is that we're waiting on a new revision of 
> your patch?

We deliberately took the discussion off Phab and onto the mailing list
to try to get more-fruitful discussion. For example in


I suggested that 'New guidelines should...' and then wrote some
content. If we don't agree on 'what new guidelines should do', then
perhaps it is not time for a Phab patch yet?

Thanks for responding to that mail! :) I responded to you, but the
discussion again did not progress. Is there just not enough interest
in this? I'm getting the impression the topic should be dropped, but
maybe I'm missing something?



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