[cfe-dev] Reflection/metaparsing implem

[David Rector via cfe-dev]

Well then for any such "volatile" stuff you’d either CLANG_NO_REFLECT it or define a CLANG_RELFECT_VOLATILE so that you’d get always get a warning if it was used.

Surely a full parallel reflection property processing system to do such filtering is going to require more maintenance than anything we’re talking about here.

> On Nov 11, 2019, at 5:25 PM, David Blaikie <dblaikie at gmail.com> wrote:
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> On Mon, Nov 11, 2019 at 2:08 PM David Rector <davrec at gmail.com <mailto:davrec at gmail.com>> wrote:
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>> On Nov 11, 2019, at 4:09 PM, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
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>> On Mon, Nov 11, 2019 at 1:01 PM James Y Knight <jyknight at google.com <mailto:jyknight at google.com>> wrote:
>> On Mon, Nov 11, 2019 at 3:03 PM David Blaikie via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote: 
>> I think that'd still be a pretty hard sell - Clang's APIs and AST are volatile & intentionally so. People writing against the Clang APIs themselves with things like clang-tidy are aware of that. If we sold this as a feature to general C++ users, I don't think there would be a way to sufficiently communicate how volatile this will be - and end up backing ourselves into a corner & limiting the ability to change Clang.
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>> Does Clang's AST API _need_ to be unstable in order to keep up with language changes, or is it just a convenience?
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>> I imagine with sufficient heroics the AST could be maintained for a particular version - and /probably/ version-over-version (so the C++23 AST was backwards compatible with the C++20 AST, etc). In the same way that the language itself is mostly backwards compatible version over version.
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>> Is this concern indicative of problems that a standardized AST API would also have, or is unique to Clang's current API?
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>> The concern is mostly indicative of a very high fidelity API - the lower level/more bare the API, the more brittle it'll be to changes (both refactoring improvements to Clang, and necessary changes to accommodate new language features). Standardizing something a little higher level could provide a lot more flexibility under the hood.
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> Volatility issues could be managed. E.g. I’ve already implem’d so that CLANG_NO_REFLECT turns off reflection for any AST public const method or field — but note that it is a crutch; any public const method is either a) eventually going to be asked to be reflected or b) probably shouldn’t be a public const method.
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> Having something be "public" within Clang's codebase and users of Clang's APIs that are explicitly documented as volatile is quite different from it being public across significant amounts of end-user C++ code.
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> It makes a fair bit of sense for a lot of things to be public within the former scope that aren't suitable to be public within the latter scope.
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> The goal is that people may ask for (a) but we may reasonably push back and say that that level of fidelity maybe isn't worth the maintenance burden to all C++ compilers indefinitely into the future.
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>  (Reflection is an opportunity to look in the mirror — better to hit the gym than to design a funhouse mirror to fix the issue.)
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> I imagine Other AST attributes — e.g. CLANG_REFLECT_DEPRECATED — could be defined so that the tool generates warnings and fixits whenever they are used.
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> Historically the C++ standard hasn't been terribly open to significant deprecation/feature removal - it's probably not practical to plan on a reflection API that would need that to maintain... maintainability. 

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