[PATCH] D122249: [Clang] Add a compatibiliy warning for non-literals in constexpr.

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman accepted this revision.
aaron.ballman added a comment.

LGTM



================
Comment at: clang/lib/Sema/SemaDeclCXX.cpp:1905
+        if (SemaRef.LangOpts.CPlusPlus2b) {
+          if (!VD->getType()->isLiteralType(SemaRef.Context))
+            SemaRef.Diag(VD->getLocation(),
----------------
hubert.reinterpretcast wrote:
> cor3ntin wrote:
> > aaron.ballman wrote:
> > > aaron.ballman wrote:
> > > > cor3ntin wrote:
> > > > > hubert.reinterpretcast wrote:
> > > > > > This seems to trigger even when the type is dependent:
> > > > > > ```
> > > > > > <stdin>:1:36: warning: definition of a variable of non-literal type in a constexpr function is incompatible with C++ standards before C++2b [-Wpre-c++2b-compat]
> > > > > > auto qq = [](auto x) { decltype(x) n; };
> > > > > >                                    ^
> > > > > > 1 warning generated.
> > > > > > ```
> > > > > > 
> > > > > > This also seems to emit even when `Kind` is not `Sema::CheckConstexprKind::Diagnose` (unlike the `static`/`thread_local` case above). Is the `CheckLiteralType` logic not reusable for this?
> > > > > You are right, thanks for noticing that, it was rather bogus.
> > > > > The reason I'm not using CheckLiteralType is to avoid duplicating a diagnostics message, as CheckLiteralType doesn't allow us to pass parameter to the diagnostic message.
> > > > > 
> > > > > It leaves us with an uncovered scenario though: We do not emit the warning on template instantiation, and I don't think there is an  easy way to do that.
> > > > > The reason I'm not using CheckLiteralType is to avoid duplicating a diagnostics message, as CheckLiteralType doesn't allow us to pass parameter to the diagnostic message.
> > > > 
> > > > Huh?
> > > > 
> > > > ```
> > > > static bool CheckLiteralType(Sema &SemaRef, Sema::CheckConstexprKind Kind,
> > > >                              SourceLocation Loc, QualType T, unsigned DiagID,
> > > >                              Ts &&...DiagArgs) {
> > > >   ...
> > > > }
> > > > ```
> > > > I would hope `DiagArgs` should do exactly that? :-)
> > > > It leaves us with an uncovered scenario though: We do not emit the warning on template instantiation, and I don't think there is an easy way to do that.
> > > 
> > > I believe the code paths that lead us here all come from `Sema::CheckConstexprFunctionDefinition()` which is called from `Sema::ActOnFinishFunctionBody()` which seems to be called when instantiating templates in `Sema::InstantiateFunctionDefinition()`, so perhaps some more investigation is needed as to why we're not reaching this for template instantiations.
> > We could add something in addition of `Sema::CheckConstexprKind::CheckValid` and `Sema::CheckConstexprKind::Diagnose`, but 
> > 
> > * not for implicit lambdas, because we should not warn on lambdas that won't be constexpr
> > * for explicit constexpr lambdas / functions, it would force us to call CheckConstexprFunctionDefinition  on instanciation, which we currently don't do, and is not free for that one warning - and we would have to not-reemit the other warnings. It seems like quite a fair amount of work for a diagnostic not enabled by default.
> > so perhaps some more investigation is needed as to why we're not reaching this for template instantiations.
> 
> @aaron.ballman, do you have any position on whether we want this investigation before moving forward with this patch?
>>so perhaps some more investigation is needed as to why we're not reaching this for template instantiations.
> @aaron.ballman, do you have any position on whether we want this investigation before moving forward with this patch?

@hubert.reinterpretcast -- I think @cor3ntin did that investigation and found that we don't make it to this code path because of https://github.com/llvm/llvm-project/blob/main/clang/lib/Sema/SemaDecl.cpp#L14961.

I think this is incremental progress, and we can handle the template instantiation cases in a follow-up.


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  https://reviews.llvm.org/D122249/new/

https://reviews.llvm.org/D122249