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

[Corentin Jabot via Phabricator via cfe-commits]

cor3ntin added inline comments.


================
Comment at: clang/lib/Sema/SemaDeclCXX.cpp:1905
+        if (SemaRef.LangOpts.CPlusPlus2b) {
+          if (!VD->getType()->isLiteralType(SemaRef.Context))
+            SemaRef.Diag(VD->getLocation(),
----------------
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.


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

https://reviews.llvm.org/D122249