[PATCH] D86369: [Sema][MSVC] warn at dynamic_cast when /GR- is given

[Zequan Wu via Phabricator via cfe-commits]

zequanwu added inline comments.


================
Comment at: clang/lib/Sema/SemaCast.cpp:895
+  if (!Self.getLangOpts().RTTIData) {
+    bool isMSVC = Self.getDiagnostics().getDiagnosticOptions().getFormat() ==
+                  DiagnosticOptions::MSVC;
----------------
hans wrote:
> zequanwu wrote:
> > hans wrote:
> > > I'm not sure isMSVC is the best name (it's not clear what aspect is MSVC exactly -- in this case it's the diagnostics format).
> > > 
> > > It's possible to target MSVC both with clang-cl and with regular clang.
> > > 
> > > For example, one could use
> > > 
> > >   clang-cl /c /tmp/a.cpp
> > > 
> > > or
> > > 
> > >   clang -c /tmp/a.cpp -target i686-pc-windows-msvc19.11.0 -fms-extensions
> > > 
> > > 
> > > My understanding is that the purpose of "isMSVC" here is to try and detect if we're using clang-cl or clang so that the diagnostic can say "/GR-" or "-fno-rtti-data". So maybe it's better to call it "isClangCL" or something like that.
> > > 
> > > Also, I don't think we should check "isMSVC" in the if-statement below. We want the warning to fire both when using clang and clang-cl: as long as -fno-rtti-data or /GR- is used, the warning makes sense.
> > > 
> > > So I think the code could be more like:
> > > 
> > > ```
> > > if (!Self.getLangOpts().RTTIData && !DestPointee->isVoidType()) {
> > >   bool isClangCL = ...;
> > >   Self.Diag(...) << isClangCL;
> > > }
> > > ```
> > MSVC will warn even if the DestPointee is void type. What I thought is if invoked by clang-cl warn regardless of DeskPointee type. If invoked by clang, warn if it's not void type. 
> > https://godbolt.org/z/475q5v. I noticed MSVC won't warn at typeid if /GR- is given. Probably I should remove the warning in typeid.
> If it's true the casting to void* doesn't need RTTI data (I think it is, but would be good to verify), then it doesn't make sense to warn. We don't have to follow MSVC's behavior when it doesn't make sense :)
> 
> Similar reasoning for typeid() - I assume it won't work with /GR- also with MSVC, so warning about it probably makes sense.
In clang, I believe that dynamic_cast to void* doesn't use RTTI data: https://godbolt.org/z/Kbr7Mq
Looks like MSVC only warns if the operand of typeid is not pointer: https://godbolt.org/z/chcMcn



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  https://reviews.llvm.org/D86369/new/

https://reviews.llvm.org/D86369