thakis added inline comments.

================
Comment at: test/SemaCXX/auto-type-from-cxx.cpp:14
@@ +13,3 @@
+  auto _ = [](__auto_type f) {}; // expected-error {{'__auto_type' not allowed 
in lambda parameter}}
+  __auto_type g = 2;
+  struct BitField { int field:2; };
----------------
comex wrote:
> thakis wrote:
> > Shouldn't this say "warning: __auto_type is a gnu extension" (since this 
> > uses -std=c++14, not -std=gnu++14)?
> Hmm... when I added `ext_auto_type` to the .td, I didn't notice the 
> difference between Extension and ExtWarn.  Since the patch currently uses 
> Extension, it won't warn by default, but will with `-pedantic`, `-Wgnu`, or 
> `-Wgnu-auto-type`.  The C test uses `-pedantic` so it gets the warning.
> 
> Is there an explicit policy on which extensions should be ExtWarn?  Looking 
> at the rest of that file, ExtWarns seem to be mostly either (a) standardized 
> extensions (which will warn if `-std` is too early) and (b) 'extensions' that 
> are more like "Clang will accept your buggy code" than features.  Most GNU 
> extensions are just Extension, so I think it makes sense to do the same for 
> `__auto_type`.
> 
> I could update the C++ test to add `-pedantic`, but arguably it makes more 
> sense to test the fact that the warning is not emitted by default.
I'm not sure either. `typeof` warns with -std=c++14 but not with -std=gnu++14. 
From a user perspective, this makes sense to me: I want to write standard c++ 
and I want the compiler to help me with that, but I don't want to get all the 
pedantic warnings that -pedantic entails. For example, consider using clang-cl 
to build Windows code, and wanting MSVC to be able to build said Windows code 
too. (This can of course go wrong with standard C++ too, but in that case I'm 
running into MSVC bugs which will eventually be fixed.) So if it's possible to 
match how typeof works, I think that'd be good. If it's not possible, this 
wouldn't be the only GNU extension allowed in -std=c++14 mode though, so it's 
not a big thing.


http://reviews.llvm.org/D12686



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