[PATCH] D36572: Implemented P0409R2 - Allow lambda capture [=, this]

John McCall via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Mon Aug 14 10:56:18 PDT 2017

rjmccall added inline comments.

Comment at: lib/Sema/SemaLambda.cpp:959
+                         ? diag::ext_equals_this_lambda_capture_cxx2a
+                         : diag::warn_cxx1z_compat_equals_this_lambda_capture);
hamzasood wrote:
> faisalv wrote:
> > Shouldn't we try and hit the 'continue' (that u deleted) if warnings (and extension warnings) are turned into errors? 
> That's an interesting scenario which admittedly I hadn't considered.
> I based this implementation on the [[ https://github.com/llvm-mirror/clang/blob/7602b13a8e8b5656afd6327d112b76b39f836e5b/lib/Sema/SemaLambda.cpp#L935 | '*this' capture handling ]] from the same loop. When a '*this' capture is seen pre-C++1z, an extension warning is emitted and then the capture is processed as normal (i.e. without consideration of that warning potentially becoming an error).
> I also looked at other places where extension warnings are emitted and I couldn't find any special handling for warnings becoming errors.
Yeah, I doubt there's a single place in the compiler where we stop processing code when warnings are turned into errors.

Comment at: test/FixIt/fixit-cxx0x.cpp:57
   (void)[&, &i, &i]{}; // expected-error 2{{'&' cannot precede a capture when the capture default is '&'}}
-  (void)[=, this]{ this->g(5); }; // expected-error{{'this' cannot be explicitly captured}}
   (void)[i, i]{ }; // expected-error{{'i' can appear only once in a capture list}}
rjmccall wrote:
> hamzasood wrote:
> > rjmccall wrote:
> > > hamzasood wrote:
> > > > rjmccall wrote:
> > > > > hamzasood wrote:
> > > > > > rjmccall wrote:
> > > > > > > Shouldn't you only be accepting this in C++2a mode?
> > > > > > I'm not sure what the system is with allowing future language features as extensions, but I noticed that [*this] capture is allowed as an extension pre-C++17 so I figured it would make sense for [=, this] to also be allowed as an extension (since the proposal mentions how it's meant to increase code clarify in the presence of [*this]).
> > > > > Surely there should at least be an on-by-default extension warning?  The behavior we're using sounds a lot more like we're treating this as a bug-fix in the standard than a new feature.  Richard, can you weigh in here?
> > > > The extension warning for this (ext_equals_this_lambda_capture_cxx2a) is on by default.
> > > Why did the diagnostic disappear from this file, then?
> > That file is for FixIt hints, which I don't think make much sense for an extension warning (and I couldn't find any other extension warnings that offer FixIt hints)
> Sure, it's reasonable for this specific test to not test the warning.  However, since I don't see anything in this test that actually turns off the warning, and since you have not in fact added any tests that verify that the warning is ever turned on, I suspect that it is actually not being emitted.
I'm sorry, I see that you've added an explicit test for the warning, but I still don't understand why the warning is not emitted in this file.  -verify normally verifies all diagnostics, and this file is tested with -std=c++11.  Is there some special behavior of -fixit that disables warnings, or ignores them in -verify mode?


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