[PATCH] D11916: [CONCEPTS] Add diagnostic; invalid tag when concept specified

[Hubert Tong via cfe-commits]

>From Aaron's description of the user experience, I think the
err_concept_decl_non_template message text is good (although
err_concept_decl_non_template might need to be renamed).

-- HT

On Tue, Aug 11, 2015 at 4:01 PM, Aaron Ballman <aaron.ballman at gmail.com>
wrote:

> On Tue, Aug 11, 2015 at 3:36 PM, Nathan Wilson <nwilson20 at gmail.com>
> wrote:
> >
> >
> > On Tue, Aug 11, 2015 at 8:54 AM, Aaron Ballman <aaron.ballman at gmail.com>
> > wrote:
> >>
> >> On Mon, Aug 10, 2015 at 3:00 PM, Nathan Wilson <nwilson20 at gmail.com>
> >> wrote:
> >> > nwilson created this revision.
> >> > nwilson added reviewers: rsmith, hubert.reinterpretcast, fraggamuffin,
> >> > faisalv, aaron.ballman.
> >> > nwilson added a subscriber: cfe-commits.
> >> >
> >> > Adding check to emit diagnostic for invalid tag when concept is
> >> > specified and associated tests.
> >> >
> >> > http://reviews.llvm.org/D11916
> >> >
> >> > Files:
> >> >   include/clang/Basic/DiagnosticSemaKinds.td
> >> >   lib/Sema/SemaDecl.cpp
> >> >   test/SemaCXX/cxx-concept-declaration.cpp
> >> >
> >> > Index: test/SemaCXX/cxx-concept-declaration.cpp
> >> > ===================================================================
> >> > --- test/SemaCXX/cxx-concept-declaration.cpp
> >> > +++ test/SemaCXX/cxx-concept-declaration.cpp
> >> > @@ -23,3 +23,13 @@
> >> >  template<typename T>
> >> >  concept bool D6; // expected-error {{variable concept declaration
> must
> >> > be initialized}}
> >> >
> >> > +// Tag
> >> > +concept class CC1 {}; // expected-error {{class cannot be marked
> >> > concept}}
> >> > +concept struct CS1 {}; // expected-error {{struct cannot be marked
> >> > concept}}
> >> > +concept union CU1 {}; // expected-error {{union cannot be marked
> >> > concept}}
> >> > +concept enum CE1 {}; // expected-error {{enum cannot be marked
> >> > concept}}
> >> > +template <typename T> concept class TCC1 {}; // expected-error
> {{class
> >> > cannot be marked concept}}
> >> > +template <typename T> concept struct TCS1 {}; // expected-error
> >> > {{struct cannot be marked concept}}
> >> > +template <typename T> concept union TCU1 {}; // expected-error
> {{union
> >> > cannot be marked concept}}
> >> > +
> >> > +extern concept bool ExtC; // expected-error {{'concept' can only
> appear
> >> > on the definition of a function template or variable template}}
> >> > Index: lib/Sema/SemaDecl.cpp
> >> > ===================================================================
> >> > --- lib/Sema/SemaDecl.cpp
> >> > +++ lib/Sema/SemaDecl.cpp
> >> > @@ -3662,6 +3662,19 @@
> >> >      return TagD;
> >> >    }
> >> >
> >> > +  if (DS.isConceptSpecified()) {
> >> > +    // C++ Concepts TS [dcl.spec.concept]p1: A concept definition
> >> > refers to
> >> > +    // either a function concept and its definition or a variable
> >> > concept and
> >> > +    // its initializer.
> >> > +    if (Tag)
> >> > +      Diag(DS.getConceptSpecLoc(), diag::err_concept_tag)
> >> > +          << GetDiagnosticTypeSpecifierID(DS.getTypeSpecType());
> >> > +    else
> >> > +      Diag(DS.getConceptSpecLoc(),
> >> > diag::err_concept_decl_non_template);
> >> > +    // Don't emit warnings after this error.
> >> > +    return TagD;
> >> > +  }
> >>
> >> I'm not certain I understand why we need two different diagnostics for
> >> this case. I think err_concept_decl_non_template is sufficiently clear
> >> for both.
> >>
> >> ~Aaron
> >
> >
> > This was based on how constexpr handles these checks.
> >
> > Perhaps someone can correct me if I'm wrong, but I believe the idea is
> that
> > when the `struct` tag exists, for example, we think the user meant to
> > declare a `struct` and not the start of a `concept` declaration. So, the
> > `concept` specifier would be erroneous and we try give a more helpful
> > diagnostic.
>
> It could just be me, but I don't find the new diagnostic particularly
> helpful. "foo cannot be marked concept" tells me why I have an error
> but does not tell me what to do about it. "'concept' can only appear
> on the definition of a function template or variable template" tells
> me what I need to do to not have the error in the first place, as well
> as why I have the error.
>
> However, others may have differing opinions on the subject.
>
> ~Aaron
>
> >
> > I would need to add/fix the test case for this, but I tend to think the
> > declaration such as `concept bool;` could be the users intention to try
> to
> > create a `concept` declaration which is where the
> > err_concept_decl_non_template comes in.
> >
> >>
> >>
> >> > +
> >> >    DiagnoseFunctionSpecifiers(DS);
> >> >
> >> >    if (DS.isFriendSpecified()) {
> >> > Index: include/clang/Basic/DiagnosticSemaKinds.td
> >> > ===================================================================
> >> > --- include/clang/Basic/DiagnosticSemaKinds.td
> >> > +++ include/clang/Basic/DiagnosticSemaKinds.td
> >> > @@ -1973,6 +1973,8 @@
> >> >    "function concept declaration must be a definition">;
> >> >  def err_var_concept_not_initialized : Error<
> >> >    "variable concept declaration must be initialized">;
> >> > +def err_concept_tag : Error<
> >> > +  "%select{class|struct|interface|union|enum}0 cannot be marked
> >> > concept">;
> >> >
> >> >  // C++11 char16_t/char32_t
> >> >  def warn_cxx98_compat_unicode_type : Warning<
> >> >
> >> >
> >
> >
>
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