[PATCH] [RFC] PR20146 - Cannot return void with qualifiers

[Richard Smith]

It's a little confusing to put "X cannot have Y" in an ExtWarn; we usually
phrase such things as "X has Y" or "ISO C does not allow X to have Y" to
make it clear that we may be allowing this despite the standard saying
otherwise. Also, the trailing %0 in the diagnostic text doesn't seem to fit
into the sentence very well. Suggestion:

  ISO C does not allow %0 qualifier%plural{1:|:s} on void return type

... and use Sema::diagnoseIgnoredQualifiers to produce a diagnostic with
the relevant qualifiers listed.

Other than the diagnostic wording, this looks good to me, thanks.

On Thu, Jul 10, 2014 at 7:57 AM, Zach Davis <zdavkeos at gmail.com> wrote:

> Thanks for the suggestions. In this patch:
>
> - The error is now a warning with a "return-qualified-void" group
> - The warning only applies to C code
> - Re-formatted the code
>
> The warning is currently on by default.
>
>
> On Tue, Jul 8, 2014 at 7:29 PM, Richard Smith <richard at metafoo.co.uk>
> wrote:
> >                                                unsigned
> FunctionChunkIndex)
> > {
> > +
> > +  // C99 6.9.1/3: The return type of a function shall be void or
> > +  // an object type other than array type.
> > +  // A return type of void cannot be qualified.
> >
> >
> > Micro-nit: no blank line at the start of a function body.
> >
> > This should only apply in C; 'const void' is explicitly a valid return
> type
> > in C++ (see for instance 6.6.3/2, "a function with the return type cv
> > void").
> >
> > Please make this an ExtWarn rather than an error, since GCC accepts it by
> > default (and doesn't even diagnose it if it's on a non-definition
> function
> > declaration), and it seems thoroughly harmless.
> >
> > On Mon, Jul 7, 2014 at 9:22 AM, Zach Davis <zdavkeos at gmail.com> wrote:
> >>
> >> Thanks for the comments.  I have:
> >>
> >> - Cleaned up the code
> >> - Made the warning an error
> >> - Moved the check into diagnoseIgnoredFunctionQualifiers()
> >> - Added 3 test cases to test/Sema/function.c
> >>
> >>
> >>
> >> On Thu, Jul 3, 2014 at 4:41 PM, Alp Toker <alp at nuanti.com> wrote:
> >> >
> >> > On 03/07/2014 22:08, Zach Davis wrote:
> >> >>
> >> >> As reported in bug 20146, a function cannot have a return type of
> >> >> 'void' with qualifiers.
> >> >>
> >> >> Clang does emit a warning that the qualifiers are ignored
> >> >> [-Wignored-qualifiers] (off by default), but according to [1] this
> >> >> code is non-conforming.
> >> >>
> >> >> The attached patch makes Clang issue a more specific warning like so:
> >> >>
> >> >>      test3.c:8:18: warning: return type of void cannot be qualified
> >> >> 'volatile void'
> >> >>      volatile void baz(void) { return; }
> >> >>                    ^
> >> >>
> >> >> [1]http://www.open-std.org/jtc1/sc22/wg14/docs/rr/dr_113.html
> >> >
> >> >
> >> > It seems fine to make this a hard error instead of a warning for C,
> and
> >> > probably C++ too. Richard?
> >> >
> >> >>
> >> >> 20146_return_qual_void.patch
> >> >>
> >> >>
> >> >> Index: lib/Sema/SemaType.cpp
> >> >> ===================================================================
> >> >> --- lib/Sema/SemaType.cpp       (revision 212275)
> >> >> +++ lib/Sema/SemaType.cpp       (working copy)
> >> >> @@ -2741,6 +2741,15 @@
> >> >>           D.setInvalidType(true);
> >> >>         }
> >> >>   +      // C99 6.9.1/3: The return type of a function shall be void
> or
> >> >> +      // an object type other than array type.
> >> >> +      // A return type of void cannot be qualified.
> >> >> +      if (T->isVoidType() && T.getCVRQualifiers()) {
> >> >> +          unsigned diagID =
> diag::warn_func_returning_qualified_void;
> >> >
> >> >
> >> > Just pass the ID directly to Diag().
> >> >
> >> >> +          S.Diag(DeclType.Loc, diagID) << T;
> >> >> +          D.setInvalidType(true);
> >> >> +      }
> >> >> +
> >> >
> >> >
> >> > How about placing this check with an early return at the top of
> >> > diagnoseIgnoredFunctionQualifiers()?
> >> >
> >> >>         // Do not allow returning half FP value.
> >> >>         // FIXME: This really should be in BuildFunctionType.
> >> >>         if (T->isHalfType()) {
> >> >> Index: include/clang/Basic/DiagnosticSemaKinds.td
> >> >> ===================================================================
> >> >> --- include/clang/Basic/DiagnosticSemaKinds.td  (revision 212275)
> >> >> +++ include/clang/Basic/DiagnosticSemaKinds.td  (working copy)
> >> >> @@ -4160,6 +4160,8 @@
> >> >>     def err_func_returning_array_function : Error<
> >> >>     "function cannot return %select{array|function}0 type %1">;
> >> >> +def warn_func_returning_qualified_void : Warning<
> >> >> +  "return type of void cannot be qualified %0">;
> >> >
> >> >
> >> > (Warnings need to have a diagnostic group / -W flag, though it doesn't
> >> > matter if you go ahead and make it an error.)
> >> >
> >> >>   def err_field_declared_as_function : Error<"field %0 declared as a
> >> >> function">;
> >> >>   def err_field_incomplete : Error<"field has incomplete type %0">;
> >> >>   def ext_variable_sized_type_in_struct : ExtWarn<
> >> >
> >> >
> >> > Test case?
> >> >
> >> > Alp.
> >> >
> >> >>
> >> >>
> >> >> _______________________________________________
> >> >> cfe-commits mailing list
> >> >> cfe-commits at cs.uiuc.edu
> >> >> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
> >> >
> >> >
> >> > --
> >> > http://www.nuanti.com
> >> > the browser experts
> >> >
> >
> >
>
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