[cfe-dev] ParseClassSpecifier: can "unions" inherit?

[steve naroff]

On Sep 4, 2008, at 12:19 PM, Nico Weber wrote:

> Hi,
>
> On 04.09.2008, at 18:01, Ted Kremenek wrote:
>
>> I just noticed the following code fragment in ParseClassSpecifier:
>>
>>  // Parse the tag portion of this.
>>  DeclTy *TagDecl = Actions.ActOnTag(CurScope, TagType, TK, StartLoc,
>> Name,
>>                                     NameLoc, Attr);
>>
>>  // Parse the optional base clause (C++ only).
>>  if (getLang().CPlusPlus && Tok.is(tok::colon)) {
>>    ParseBaseClause(TagDecl);
>>  }
>>
>> It appears from the code that the 'if' branch can be taken even if  
>> the
>> tag decl is a union.  Is this valid in the C++ grammar?  That is, can
>> there be a "base clause" for unions in C++, and if so, what does that
>> mean?
>
> it seems not to be allowed -- at least both gcc and clang reject it.
> Clang does it in ActOnBaseSpecifier in Sema:
>
>   // C++ [class.union]p1:
>   //   A union shall not be used as a base class.
>   if (BaseType->isUnionType()) {
>     Diag(BaseLoc, diag::err_union_as_base_class, SpecifierRange);
>     return;
>   }
>

That's odd - it seems like this should be diagnosed by the parser.  
Since I'm not very familiar with all the "challenges" of implementing C 
++:-), I'll understand if this isn't convenient.

snaroff

> Nico
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