[cfe-dev] When does ~decltype(expr) make sense ?

[Richard Smith]

[class.dtor] (12.4)/1:

"A declaration of a destructor uses a function declarator (8.3.5) of the
form

    ptr-declarator ( parameter-declaration-clause ) exception-specificationopt
attribute-specifier-seqopt

where the ptr-declarator consists solely of an id-expression, an optional
attribute-specifier-seq, and optional surrounding parentheses, and the
id-expression has one of the following forms:

-- in a member-declaration that belongs to the member-specification of a
class but is not a friend declaration (11.3), the id-expression is ~class-name
and the class-name is the injected-class-name (Clause 9) of the
immediately-enclosing class;

[... bullets 2 and 3 do not apply ...]"

A decltype-specifier is never a class-name, so ~decltype(...) cannot be
used to declare a destructor.

On Sun, Feb 22, 2015 at 12:51 PM, David Blaikie <dblaikie at gmail.com> wrote:

>
>
> On Sun, Feb 22, 2015 at 7:56 AM, Manasij Mukherjee <manasij7479 at gmail.com>
> wrote:
>
>> Hi,
>> I am trying to fix this bug,
>> http://llvm.org/bugs/show_bug.cgi?id=22508
>>
>> In short,
>> struct A
>> {
>>     ~decltype(auto){}
>> };
>> is accepted when it should not be.
>>
>> In Parser::ParseUnqualifiedId (ParseExprCXX.cpp:2488 ),
>>
>> // Parse the '~'.
>>     SourceLocation TildeLoc = ConsumeToken();
>>
>>     if (SS.isEmpty() && Tok.is(tok::kw_decltype)) {
>>       DeclSpec DS(AttrFactory);
>>       SourceLocation EndLoc = ParseDecltypeSpecifier(DS);
>>       if (ParsedType Type = Actions.getDestructorType(DS, ObjectType)) {
>>         Result.setDestructorName(TildeLoc, Type, EndLoc);
>>         return false;
>>       }
>>       return true;
>>     }
>>
>> After this, clang handles the identifier case.
>>
>> Removing this block results in clang reporting the correct diagnostic:
>> "error: expected a class name after '~' to name a destructor"
>>
>> Why is this block present ?
>> Can anyone point out what is the expected behavior ?
>>
>
> 5.1.1 [expr.prim.general] in paragraph 8 states that "The form ~
> decltype-specifier also denotes the destructor, but it shall not be used as
> the unqualified-id in a qualified-id."
>
> I implemented this a while ago, but it looks like (r146155) it was for
> expressions (x.~decltype(*x)(), for example) not necessarily for dtor
> declarations.
>
> I can't find any particular words in the spec about taht, but I could
> believe it's not intended (I didn't test it, at least) for dtor
> declarations/definitions to use this syntax.
>
> http://llvm.org/bugs/show_bug.cgi?id=15801 was also filed about this
>
>
>>
>> Thanks
>> Manasij Mukherjee
>>
>>
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>>
>>
>
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