Is using 'decltype' in the declaration of pointer to member valid?

Question

Imagine for some strange reason I write this:

int main()
{
    struct S
    {        
        int i;
    } var;


    int decltype(var)::* pint = &decltype(var)::i;
}

GCC seems to compile it fine although Clang fails with some indeterminate syntax-related error message.

So what does the holy ISO paper says about this - is this valid or not?

Answer 1

This is actually a known bug in Clang.

The code is valid.

N4140 [dcl.mptr]/1:

In a declaration T D where D has the form

nested-name-specifier * attribute-specifier-seq_opt cv-qualifier-seq_optD1

and the nested-name-specifier denotes a class, and the type of the identifier in the declaration T D1 is “derived-declarator-type-list T”, then the type of the identifier of D is “derived-declarator-type-list cv-qualifier-seq pointer to member of class nested-name-specifier of type T”. The optional attribute-specifier-seq (7.6.1) appertains to the pointer-to-member.

In this definition we are interested in nested-name-specifier, and it's defined at [expr.prim.general]/8 as (emphasis mine):

nested-name-specifier:

:: type-name ::
namespace-name ::
decltype-specifier ::
nested-name-specifier identifier ::
nested-name-specifier template_opt simple-template-id ::