clang doesn't see base class constructors pulled in via type alias

Question

This code is accepted by GCC (several versions) and refused by clang (also several versions):

struct ACS{};

template<typename Dep>
struct AD{};

template<typename Der, typename Dep>
struct PA{
    explicit PA(ACS&) {}
};

template<typename AT, typename ADep>
struct AA : public PA< AA<AT, ADep>, AD<ADep> >
{
    using base = PA< AA<AT, ADep>, AD<ADep> >;
    using base::PA;
    // accepted by both: using base::base;
};

int main()
{
    ACS acs;
    AA<int, double> aa{acs};
}

To me, this looks like the same situation as clang doesn't see base class constructors pulled in via typedef. However, instead of a "no matching constructor" error message, clang issues this error message:

<source>:15:17: error: dependent using declaration resolved to type without 'typename'
using base::PA;
            ^

Apparently, clang expects the typename keyword. The hint suggests adding it before PA, but this:

using base:: typename PA;

is silly, and is rejected by both compilers. However, the more sensible alternative:

using typename base::PA;

is accepted without a syntax error, but now clang complains:

<source>:22:21: error: no matching constructor for initialization of 'AA<int, double>'
AA<int, double> aa{acs};
                ^ ~~~~~

In contrast, using base::base is accepted by both compilers, even without a typename. In fact, using typename base::base is accepted only by GCC; clang complains:

<source>:18:27: error: typename is allowed for identifiers only
using typename base::base;
      ~~~~~~~~~       ^

What's going on? Why is using base::PA different from using base::base? Isn't base an alias of PA?

That is accepted by both (unless I also put a typename in front of the type; then clang complains). — Bulletmagnet
– Bulletmagnet, Commented Feb 10 at 21:01
A shorter way to define base is using base = AA::PA;. You can also write using AA::PA::PA; - both are accepted by clang. — Eugene
– Eugene, Commented Feb 11 at 0:57
using base = AA::PA may be shorter, but it's ill-formed: invalid use of incomplete type 'struct AA<AT, ADep>' godbolt.org/z/r5q68MTjq — Bulletmagnet
– Bulletmagnet, Commented Feb 11 at 8:02

Davis Herring · Accepted Answer · 2025-02-10 23:27:16Z

3

Because dependent names (like PA) aren’t resolved until instantiation, using base::PA is presumed not to name a constructor; this decision must be made early because it affects parsing of the rest of the derived class template. Repeating the same name is a special case to allow inheriting dependent constructors at all; it’s very special in that base is of course not looked up in base where it wouldn’t find anything.

answered Feb 10 at 23:27

Davis Herring

42.1k4 gold badges59 silver badges92 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

clang doesn't see base class constructors pulled in via type alias

1 Answer 1

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related