This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-12-19

1894. typedef-names and using-declarations

[Accepted at the November, 2020 meeting as part of paper P1787R6 and moved to DR at the February, 2021 meeting.]

The resolution of issue 407 does not cover cases involving using-declarations. For example:

  namespace A { struct S {}; }
  namespace B {
    // This is valid per issue 407
    using A::S;
    typedef A::S S;
    struct S s;
  }
  namespace C {
    // The typedef does not redefine the name S in this
    // scope, so issue 407's resolution does not apply.
    typedef A::S S;
    using A::S;
    // The name lookup here isn't ambiguous, because it only finds one
    // entity, but it finds both a typedef-name and a non-typedef-name referring
    // to that entity, so the standard doesn't appear to say whether this is valid.
    struct S s;
  }

The same issue appears with using-directives:

  namespace D { typedef A::S S; }
  namespace E {
    using namespace A;
    using namespace D;
    struct S s; // ok? issue 407 doesn't apply here either
  }

One possibility might be to remove the rule that a typedef-name declaration redefines an already-defined name and instead rely on struct stat-style hiding, taking the non-typedef-name if name lookup finds both and they refer to the same type.

Notes from the June, 2014 meeting:

CWG felt that these examples should be well-formed.