This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 119e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2026-05-20

3190. Ambiguous lookup for type aliases in reflection

Consider:

  namespace A {
    using T = int;
  }
  namespace B {
    using T = int;
  }
  using namespace A;
  using namespace B;
  T t;                // #1, OK
  auto r = ^^T;       // #2, ???

While 6.1 [basic.pre] paragraph 8 clarifies that #1 is well-formed by considering the underlying type, the treatment of #2 is unclear.

Possible resolution (option 2) [SUPERSEDED]:

Change in 7.6.2.10 [expr.reflect] paragraph 5 as follows:

• ...
• Otherwise, if lookup finds a namespace alias (9.9.3 [namespace.alias]), R represents that an unspecified namespace alias among the lookup results.
• ...
• Otherwise, if lookup finds a type alias A, R represents the underlying entity of A if A was introduced by the declaration of a template parameter; otherwise, R represents A an unspecified type alias among the lookup results.
  [ Example:

    namespace A {
      using T = int;
    }
    namespace B {
      using T = int;
    }
    using namespace A;
    using namespace B;
    auto r = ^^T;       // OK, represents either A::T or B::T
  

  -- end example ]
• ...

Proposed resolution (approved by CWG 2026-05-19):

Change in 7.6.2.10 [expr.reflect] paragraph 5 as follows:

• ...
• Otherwise, if lookup finds a namespace alias (9.9.3 [namespace.alias]), all declarations found by name lookup shall have the same target scope, and R represents that such a namespace alias.
• ...
• Otherwise, if lookup finds a type alias A,:
  • If any declaration found by name lookup introduced a template parameter T, R represents the underlying entity of T. A if A was introduced by the declaration of a template parameter; otherwise,
  • Otherwise, all declarations found by name lookup shall have the same target scope, and R represents A such a type alias.
  [ Example:

    namespace A {
      using T = int;
    }
    namespace B {
      using T = int;
    }
    using namespace A;
    using namespace B;
    auto r = ^^T;       // error: lookup finds type aliases with different target scopes
  

  -- end example ]
• ...

CWG 2026-05-19

Seeking EWG's approval via paper issue #2795.