This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 110b. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2023-01-15


1828. nested-name-specifier ambiguity

Section: 6.5.5  [basic.lookup.qual]     Status: CD6     Submitter: Richard Smith     Date: 2014-01-08

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

Issue 125 concerned an example like

  friend A::B::C();

which might be parsed as either

  friend A (::B::C)();

or

  friend A::B (::C)();

Its resolution attempted to make such constructs unambiguously ill-formed by allowing any identifier, not just namespaces and types, to appear in a nested-name-specifier, apparently on the assumption that C in this case would become part of an ill-formed nested-name-specifier instead of being taken as the unqualified-id in a qualified-id. Unfortunately, the current specification does not implement that intent, leaving both parses as valid possibilities.

A different approach might be to adjust the specification of the lookup of names appearing in nested-name-specifiers from

If a :: scope resolution operator in a nested-name-specifier is not preceded by a decltype-specifier, lookup of the name preceding that :: considers only namespaces, types, and templates whose specializations are types. If the name found does not designate a namespace or a class, enumeration, or dependent type, the program is ill-formed.

to

Lookup of an identifier followed by a :: scope resolution operator considers only namespaces, types, and templates whose specializations are types. If an identifer, template-id, or decltype-specifier is followed by a :: scope resolution operator, the name shall designate a namespace, class, enumeration, or dependent type, and shall form part of a nested-name-specifier.

This approach would also remove the need for deferred lookup for template-ids and thus resolve issue 1771.