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


1318. Syntactic ambiguities with final

Section: Clause 11  [class]     Status: CD3     Submitter: Johannes Schaub     Date: 2011-05-14

[Moved to DR at the April, 2013 meeting.]

The ambiguity in an example like

  struct A final { };

is resolved by 5.10 [lex.name] paragraph 2 to be the declaration of a variable named final:

any ambiguity as to whether a given identifier has a special meaning is resolved to interpret the token as a regular identifier.

Similarly, in an example like

  struct C { constexpr operator int() { return 5; } };
  struct A {
   struct B final : C{};
  };

final is taken as the name of a bit-field member rather than as the name of a nested class.

Proposed resolution (October, 2012):

  1. Change 5.10 [lex.name] paragraph 2 as follows:

  2. The identifiers in Table 3 have a special meaning when appearing in a certain context. When referred to in the grammar, these identifiers are used explicitly rather than using the identifier grammar production. Unless otherwise specified, any ambiguity as to whether a given identifier has a special meaning is resolved to interpret the token as a regular identifier.
  3. Change Clause 11 [class] paragraph 3 as follows:

  4. If a class is marked with the class-virt-specifier final and it appears as a base-type-specifier in a base-clause (11.7 [class.derived]), the program is ill-formed. Whenever a class-key is followed by a class-head-name, the identifier final, and a colon or left brace, final is interpreted as a class-virt-specifier. [Example:

      struct A;
      struct A final {};      // OK: definition of struct A,
                              // not value-initialization of variable final
    
      struct X {
       struct C { constexpr operator int() { return 5; } };
       struct B final : C{};  // OK: definition of nested class B,
                              // not declaration of a bit-field member final
      };
    

    end example]