To: J3                                                     J3/26-128r1
From: Malcolm Cohen
Subject: Edits for C23 enum interoperability
Date: 2026-February-26
Reference: 26-122r1, 26-007.


1. Introduction

This paper has the edits to add interoperability with the new enum abilities
in C23 (ISO/IEC-9899:2024). See 26-122r1 for background discussion,
requirements, specifications and syntax.

2. Edits to 26-007

[76:26-27] 7.4.1 Classification and specification,
           C727 after R717 kind-selector,
           Delete "(R717)",
           After "of" insert "the",
           Before "shall be" insert "in a kind-selector",
           After "on the processor" insert "for the type being specified".
           That makes the whole constraint read:
   "The value of the scalar-int-constant-expr in a kind-selector shall be
    nonnegative and shall specify a representation method that exists on the
    processor for the type being specified."
{This is just editorial. The existing text is not really wrong, but it makes
 the reader work harder than necessary.}

[109:5-7] 7.6.1 Interoperable enumerations and enumtypes, R771 enum-def-stmt,
          in each production insert "[ \si{kind-selector} ]" after the keyword
          ENUM.
{EDITOR: If that makes a line too long, consider using a narrower BNF macro.}

[109:13+] Same subclause, after C7137 "An enum-type-name", insert constraint
   "C7137a If kind-selector appears in an enum-def-stmt of an enum-def, each
           named-constant in an \si{enumerator} of that definition shall have
           a value that is representable in the specified representation
           method."
{Careful wording so that it applies when there is no expression, but the
 auto-increment overflows.}

[109:18-20] Same subclause, p2, near the end,
         After "order as specified by the enum-def",
         insert ", and with a C integer type specifier that interoperates
                 with the kind of integer specified by kind-selector if it
                 appears, and without a C integer type specifier if
                 kind-selector does not appear".
         That makes the sentence rather unwieldy, so the editor should
         split the two conditions into two bullets, making the whole
         sentence read:
   "The corresponding C enumerated type is the type that would be declared by
    a C enumeration specifier (ISO/IEC 9899:2024, 6.7.3.3)
        - that specified C enumeration constants with the same values as those
          specified by the enum-def, in the same order as specified by the
          enum-def, and
        - with a C integer type specifier that interoperates with the kind of
          integer specified by kind-selector if it appears, and without a C
          integer type specifier if kind-selector does not appear."
{I have made the assumption here that we want to retain our requirement for
corresponding values; C certainly used to require that for type compatibility,
but I have not checked C23 to see if it still does.}

ALTERNATIVE [109:18-20]
(if we think that a kind-specifier obviates the value sequence requirement)
Make the sentence look like this:
   "The corresponding C enumerated type is the type that would be declared by
    a C enumeration specifier (ISO/IEC 9899:2024, 6.7.3.3)
        - with a C integer type specifier that interoperates with the kind of
          integer specified by kind-selector if it appears, or
        - without a C integer type specifier if kind-selector does not appear,
          and that specified C enumeration constants with the same values as
          those specified by the enum-def, in the same order as specified by
          the enum-def."
{We should only do this if C23 has dropped the value-sequence requirement when
 an integer type specifier appears in the enumerated type definition.}

[109:20] Split the accessibility sentence at the end of p2 into a new
         paragraph. It would appear after the bullet list anyway, so might as
         well be a new para to avoid confusion.

[110:15+14+] Between NOTE 4 "Example of an interoperable..."
             and NOTE 5 "There is no difference..."
             Insert new NOTE:
   "NOTE 4a
    Specifying a kind-selector facilitates interoperation with the new syntax
    for enums introduced by ISO/IEC-9899:2024. For example, for the enumerator

         enum my_long_enum : long {
            red = 4,
            blue = 9,
            yellow
         };

      the corresponding Fortran interoperable enumeration could be defined by

         ENUM (KIND=C_LONG), BIND (C) :: my_long_enum
            ENUMERATOR :: red = 4, blue = 9
            ENUMERATOR yellow
         END ENUM
    \end{note}"

==END==