To: J3                                                     J3/26-108
From: Malcolm Cohen
Subject: US16 Default kinds, syntax
Date: 2026-January-11
References: 23-199r1, 25-191, 25-191r2.


1. Introduction

This paper has the syntax for US16, Default kinds.

See 23-199r1 for an introduction to this feature.
25-191r2 has the formal requirements and specifications.
25-191 has some additional discussion and background.


2. Syntax

Y1. The form of the new statement shall be
    default-kind-stmt is DEFAULT KIND ( default-kind-spec-list )
    default-kind-spec is basic-intrinsic-type-name = int-constant-expr
    basic-intrinsic-type-name is REAL or INTEGER or LOGICAL or CHARACTER

{Used the long-winded "basic-intrinsic-type-name", to avoid confusion
 as COMPLEX is not included.}

ALTERNATIVE Y1:
    As above, but with the statement having the form
        DEFAULT KIND :: default-kind-spec-list
    This form avoids the parentheses, but the double-colon is mandatory
    to avoid fixed-form ambiguity with an assignment statement.

Y2. The int-constant-expr in a default-kind-spec shall not depend on a
    property of the default kind of a type, unless that type has its
    default kind previously specified in the scoping unit or its default
    kind is not specified in the scoping unit.

Y3. A default-kind-stmt shall appear after any USE statement, but before
    any IMPLICIT statement.

Y4. Constraint: The same basic-intrinsic-type-name in a default-kind-stmt
                shall not appear more than once in a scoping unit.

Y5. More than one default-kind-stmt may appear, subject to Y3 and Y4.

Y6. A default-kind-stmt shall appear only in the specification part of a
    program unit, interface body, or nested subprogram, and applies to all
    nested scoping units other than interface bodies.

{This is the same as for IMPLICIT.}

Y7. The default kinds can be imported from a single module with a USE
    statement that has the ", DEFAULT_KINDS" clause. Only one such import
    is permitted, and if used, no default-kind-stmt shall appear in the
    scoping unit.
    The ", DEFAULT_KINDS" clause is only permitted in a scoping unit that
    would otherwise be permitted to have a default-kind-stmt.

    This changes the beginning of the USE statement syntax from
        USE [ [ , module-nature ] :: ]
    to
        USE [ [ , module-nature ] [ , DEFAULT_KINDS ] :: ]

Y8. The default kind when no default kind specification is in effect is:
        default real kind = single precision real kind
        default integer kind = standard integer kind
        default logical kind = standard logical kind
        default character kind = system character kind

{Previously proposed terminology for INTEGER and LOGICAL was "default range"
 and "default size" respectively, but it's better to just say standard.}

{Note: We could say "standard character" instead of "system character", but
       I think the latter is more evocative and thus better.}

    Standard integer kind and standard logical kind are defined to have the
    same storage size as single precision real kind.

Y9. ISO_FORTRAN_ENV shall have named constants as follows:

    SINGLE_PRECISION    - kind of single precision real
    DOUBLE_PRECISION    - kind of double precision real
    STANDARD_INTEGER    - kind of standard integer
    STANDARD_LOGICAL    - kind of standard logical
    SYSTEM_CHARACTER    - kind of system character

Y10. Standard generic intrinsic functions that return a default kind shall
     return the user-specified default kind if active, except for operating
     system interactions with type CHARACTER which will always be system
     character kind.
{Note: This is achieved by leaving the wording as is in most cases.}
     The exceptions are thus:
        DATE_AND_TIME: the DATE, TIME, and ZONE arguments.
        EXECUTE_COMMAND_LINE: the COMMAND and CMDMSG arguments.
        GET_COMMAND: the COMMAND and ERRMSG arguments.
        GET_COMMAND_ARGUMENT: the VALUE and ERRMSG arguments.
        GET_ENVIRONMENT_VARIABLE: the NAME, VALUE, and ERRMSG arguments.
        MOVE_ALLOC: the ERRMSG argument.
        RANDOM_SEED: the GET and PUT arguments. The SIZE argument should
                     be changed to allow any kind of integer with a decimal
                     exponent range of at least four.

     There is also:
        DBLE - will continue to return double precision real
        DPROD - will continue to return double precision real
        SYSTEM_CLOCK - the "no smaller than default integer" wording will
                       be changed to "no smaller than standard integer".

Y11. Standard specific intrinsic functions that return default real or
     integer shall return single precision real and default range integer.

Y12. Procedures in standard intrinsic modules that have an argument of default
     kind, or a return value of default kind, will remain with those kinds
     i.e. the wording will be changed to standard kind.

{RATIONALE: It would be unreasonable to require all the possible variations of
            possible kind specifications for the intrinsic modules to be
            implemented, and some of the possibilities would not in fact work
            due to 8-bit integer being too small.}

Y13. Input/output specifiers that return default kind integer, logical, or
     character shall be unaffected; that is, the wording will be changed to
     standard integer, standard logical, and system character.

Y14. That the arguments of defined input/output procedures shall be standard
     integer and system character, as they are now.

Y15. That the ERRMSG= specifiers in other statements, such as ALLOCATE, shall
     be system character kind.

===END===