To: J3                                                     J3/20-152
From: Robert Corbett
Subject: Optional comma in type declaration
References: 18-007r1
Date: 2020-October-11

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NUMBER: F18/021
TITLE: Optional comma in type declaration
DEFECT TYPE: Erratum
STATUS: Submitted

QUESTION:

The program

      PROGRAM EXAMPLE1
        CHARACTER*1, C(SUM([0, 1]))
      END

is a conformant Fortran 2018 program.  The program

      PROGRAM EXAMPLE2
        CHARACTER*1 C(SUM([INTEGER:: 0, 1]))
      END

is also conformant.  The similar program

      PROGRAM EXAMPLE3
        CHARACTER*1, C(SUM([INTEGER :: 0, 1]))
      END

violates constraint C725 of 18-007r1.  The constraint
states

    C725 (R722)  The optional comma in a
         \si{length-selector} is permitted only if no
         double colon separator appears in the
         \si{type-declaration statement}.

Q: Should the optional comma be permitted when the
   \si{type-decalaration-statement} includes a double
   colon separator in an expression?

ANSWER:

A: Yes.

The Fortran 90 and Fortran 95 standards do not allow a
double colon separator to appear in an expression, and so
the problem does not arise in those standards.  When type
specifications were added to array constructors in
Fortran 2003, this case was overlooked.

Constraint 725 is too clever.  Its intended purpose is not
obvious.  It is enforcing a local condition using a global
rule.  The intent is to allow the optional comma in cases
similar to those for the optional comma in a DO-statement.

An edit to 18-007r1 is provided.

EDIT to 18-007r1:
[7.4.4.2 "Character type specifier" p3+, page 62:14-15]
Replace "only if ... \si{type-declaration-statement}."
with
  "only if the first nonblank character following the comma
   in the \si{type-declaration-statement} is a
   \si{letter}."

The amended constraint is

    C725 (R722)  The optional comma in a
         \si{length-specifier} is permitted only if the
         first nonblank character following the comma in
         the \si{type-declaration-statement} is a
         \si{letter}.

In the Fortran 202X standard, constraints C724 and C725
could be fused.  That might eliminate some repetition.


SUBMITTED BY: Robert Corbett

HISTORY: 20-nnn m222 Submitted

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