J3/16-229
To: J3
From: Malcolm Cohen
Subject: Clarify type-bound procedure duplication
Date: 2016 June 08
Reference: 16-171r1


1. Introduction

The standard does not clearly prohibit duplicate type-bound procedure
definition.  This should be clarified.


2. Specific type-bound procedures

Consider
  TYPE T
  CONTAINS
    PROCEDURE :: P => MP ! (a)
    PROCEDURE :: P => MP ! (b)
  END TYPE

The type-bound procedure declaration at (a) declares that P is a
type-bound procedure in the scope of T.  This fact is an "attribute"
that determines the uses of P.

Accordingly,C514
  "An entity shall not be explicitly given any attribute more than once
   in a scoping unit."
means that the duplicate declaration at (b) is invalid.  However, this is
far from obvious, so an explicit constraint should be added.


3. Generic type-bound procedures

Consider
  TYPE T
  CONTAINS
    PROCEDURE :: P
    GENERIC :: G=>P, G=>P
  END TYPE

This cannot be valid, as
  "A GENERIC statement declares a generic type-bound procedure, which is a
   type-bound generic interface for its specific type-bound procedures."
but the list of its type-bound procedures has P occurring twice, and P
cannot be distinguished from itself, so the list cannot satisfy the
requirements for generic interfaces.

Again, this is far from obvious and should be clarified.


4. EDITS to 16-007r1:

[75:16+] 4.5.5 Type-bound procedures, after C470 insert new constraint:
  "C470a A <binding-name> in a <type-bound-proc-decl> in a derived type
         definition shall not be the same as any other <binding-name>
         within that derived type definition.".
{Clearly prohibit duplicative specific tbp declaration.}

[75:23+] same subclause, after "C472 (R451) Each <binding-name>...type."
         insert new constraint
  "C472a A <binding-name> in a type-bound GENERIC statement shall
         not specify a specific binding that was inherited or specified
         previously for the same generic identifier in that derived type
         definition.".
{Clearly prohibit duplicative generic tbp declaration.}

===END===