To: J3                                                     J3/21-113
From: Malcolm Cohen
Subject: Interpretation of CLASS(*) overloading
Date: 2021-February-21

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NUMBER: F18/023
TITLE: CLASS(*) ambiguous operator overloading
KEYWORDS: CLASS(*) generic OPERATOR
DEFECT TYPE: Erratum
STATUS: J3 consideraton in progress

QUESTION:

Consider the program

  Module m
    Interface Operator(==)
      Module Procedure mp
    End Interface
  Contains
    Logical Function mp(a,b)
      Class(*),Intent(In) :: a,b
      mp = .False.
      Select Type(a)
      Type Is (Integer)
        Select Type(b)
        Type Is (Integer)
          mp = .True.
        End Select
      End Select
    End Function
  End Module
  Program ambiguous
    Use m
    If (13==999) Then
      Print *,'Invoked the user function'
    Else
      Print *,'Did not invoke the user function'
    End If
  End Program

Is this program valid?
If so, does it invoke the user function or the intrinsic operation?

It is clear from the standard that the user is not supposed to be able
to override intrinsic operators, merely extend them. The last sentence
of 15.4.3.4.2p1 "Defined operations" says:
  "If the operator is an intrinsic-operator (R608), the number of dummy
   arguments shall be consistent with the intrinsic uses of that
   operator, and the types, kind type parameters, or ranks of the dummy
   arguments shall differ from those required for the intrinsic
   operation (10.1.5)."

However, CLASS(*) encompasses these while "differing".

ANSWER:

The program is not conforming because it is ambiguous.

However, the interface was intended to be forbidden; as noted, the
standard clearly intended to prohibit overriding intrinsic operations.
An edit is provided to correct this oversight.

EDITS to 18-007r1:

[295:11] 15.4.3.4.2 Defined operations, p1, last sentence,
    After "(10.1.5)"
    insert ", treating a CLASS(*) dummy argument as not differing in
            type or kind".
{A bit ugly, but we can't use "distinguishable" because operations have
 operands not arguments."

SUBMITTED BY: Paul Richard Thomas

HISTORY: 21-nnn   m223  Submitted

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