To: J3                                                     J3/26-183r1
From: Malcolm Cohen
Subject: Invalid branching
Date: 2026-June-18

1. Introduction

Our rules for branching within constructs that one is not permitted to branch
out of, viz CHANGE TEAM, CRITICAL, and DO CONCURRENT, on the face of it permit
branching to the beginning statement of the construct. But, that statement is
not part of the block, and branching will in fact re-enter the construct thus
effectively branching out.

This paper contains edits to correct this.

This is already interp F23/019 (see 26-111), so will be fixed in the upcoming
Corrigendum 2, which is expected before the DIS stage. Even without that fix,
the absence of unambiguous semantics means this does not need to be considered
to be a compatibility problem.


2. Approach

We could state in 11.1.2.1 that for the purposes of branching, the initial
statement of a construct is not considered to be part of the construct. That
is the approach suggested by F23/019. That interp assumes that DO CONCURRENT
and CRITICAL are already handled correctly, but it is hard to find normative
text that clearly states this.

As this is perhaps too subtle an approach, so instead this paper proposes
inserting explicit prohibitions in each place.


2. Edits to 26-007r1

[220:14] 11.1.5.1 Purpose and form of the CHANGE TEAM construct, C1111 which
         begins "A branch", insert before the full stop
            "or that is the CHANGE TEAM statement of the construct",
         making that whole constraint read:
   "C1111 A branch (11.2) within a CHANGE TEAM construct shall not have a
          branch target that is outside the construct or that is the CHANGE
          TEAM statement of the construct."

[222:22] 11.1.6 CRITICAL construct, C1120 which begins "A branch",
         insert before the full stop
            "or that is the CRITICAL statement of the construct",
         making the whole constraint read:
   "C1120 A branch (11.2) within a CRITICAL construct shall not have a branch
          target that is outside the construct or that is the CRITICAL
          statement of the construct."

[228:30] 11.1.7.5 Additional semantics for DO CONCURRENT constructs, C1142
         which begins "A branch", insert before the full stop
            "or that is the DO statement of the construct",
         making that whole constraint read:
   "C1142 A branch (11.2) within a DO CONCURRENT construct shall not have a
          branch target that is outside the construct or that is the DO
          statement of the construct."

[244:8+] 11.2.1 Branch concepts, Table 11.2,
         insert "change-team-stmt" and "end-change-team-stmt",
         in between end-block-stmt and critical-stmt.

===END===