[2026-06 CWG Motion 2] P3596R3 Undefined Behavior and IFNDR Annexes by notadragon · Pull Request #9107 · cplusplus/draft

notadragon · 2026-06-13T20:33:49Z

Fixes #9069
Also fixes cplusplus/papers#2623

This is from the branch from which the paper was generated, with a small number of editorial changes already made:

Most notable, after rendering the standard the annex titles were very large and wrapping to multiple lines. After discussion, Jens and I shortened them by removing "Enumeration of" from each one.
One set of comments was fixed to use "IFNDR" the way the rest of the annex does instead of the (comma-less) "ill formed no diagnostic required"
One bug that was discovered during wording review of another paper was fixed, but that entry will also be removed when that paper lands in a later motion.
The \lastcorechapter macro was used where it was supposed to be used because my paper rendering couldn't handle that when the P paper was generated.

shafik

Finished UB section.

shafik · 2026-06-19T22:24:45Z

+\end{codeblock}
+\end{example}
+
+\ubdescription{conv.fpint.float.not.represented}


Super minor but I think this one should be swapped w/ conv.fpint.int.not.represented b/c in 7.3.11 we have E.3.8 first and then E.3.7 but again super minor nit.

will rearrange.

shafik · 2026-06-19T22:31:58Z

+
+\pnum
+Evaluating a \keyword{dynamic_cast} on a reference that
+denotes an object (of polymorphic type) of the wrong type or an object


Suggested change

denotes an object (of polymorphic type) of the wrong type or an object

denotes an object (of polymorphic type) of the similar type or an object

I am not sure but looking at the core wording, I think that is more correct?

"of the similar type" is wrong, similar type would be fine. "wrong" type could be interpreted broadly as "any type that is wrong for doing this" but we can be more precise and say "of a type that is not similar" . I will change to that.

shafik · 2026-06-19T22:50:30Z

+\pnum
+\begin{example}
+\begin{codeblock}
+struct S {


You don't indicate the UB in the example and why didn't you use the simpler example from 7.6.6? Maybe this was my original example?

i will compact this example slightly and add a comment about the UB. Not sure who the example dates back to (probably you). I would rather not rewrite examples entirely in this phase so will leave switching to clearer/simpler examples for followup efforts.

shafik · 2026-06-19T22:53:08Z

+\ubdescription{expr.assign.overlap}
+
+\pnum
+Overlap in the storage between the source and destination may result in undefined behavior.


Should we be using may here?

shafik · 2026-06-19T23:10:11Z

+\ubdescription{class.base.init.mem.fun}
+
+\pnum
+It is undefined behavior to call a member function before all the \grammarterm{mem-initializer}s for base classes have completed.


It looks like there are two contracts cases added that we are missing. This feels minor and we can add afterwards too.

I am prepping a PR with new/missing entries and any entries that need major updating after all of the Brno motions land, I will make note to include this in that PR (and the paper that will be generated from that PR). I think it should wait until after all of the Brno motions land.

jensmaurer · 2026-06-21T05:41:44Z

@notadragon, please squash all commits into the first one; we don't need the details of the genesis preserved for posterity.

jensmaurer · 2026-06-21T05:43:11Z

@notadragon, do we have "ubx" as part of a visible label in the Annex headings now? We shouldn't; those visible labels should be "ub:something". (Similarly with ifndrx, if any).

shafik

Finished reviewing IFNDR annex. Really minor comments. I think we are in good shape here.

shafik · 2026-06-21T22:12:08Z

+\ifndrdescription{module.unit.named.module.no.partition}
+
+\pnum
+Having multiple primary module interface units


I am not sure this description matches what the wording says but I may just not be reading it correctly.

The wording defines "primary module interface unit" and says, effectively, "A named module shall contain one module interface unit that meets that definition, no diagnostic required". As far as I can tell that's what the annex entry is saying as well, though i'm happy to adjust if it's wrong or there's clearer wording. Leaving this as-is for now.

notadragon · 2026-06-22T14:40:55Z

@notadragon, do we have "ubx" as part of a visible label in the Annex headings now? We shouldn't; those visible labels should be "ub:something". (Similarly with ifndrx, if any).

No, I swapped which label is "ub:" and which is "ubx:", and I also made the "stable names" shown in the annex start with "ub:" and ifndr:.

Here's some screenshots of my locally built copy of the standard for review:

notadragon · 2026-06-22T16:37:05Z

@jensmaurer @shafik I've addressed, I believe, all of your comments.

shafik · 2026-06-22T16:55:00Z

@jensmaurer @shafik I've addressed, I believe, all of your comments.

Thank you so much for all this work to get the annexes landed in the draft! I am happy with direction in your replies.

notadragon mentioned this pull request Jun 13, 2026

[2026-06 CWG Motion 14] P3424R2 Deallocation Functions with Throwing Exception Specification Are Ill-formed #9081

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eisenwave added this to the post-2026-06 milestone Jun 14, 2026

shafik reviewed Jun 19, 2026

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shafik reviewed Jun 22, 2026

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joshuaberne added 3 commits June 22, 2026 12:04

P3596R3 Undefined Behavior and IFNDR Annexes

edce8b0

fixup: minor changes and reordering

b474340

fixup: formatting fixes

3e05f52

notadragon force-pushed the ub-ifndr-cwgreview1 branch from c26a110 to 3e05f52 Compare June 22, 2026 16:36

	denotes an object (of polymorphic type) of the wrong type or an object
	denotes an object (of polymorphic type) of the similar type or an object

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notadragon commented Jun 13, 2026

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jensmaurer commented Jun 21, 2026

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notadragon commented Jun 22, 2026

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shafik commented Jun 22, 2026

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