P0870 A proposal for a type trait to detect narrowing conversions

[wg21bot]

P0870R1 A proposal for a type trait to detect narrowing conversions (Giuseppe D'Angelo)

[erichkeane]

EWG-I in Prague

Forward D0870R2 to EWG, as we believe that EWGI cannot provide any further useful feedback to the author.

SF	F	N	A	SA
5	4	0	0	0

[jfbastien]

EWG Prague Friday afternoon:

D0870R2 A proposal for a type trait to detect narrowing conversions

Send to SG6 Numerics for input.

SF	F	N	A	SA
4	4	6	3	0

We have no concerns with this, only send this paper back to us if SG6 or LEWG / LWG / CWG ask.

SF	F	N	A	SA
3	13	2	0	0

[brycelelbach]

Prague 2020-02 LEWGI Minutes

P0870R1 std::is_narrowing_conversion: Direction Review

Chair: Billy Baker

Champion: Marc Mutz

Minute Taker: David Olsen

Start Review: 2020-02-11 08:39

You mentioned that there are proposed changes to what are narrowing conversions in flight? Do we expect the definition to change again in the future?

Does EWG(I) need to see this?

Start Polling: 9:14

POLL: We should promise more committee time to pursuing P0870, knowing that our time is scarce and this will leave less time for other work.

Strongly For	Weakly For	Neutral	Weakly Against	Strongly Against
2	10	2	1	0

Attendance: 20

# of Authors: 1

Author Position: SF

End: 9:16

CONSENSUS: Bring a revision of P0870R1 (std::is_narrowing_conversion) to EWGI for review, and then revise, with the guidance below, and return to LEWGI for further design review.

• Paper authors are encouraged to provide a better example of an implementation.
• Paper authors are encouraged to provide a revision history.
• Paper authors are encouraged to provide additional references in P0870 to papers related to narrowing conversions that were mentioned during the discussion such as P1818.

[wg21bot]

P0870R2 A proposal for a type trait to detect narrowing conversions (Giuseppe D'Angelo)

[wg21bot]

P0870R3 A proposal for a type trait to detect narrowing conversions (Giuseppe D'Angelo)

[brycelelbach]

2022-09-13 Joint Library Evolution and Numerics Telecon

P0870R4: Type trait to detect narrowing conversions

2022-09-13 Joint Library Evolution and Numerics Telecon Minutes

Chair: Bryce Adelstein Lelbach

Minute Taker: Mark Hoemmen

Champion: Giuseppe D'Angelo

Start: 2022-09-13 11:05 Eastern

Does this paper have:

• Examples?
  • Yes.
• Field experience?
  • Implementation experience?
    • Yes, within Qt and as a patch to libstdc++.
  • Usage experience?
    • Yes, within Qt and as a patch to libstdc++.
  • Deployment experience?
    • Yes, within Qt and as a patch to libstdc++.
• Discussion of prior art?
  • Yes.
• Changes Library Evolution previously requested?
  • Yes.
• Wording?
  • Yes, but the Numerics group has suggested it could be improved.
• Feature test macro?
  • Yes.
• Freestanding?
  • The intention is yes.

Should future things like optional use is_convertible_without_narrowing?

Changing optional to use is_convertible_without_narrowing would be a breaking change - for example you can construct an optional<int> from a double today.

A variant<int> can't be constructed from a double, because it doesn't allow construction from things that narrow. Same for expected.

Related: P2509R0 (Type trait to detect value-preserving conversions).

POLL: Send P0870R4 (Type trait to detect narrowing conversions) to Library for C++26 classified as B3 - addition, to be confirmed with a Library Evolution electronic poll.

Strongly Favor	Weakly Favor	Neutral	Weakly Against	Strongly Against
7	6	1	0	0

Attendance: 18

# of Authors: 1

Author Position: SF

Outcome: Strong consensus in favor.

End: 11:38

Summary

This is a proposal for a new trait to detect narrowing conversions. As narrowing conversions themselves are not extensible to user-defined arithmetic types, the author's intent is for this proposal to just cover builtin types.

There was some discussion about how we might use this trait within the Standard Library, both for existing features and future features. Some proposed that we could change existing types, like optional and variant, to use this trait, but others pointed out this would be a breaking change in semantics.

We also briefly talked about a related proposal, P2509 (Type trait to detect value-preserving conversions).

Library Evolution feels this feature is useful and we believe the proposal is well-baked.

Next Steps

Take a Library Evolution electronic poll to send P0870R4 (Type trait to detect narrowing conversions) to Library for C++26 classified as B3 - addition.

[cor3ntin]

2022-10 Library Evolution Electronic Poll Outcomes

Send [P0870R4] is_convertible_without_narrowing to Library Working Group for C++26, classified as an addition ([P0592R4] bucket 3 item).

Strongly Favor	Weakly Favor	Neutral	Weakly Against	Strongly Against
9	6	1	0	0

Outcome: Strong consensus in favor.

[dangelog]

@cor3ntin I believe the above is a C&P mistake? I've got 9/6/1/0/0 (the outcomes paper isn't published yet, so I can't check against an authoritative reference...)

[cor3ntin]

Indeed, thanks for noticing.

[wg21bot]

P0870R5 A proposal for a type trait to detect narrowing conversions (Giuseppe D'Angelo)

[dangelog]

Hi @brycelelbach @cor3ntin ; during the review of this paper in Issaquah it's been requested whether LEWG could confirm a design choice.

To this end, I've uploaded R5. It does not change any design / behavior from R4, but it adds a paragraph that elaborates on the design choice mentioned above: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p0870r5.html#ch5.9

I guess this paper should go back to LWG only if LEWG can confirm that the choice was the intended one.

[brycelelbach]

This paper is now in the custody of Library Evolution.

[brycelelbach]

@inbal2l and @cor3ntin this can be handled with a mailing list review. It should get higher priority than others, as it's just a design clarification.