P1018R6
Language Evolution status after Prague 2020

1. Executive summary

• Finalize ballot resolution for C++20, to address National Body comments in [N4844].

• Start work on features for C++23 and later.

• Joins session with LEWG on ABI, based on P1863R1.

2. Paper of note

• P1000R4 C++ IS schedule

• P0592R4 To boldly suggest an overall plan for C++23

• P1999R0 Process: 2×-🇨🇿 evolutionary material via a Tentatively Ready status

• P2118R0 Documenting Core Undefined or Unspecified Behavior

3. Tentatively ready papers

Following our process in P1999, here are the papers that EWG considers tentatively ready for CWG. We’ll take a brief look at the next meeting, and if nothing particular concerns anyone, send them to CWG.

• P1847R2 Make declaration order layout mandated

• P2025R0 Guaranteed copy elision for named return objects

• P1949R2 C++ Identifier Syntax using Unicode Standard Annex 31

You can follow this list on GitHub.

4. ABI discussion

We held a joint sessions with LEWG to discuss ABI, based on P1863R1, The outcome of the discussion was as follows:

• To the best of our ability, we should promise users that we won’t break ABI, ever
  Wasn’t contended: we disagree with this statement and might break ABI in the future.

• From now on, we should consider incremental ABI for every C++ release
  Received extremely positive support, with a small minority disagreeing strongly.

• We should consider a big ABI break for C++23
  Was extremely contended, with a few more people in favor than against. This was insufficient to call consensus.

• We should consider a big ABI break for C++SOMETHING
  Was positive enough to call consensus, but still had a quite substantial opposition including many disagreeing strongly. Were we to do a big ABI break we would need to work very hard on consensus building. Indeed, the number of people disagreeing strongly on a poll for a concrete change would block consensus.

• When we are unable to resolve a conflict between performance and ABI compatibility, we should prioritize performance
  Was still more positive, but also had a quite substantial opposition including many disagreeing strongly. Again, we should consider performance over ABI but work extremely hard towards consensus building when doing so.

5. National body comments

• P2003R0 Fixing Internal and External Linkage Entities in Header Units #740

• P2014R0 Proposed resolution for US061/US062 - aligned allocation of coroutine frames #750

• P1884R0 Private Module Partition: An Inconsistent Boundary #729

• P2100R0 Keep unhandled_exception of a promise type mandatory - a response to US062 and FR066

• P2104R0 GB046 Allow caching of evaluations of concept specializations #45

6. C++23 discussions

We discussed a few papers which could make it to C++23:

• P2085R0 Consistent defaulted comparisons

• P0592R4 To boldly suggest an overall plan for C++23

• P1999R0 Process proposal: double-check evolutionary material via a Tentatively Ready status

• P1468R3 Fixed-layout floating-point type aliases

• P1467R3 Extended floating-point types

• P1371R2 Pattern Matching

• P1000R4 C++ IS schedule

• P1726R2 Pointer lifetime-end zap

• P2092R0 Disambiguating Nested-Requirements

• P1040R5 std::embed

• P1677R2 Cancellation is not an Error

• P1401R2 Narrowing contextual conversions to bool

• P0876R10 fiber_context - fibers without scheduler

• P0847R4 Deducing this

• P2082R1 Fixing CTAD for aggregates

• P1774R3 Portable assumptions

• P2118R0 Documenting Core Undefined or Unspecified Behavior

• P0849R2 auto(x): decay-copy in the language

• P2036R0 Changing scope for lambda trailing-return-type

• P2071R0 Named universal character escapes

• P1900R0 Concepts-Adjacent Problems

• P1847R2 Make declaration order layout mandated

• P1393R0 A General Property Customization Mechanism

• P2026R0 A Constituent Study Group for Safety-Critical Applications

• P1938R0 if consteval

• P1955R0 Top Level Is Constant Evaluated

• P2041R0 Deleting variable templates

• P0870R2 A proposal for a type trait to detect narrowing conversions

• P2025R0 Guaranteed copy elision for named return objects

• P2013R0 Freestanding Language: Optional ::operator new

• P1949R2 C++ Identifier Syntax using Unicode Standard Annex 31

The following papers were scheduled for discussion, but authors requested to delay until the next meeting:

• P1967R1 #embed - a simple, scannable preprocessor-based resource acquisition method

• P1046R2 Automatically Generate More Operators

• P2049R0 Constraint refinement for special-cased functions

The following papers were scheduled for discussion, but were seen in SG7 Reflection who decided to table them for now:

• P1733R0 User-friendly and Evolution-friendly Reflection: A Compromise

• P2089R0 Function parameter constraints are fragile

7. Near-future EWG plans

We will continue to work on C++23, prioritizing according to P0592.