P3969 R2 Fixing std::bit_cast of types with padding bits

[wg21bot]

P3969R0 Fixing std::bit_cast of types with padding bits (Jan Schultke)

[hanickadot]

EWG in Croydon:

EWG prefers adding another flavor of bit_cast (padding zero-ing) in addition to existing bit_cast (which makes reading padding bits a library UB)

SF	F	N	A	SA
4	3	7	10	3

Result: not consensus

EWG encourages more work in direction P3969R0

SF	F	N	A	SA
12	16	3	1	0

Result: consensus

[inbal2l]

(out of order)

2026-03-10 Library Evolution Telecon

P3969R0: Fixing std::bit_cast of types with padding bits

2026-03-10 Library Evolution Telecon Minutes

Champion: Jan Schultke
Chair: Andreas
Minute Taker: Bronek

Summary

ACTIONs:

• Define behaviour of constexpr version on union, and of struct encapsulating union.
• Measure performance
• Have a table of results on different evaluations

POLL: We think that changing the current undefined behaviour of bit_cast to silently zeroing padding bits is problematic.

SF	F	N	A	SA
6	6	1	0	0

Attendance: 19
Author's Position: SF
Outcome: Consensus in favour.

ACTION: Submit an issue for LWG that emits the “UB” from bit_cast’s wording in constexpr case (or however LWG handles “UB” in functions transferred into constexpr) (wording bug).

POLL: The undefined behavior in bit_cast should be diagnosable during constexpr.

Attendance: 19
Author's Position: N/A
Outcome: No objection to unanimous consent.

Next Steps

• Room supported keeping the current behaviour of bit_cast (changing the current undefined behaviour of bit_cast to silently zeroing padding bits is problematic), but was open to additional functions with different behaviour (zeroing padding bits, etc.)
• EWG was tagged for additional input on the language side
• Next revision should include:
  • Define behaviour of constexpr version on union, and of struct encapsulating union.
  • Measure performance
  • Have a table of results on different evaluations
  • The additional functions should have the ability to diagnose UB during constexpr

[wg21bot]

P3969R1 Fixing std::bit_cast of types with padding bits (Jan Schultke)