D459 - A call trace records what an answer was, not only what was asked

Status: decided Date: 2026-09-01

Recording a guest library call records the value the implementation answered alongside the arguments the guest passed. A call whose answer is not yet known, or is not known to be safe to read, reports unknown rather than a default value.

Why: An implemented call answering a wrong value the guest goes on to trust is a real and common class of fault, and until the answer itself is recorded a reader has to reconstruct it by reading the implementation's code and guessing. Any value can be a legitimate answer, including zero, so a slot that never got an answer must be distinguishable from one that answered zero.

Rejected: recording only a boolean success/failure - loses exactly the value that turns out to be the wrong one.