FA-42458 / Listener lifecycle / Member archive

Combine-latest clears a seen marker when the other source updates · case 03

Combine-latest clears a seen marker when the other source updates; the returned lifecycle plan violates the stated transition contract.

Member previewVariant 3 · 3 implementations · 8 checks per implementation

Case contract

Plan a two-input combine-latest observer. A value may be None, so readiness comes from separate seen flags. Updating one side preserves the other. Emit a pair only after both inputs have been seen. A completed input with no value makes a pair impossible; completion after a value keeps its last value usable. Any source error terminates both subscriptions. Inputs are finite state snapshots, including transitional and stale states; output fields are the exact independent actions/metadata defined by the corrected expression table. Numeric fixture capacities and counts vary with N. Field definitions, each independently computed from the input snapshot: ready = s['left_seen'] and s['right_seen']; left = s['value'] if s['side']=='left' else s['left']; right = s['value'] if s['side']=='right' else s['right']; left_seen = s['left_seen'] or s['side']=='left'; right_seen = s['right_seen'] or s['side']=='right'; impossible = (s['left_done'] and not s['left_seen']) or (s['right_done'] and not s['right_seen']); complete = s['left_done'] and s['right_done']; pair = [s['left'],s['right']]; cancel_both = ['left','right'] if s['error'] is not None else []; usable_left = s['left_seen']

Why this case matters

Observer implementations need to preserve this state while subscriptions enter, execute, and leave their lifecycle.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
transition 1	`{"cancel_both": [], "complete": true, "impossible": true, "left": "L", "left_seen": false, "pair": ["L", null], "ready": false, "right": null, "right_seen": true, "usable_left": true}`	`{"cancel_both": [], "complete": true, "impossible": true, "left": "L", "left_seen": true, "pair": ["L", null], "ready": false, "right": null, "right_seen": true, "usable_left": true}`	Failed

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