FA-4051 / Parsing / Open access

A prefix AST silently ignores an unconsumed token · case 01

A prefix AST silently ignores an unconsumed token.

Verified by executionVariant 1 · 4 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The implementation applies an operation whose text or grammar semantics violate this contract: Parse binary +/* prefix expressions with other tokens as leaves; reject missing operands and unconsumed tokens.

VERIFIED REPAIR

Implement the complete stated contract, including the boundary fixtures: Parse binary +/* prefix expressions with other tokens as leaves; reject missing operands and unconsumed tokens.

Unsuccessful approach: The attempted repair handles the primary example but still violates a separate boundary of the same contract.

Case contract

Parse binary +/* prefix expressions with other tokens as leaves; reject missing operands and unconsumed tokens.

Why this case matters

A local executable model for consumers of structured text; oracle values are authored literals, not outputs copied from the repaired implementation.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
import re, unicodedata, json, csv, io, html, base64, binascii, codecs, struct
from urllib.parse import quote, unquote, unquote_to_bytes, urlencode, parse_qsl, urlsplit, urlunsplit
from email.header import decode_header, make_header
from email.utils import getaddresses
from xml.etree import ElementTree as ET
import shlex, string, textwrap

N = 1
observations = []
def solve(x):
    try:
        def parse(i):
            if i >= len(x): raise ValueError('missing operand')
            t=x[i]
            if t not in ('+','*'): return t,i+1
            a,j=parse(i+1)
            b,k=parse(j)
            return [t,a,b],k
        try:
            tree,end=parse(0)
        except ValueError:
            return None
        return tree
    except Exception as exc:
        return {"error": type(exc).__name__}
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('boundary 1', solve(['+', 'a', 'b', 'c']), None)
check('boundary 2', solve(['+', 'a', '*', 'b', 'c']), ['+', 'a', ['*', 'b', 'c']])
check('boundary 3', solve(['a']), 'a')
check('boundary 4', solve(['+', 'a']), None)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
boundary 1	`['+', 'a', 'b']`	`None`	Failed
boundary 2	`['+', 'a', ['*', 'b', 'c']]`	`['+', 'a', ['*', 'b', 'c']]`	Passed
boundary 3	`a`	`a`	Passed
boundary 4	`None`	`None`	Passed

SHA-256 / 9d747ccca334b0385a0eb0f865bfbd316d1be292179e959ed48419bd018274c6

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
import re, unicodedata, json, csv, io, html, base64, binascii, codecs, struct
from urllib.parse import quote, unquote, unquote_to_bytes, urlencode, parse_qsl, urlsplit, urlunsplit
from email.header import decode_header, make_header
from email.utils import getaddresses
from xml.etree import ElementTree as ET
import shlex, string, textwrap

N = 1
observations = []
def solve(x):
    try:
        def parse(i):
            if i >= len(x): raise ValueError('missing operand')
            t=x[i]
            if t not in ('+','*'): return t,i+1
            a,j=parse(i+1)
            b,k=parse(j)
            return [t,a,b],k
        try:
            tree,end=parse(0)
        except ValueError:
            return None
        return tree if len(x)<=3 else None
    except Exception as exc:
        return {"error": type(exc).__name__}
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('boundary 1', solve(['+', 'a', 'b', 'c']), None)
check('boundary 2', solve(['+', 'a', '*', 'b', 'c']), ['+', 'a', ['*', 'b', 'c']])
check('boundary 3', solve(['a']), 'a')
check('boundary 4', solve(['+', 'a']), None)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
boundary 1	`None`	`None`	Passed
boundary 2	`None`	`['+', 'a', ['*', 'b', 'c']]`	Failed
boundary 3	`a`	`a`	Passed
boundary 4	`None`	`None`	Passed

SHA-256 / 81c884b2234fa45695446d78e444298036ba12a43d1c278f157d73642449bc35

3 / The verified repair

Exit 0

"""Failure Map reference implementation. Python standard library only."""
import json
import re, unicodedata, json, csv, io, html, base64, binascii, codecs, struct
from urllib.parse import quote, unquote, unquote_to_bytes, urlencode, parse_qsl, urlsplit, urlunsplit
from email.header import decode_header, make_header
from email.utils import getaddresses
from xml.etree import ElementTree as ET
import shlex, string, textwrap

N = 1
observations = []
def solve(x):
    try:
        def parse(i):
            if i >= len(x): raise ValueError('missing operand')
            t=x[i]
            if t not in ('+','*'): return t,i+1
            a,j=parse(i+1)
            b,k=parse(j)
            return [t,a,b],k
        try:
            tree,end=parse(0)
        except ValueError:
            return None
        return tree if end==len(x) else None
    except Exception as exc:
        return {"error": type(exc).__name__}
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('boundary 1', solve(['+', 'a', 'b', 'c']), None)
check('boundary 2', solve(['+', 'a', '*', 'b', 'c']), ['+', 'a', ['*', 'b', 'c']])
check('boundary 3', solve(['a']), 'a')
check('boundary 4', solve(['+', 'a']), None)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
boundary 1	`None`	`None`	Passed
boundary 2	`['+', 'a', ['*', 'b', 'c']]`	`['+', 'a', ['*', 'b', 'c']]`	Passed
boundary 3	`a`	`a`	Passed
boundary 4	`None`	`None`	Passed

SHA-256 / 2af3155983eb761d3103ec112aa356856875fb5766e6e3dafc0f002373b09bd2

Verification & scope

This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.

Observations recorded using Python 3.12.14 at 2026-09-29T14:37:34.780697+00:00.

Case digest / a8530e43fb1b540c9d48c7bb842939daee1465381856920bd29b28dba32e4537