Restore parser state when an optional clause fails to parse

Summary#

An external GitHub parser fix showed that consuming part of an optional construct without validating the whole construct can silently accept invalid input. When parsing speculative branches, snapshot the cursor and restore it on failure so the caller can either continue correctly or raise the intended syntax error.

Problem#

A parser advanced past a keyword for an optional clause, then failed to parse the required payload and left the input position corrupted, producing silent misbehavior instead of a clean error.

Solution#

Save parser position before entering optional syntax, parse the full clause atomically, and roll back state if any required subparse fails.

Failure Modes#

Invalid statements are accepted with altered semantics
Later parser stages report confusing downstream errors
Short-circuit logic consumes tokens that the caller can no longer recover

Sources#

https://github.com/tensorflow/tensorflow/pull/123870
https://github.com/tensorflow/tensorflow/pull/123810
https://github.com/tensorflow/tensorflow/pull/123660
https://github.com/tensorflow/tensorflow/pull/123857
https://github.com/tensorflow/tensorflow/pull/123872
https://github.com/tensorflow/tensorflow/pull/123868
https://github.com/tensorflow/tensorflow/pull/123801
https://github.com/tensorflow/tensorflow/pull/123732
https://github.com/tensorflow/tensorflow/pull/122987
https://github.com/tensorflow/tensorflow/pull/123711
https://github.com/tensorflow/tensorflow/pull/123722
https://github.com/tensorflow/tensorflow/pull/123347
https://github.com/tensorflow/tensorflow/pull/123781
https://github.com/tensorflow/tensorflow/pull/115458
https://github.com/huggingface/transformers/pull/47336
https://github.com/huggingface/transformers/pull/47481
https://github.com/huggingface/transformers/pull/47403
https://github.com/huggingface/transformers/pull/47422
https://github.com/huggingface/transformers/pull/46867
https://github.com/huggingface/transformers/pull/47492
https://github.com/huggingface/transformers/pull/47483
https://github.com/huggingface/transformers/pull/46795
https://github.com/huggingface/transformers/pull/47460
https://github.com/huggingface/transformers/pull/46267
https://github.com/huggingface/transformers/pull/46903
https://github.com/microsoft/ML-For-Beginners/pull/1002
https://github.com/microsoft/ML-For-Beginners/pull/1001
https://github.com/microsoft/ML-For-Beginners/pull/1000
https://github.com/microsoft/ML-For-Beginners/pull/994
https://github.com/microsoft/ML-For-Beginners/pull/991
https://github.com/microsoft/ML-For-Beginners/pull/990
https://github.com/microsoft/ML-For-Beginners/pull/989
https://github.com/ClickHouse/ClickHouse/pull/111159
https://github.com/ClickHouse/ClickHouse/pull/111342
https://github.com/ClickHouse/ClickHouse/pull/111678
https://github.com/ClickHouse/ClickHouse/pull/111679
https://github.com/ClickHouse/ClickHouse/pull/111680
https://github.com/ClickHouse/ClickHouse/pull/111681
https://github.com/ClickHouse/ClickHouse/pull/110460
https://github.com/ClickHouse/ClickHouse/pull/111682
https://github.com/ClickHouse/ClickHouse/pull/89180
https://github.com/ClickHouse/ClickHouse/pull/98669
https://github.com/ClickHouse/ClickHouse/pull/110710
https://github.com/vercel/turborepo/pull/13453
https://github.com/vercel/turborepo/pull/13443
https://github.com/vercel/turborepo/pull/13447
https://github.com/vercel/turborepo/pull/13452
https://github.com/vercel/turborepo/pull/13442
https://github.com/vercel/turborepo/pull/13450
https://github.com/vercel/turborepo/pull/13441
https://github.com/vercel/turborepo/pull/13440
https://github.com/vercel/turborepo/pull/13446
mined_at: 2026-07-24T01:10:11Z

Sagwan Revalidation 2026-07-24T01:48:28Z#

verdict: ok
note: 선택 구문 실패 시 상태 롤백 원칙은 현재도 유효한 파서 관행이다.

Sagwan Revalidation 2026-07-26T04:13:55Z#

verdict: ok
note: 선택 구문 파싱 실패 시 상태 복원 원칙은 여전히 최신 관행과 일치함

Sagwan Revalidation 2026-07-28T11:58:42Z#

verdict: ok
note: 선택 구문 실패 시 상태 롤백 원칙은 여전히 최신 파서 관행과 일치함

Sagwan Revalidation 2026-07-30T16:15:25Z#

verdict: ok
note: 파서 상태 롤백 원칙은 여전히 유효하며 최신 관행과 충돌 없음.

Sagwan Revalidation 2026-08-02T02:38:03Z#

verdict: ok
note: 선택 구문 실패 시 파서 상태 롤백 원칙은 여전히 표준 관행입니다.

Sagwan Revalidation 2026-08-06T02:37:45Z#

verdict: ok
note: [chatgpt HTTP 401] {

Sagwan Revalidation 2026-08-08T12:01:02Z#

verdict: ok
note: [chatgpt HTTP 401] {

Sagwan Revalidation 2026-08-11T00:19:03Z#

verdict: ok
note: [chatgpt HTTP 401] {

Sagwan Revalidation 2026-08-13T12:03:41Z#

verdict: ok
note: [chatgpt HTTP 401] {

Sagwan Revalidation 2026-08-16T01:22:45Z#

verdict: ok
note: [chatgpt HTTP 401] {

Sagwan Revalidation 2026-08-18T12:54:05Z#

verdict: ok
note: [chatgpt HTTP 401] {

Sagwan Revalidation 2026-08-21T01:26:08Z#

verdict: ok
note: [chatgpt HTTP 401] {

Sagwan Revalidation 2026-08-23T14:03:11Z#

verdict: ok
note: [chatgpt HTTP 401] {

Sagwan Revalidation 2026-08-26T02:20:44Z#

verdict: ok
note: [chatgpt HTTP 401] {

Sagwan Revalidation 2026-08-28T14:29:56Z#

verdict: ok
note: [chatgpt HTTP 401] {

Sagwan Revalidation 2026-08-31T03:18:12Z#

verdict: ok
note: [chatgpt HTTP 401] {

Sagwan Revalidation 2026-09-02T17:28:02Z#

verdict: ok
note: 선택 구문 실패 시 상태 복원 원칙은 여전히 유효한 파서 관행입니다.

Sagwan Revalidation 2026-09-08T20:32:55Z#

verdict: ok
note: [chatgpt HTTP 404] {

Sagwan Revalidation 2026-09-11T11:01:54Z#

verdict: ok
note: 투기적 파싱 시 커서 스냅샷·복원 패턴은 PEG/재귀하강 파서 전반에 걸친 불변 원칙으로 현재도 유효하다.

Sagwan Revalidation 2026-09-14T06:35:23Z#

verdict: ok
note: 파서 커서 스냅샷 후 실패 시 롤백하는 기법은 시대를 타지 않는 기본 원칙이며 현재 practice와 완전히 일치한다.