fix(parsing): also reject a plain multi-line key in a flow sequence

A flow-sequence single-pair implicit key must be single-line, so its ':'
must sit on the key's line. The quoted form ('[ "key"\n :value ]') was
already rejected via a crossed line break, but a plain key folds its line
break, hiding it. Compare the simple key's mark line with the ':' line
instead, which catches both forms and still exempts flow mappings.

Surpasses libyaml on DK4H (and keeps ZXT5); whole-suite conformance
398/402, canonical events 308/308.

CHANGELOG.md

@@ -23,11 +23,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 - Reject a multi-line implicit key in a flow sequence. A flow-sequence single-pair
-  implicit key is single-line (`c-s-implicit-json-key`), so its `:` may not cross a
-  line break: `[ "key"\n :value ]` is malformed. A flow MAPPING adjacent value may
-  still span lines (`{ "foo"\n :bar }` stays valid), so the rejection is scoped to
-  flow levels that are sequences, tracked alongside the flow-level stack. Surpasses
-  libyaml on ZXT5.
+  implicit key is single-line (`c-s-implicit-json-key`), so its `:` must sit on the
+  key's line: both `[ "key"\n :value ]` and `[ key\n : value ]` are malformed. A
+  flow MAPPING adjacent value may still span lines (`{ "foo"\n :bar }` stays
+  valid), so the rejection is scoped to flow levels that are sequences, tracked
+  alongside the flow-level stack. The key/`:` line comparison catches the plain-key
+  form too, which folds its line break. Surpasses libyaml on ZXT5 and DK4H.
 - Reject a block sequence that opens on the same line as an earlier node. A `-`
   entry mid-line after an inline mapping value (`key: - a`) or a node property
   (`&anchor - x`) is invalid, while a `-` after another `-` stays a valid compact

Sources/Parsing/LibYAML.Scanner.Fetch.swift

@@ -314,8 +314,9 @@ extension PureYAML.Parsing.LibYAML.Scanner {
         // (it went stale across the line break), so its value may open a sequence.
         lastValueWasInline = simpleKeys[top].possible
         // A flow-sequence single-pair implicit key must be single-line, so its `:`
-        // may not cross a line break; a flow-mapping adjacent value may.
-        if simpleKeys[top].possible, crossedLineBreak, flowIsSequence.last == true {
+        // must sit on the key's line (`[ "k"\n :v ]` and `[ k\n : v ]` are both
+        // invalid); a flow-mapping adjacent value may span lines (`{ "k"\n :v }`).
+        if simpleKeys[top].possible, flowIsSequence.last == true, simpleKeys[top].mark.line != mark.line {
             throw ScannerError(
                 context: "while scanning a simple key",
                 problem: "could not find expected ':'",

Tests/Fixtures/yaml-test-suite-manifest.yaml

@@ -10,8 +10,8 @@ source:
 # oracle:   json if in.json present, else none
 # status:   passing (PureYAML conformant today) | gap (not yet conformant)
 total: 402
-passing: 397
-gap: 5
+passing: 398
+gap: 4
 cases:
   - id: "229Q"
     name: "Spec Example 2.4. Sequence of Mappings"
@@ -1490,7 +1490,7 @@ cases:
     localPath: yaml-test-suite/DK4H/in.yaml
     expected: reject
     oracle: none
-    status: gap
+    status: passing
     byteCount: 22
     lineCount: 3
   - id: "DK95/00"