fix(parsing): parse a leading ':' flow-sequence entry as an empty-key pair

A flow sequence entry that begins with ':' (e.g. '[ : v ]') is a
single-pair mapping with an empty key, which YAML 1.2 permits but libyaml
rejects by handing the leading VALUE token to its node parser.

Open the implicit single-pair mapping on a leading ':' in the
flow-sequence-entry parser, and leave the VALUE token for the mapping
value state to consume so the value is parsed rather than dropped.
Surpasses libyaml on CFD4. Port score 298 -> 299.

CHANGELOG.md

@@ -22,6 +22,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- Parse a flow sequence entry that begins with `:` as a single-pair mapping with
+  an empty key (`[ : v ]`). libyaml hands the leading VALUE token to its node
+  parser and fails. The flow-sequence-entry parser now opens the implicit mapping
+  on a leading `:` and leaves the indicator for the value state, so the value is
+  parsed rather than dropped. Surpasses libyaml on CFD4.
 - Apply the YAML 1.2 flow `:` rule instead of libyaml's "always a value
   indicator in flow". In a flow collection a `:` only ends a node when it is
   followed by whitespace, the end of input, or a flow indicator, or when it is

Sources/Parsing/LibYAML.Parser.Flow.swift

@@ -24,6 +24,12 @@ extension PureYAML.Parsing.LibYAML.Parser {
                 skipToken()
                 return .mappingStart(anchor: nil, tag: nil, style: .flow, mark: token.startMark)
             }
+            if isValue(token) {
+                // A leading `:` opens a single-pair mapping with an empty key
+                // (`[ : v ]`). Keep the VALUE token for the mapping-key state.
+                state = .flowSequenceEntryMappingKey
+                return .mappingStart(anchor: nil, tag: nil, style: .flow, mark: token.startMark)
+            }
             if !isFlowSequenceEnd(token) {
                 states.append(.flowSequenceEntry)
                 return try parseNode(block: false, indentlessSequence: false)
@@ -42,7 +48,9 @@ extension PureYAML.Parsing.LibYAML.Parser {
             states.append(.flowSequenceEntryMappingValue)
             return try parseNode(block: false, indentlessSequence: false)
         }
-        if isFlowSequenceEnd(token) {
+        // An empty key: leave a VALUE token for the value state to consume (so the
+        // value is parsed); a closing `]` ends the pair with an empty value too.
+        if isValue(token) || isFlowSequenceEnd(token) {
             state = .flowSequenceEntryMappingValue
             return processEmptyScalar(at: token.startMark)
         }

Tests/CanonicalEventConformanceTests.swift

@@ -12,7 +12,7 @@ struct CanonicalEventConformanceTests {
     /// Valid suite cases whose event stream PureYAML does NOT yet emit exactly.
     /// This is the authoritative gap set: a fix removes an id (and the test fails
     /// until it is removed), a regression adds one (and the test fails). Of these,
-    /// 8 currently throw on valid input and 2 emit a wrong event stream.
+    /// 7 currently throw on valid input and 2 emit a wrong event stream.
     static let knownEventGaps: Set<String> = [
         // Gap set for the ported libyaml engine. Verified against the libyaml
         // reference dumper: on all but four of these the port matches libyaml
@@ -22,7 +22,7 @@ struct CanonicalEventConformanceTests {
         //
         // false-rejects (parseEvents throws on valid input, as libyaml does)
         "6BCT", "6CA3",
-        "A2M4", "CFD4", "DK95/00",
+        "A2M4", "DK95/00",
         "DK95/04",
         "Q5MG",
         "Y79Y/010",