fix(parsing): reject a lone dash before a flow indicator

In a flow collection a '-' that begins a plain scalar must be followed by
a non-indicator character. Immediately before a flow indicator ('[-]',
'[-, -]', '{a: -}') the dash is not a valid scalar, but the scanner
accepted it as the scalar '-'.

scanToken now rejects a '-' in flow context when the next character is a
flow indicator, before falling through to plain-scalar scanning. A dash
followed by ordinary content stays valid: negative numbers, a dash inside
a scalar, a dash starting a word, and a quoted dash are unaffected.

Measured against the official YAML test-suite, parse-level conformance
rises from 273/333 (82.0%) to 275/333 (82.6%) with no regression.

CHANGELOG.md

@@ -23,6 +23,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- A `-` immediately before a flow indicator is no longer treated as a scalar. In a flow collection
+  a `-` that begins a plain scalar must be followed by a non-indicator character, so `[-]`,
+  `[-, -]`, and `{a: -}` are now rejected. A dash followed by ordinary content is unaffected:
+  negative numbers (`[-1, -2]`), a dash inside a scalar (`a-b`), a dash starting a word (`-x`), and
+  a quoted dash all stay valid. Lifts official test-suite parse-level conformance from 273/333
+  (82.0%) to 275/333 (82.6%), with no regression.
 - A block scalar leading blank line that is more indented than the content is now rejected. The
   spec forbids a leading empty line from containing more spaces than the first non-empty line, so a
   block scalar whose blank lines are wider than its content (`|` then a 5-space blank then 2-space

Sources/Parsing/Scanner.swift

@@ -164,6 +164,16 @@ extension PureYAML.Parsing.Scanner {
         if try scanNodePropertyToken(character, state: &state) {
             return
         }
+        // In a flow collection, a `-` that begins a plain scalar must be followed by a non-indicator
+        // character. Immediately before a flow indicator (`[-]`, `[-, -]`) it is not a valid scalar.
+        if state.flowDepth > 0, character == "-", let next = state.reader.peek(offset: 1), isFlowDelimiter(next) {
+            throw PureYAML.Parsing.ParseError.unexpectedToken(
+                expected: "a flow value",
+                actual: "-",
+                line: state.reader.mark.line,
+                column: state.reader.mark.column,
+            )
+        }
         scanPlainScalar(&state)
     }
 

Tests/FlowDashScalarTests.swift

@@ -0,0 +1,26 @@
+@testable import PureYAML
+import Testing
+
+@Suite("A dash before a flow indicator")
+struct FlowDashScalarTests {
+    @Test("A `-` immediately before a flow indicator is not a valid scalar", arguments: [
+        "[-]\n", // a lone dash before `]`
+        "---\n- [-, -]\n", // dashes before `,` and `]`
+        "{a: -}\n", // a lone dash before `}`
+    ])
+    func test_dashBeforeFlowIndicatorRejected(_ yaml: String) throws {
+        #expect(throws: PureYAML.Parsing.ParseError.self) {
+            try PureYAML.parse(yaml)
+        }
+    }
+
+    @Test("A dash followed by a non-indicator character stays a valid scalar", arguments: [
+        "[-1, -2]\n", // negative numbers
+        "[a-b, c-d]\n", // a dash inside a scalar
+        "[-x, -y]\n", // a dash starting a word
+        "[\"-\", '-']\n", // a quoted dash
+    ])
+    func test_validDashScalars(_ yaml: String) throws {
+        _ = try PureYAML.parse(yaml)
+    }
+}