4.3: a paragraph line reading as a link reference definition escapes its opening bracket

corpus/round-trip/combinations/link-definition-line.json

@@ -0,0 +1,76 @@
+{
+  "content": [
+    {
+      "content": [
+        {
+          "text": "[",
+          "type": "text"
+        },
+        {
+          "attrs": {
+            "timestamp": "]:a"
+          },
+          "type": "date"
+        }
+      ],
+      "type": "paragraph"
+    },
+    {
+      "content": [
+        {
+          "content": [
+            {
+              "text": "[",
+              "type": "text"
+            },
+            {
+              "marks": [
+                {
+                  "type": "code"
+                }
+              ],
+              "text": "]: a",
+              "type": "text"
+            }
+          ],
+          "type": "paragraph"
+        }
+      ],
+      "type": "blockquote"
+    },
+    {
+      "content": [
+        {
+          "content": [
+            {
+              "content": [
+                {
+                  "text": "[",
+                  "type": "text"
+                },
+                {
+                  "attrs": {
+                    "timestamp": "]:a"
+                  },
+                  "type": "date"
+                },
+                {
+                  "type": "hardBreak"
+                },
+                {
+                  "text": "b",
+                  "type": "text"
+                }
+              ],
+              "type": "paragraph"
+            }
+          ],
+          "type": "listItem"
+        }
+      ],
+      "type": "bulletList"
+    }
+  ],
+  "type": "doc",
+  "version": 1
+}

corpus/round-trip/combinations/link-definition-line.md

@@ -0,0 +1,6 @@
+\[:date{timestamp="]:a"}
+
+> \[`]: a`
+
+- \[:date{timestamp="]:a"}\
+  b

spec/flavour.md

@@ -54,6 +54,8 @@ normalizes to it through the round-trip.
 - Entity references in input decode to their characters; output backslash-escapes only where text
   would otherwise parse as syntax, scanning the assembled line rather than each text node: escape
   the leading delimiter of a construct that would otherwise open, re-scan from there, and repeat.
+  A paragraph's opening `[` escapes wherever its line reads as a link reference definition, which
+  resolves before any inline construct binds: a `]` inside a code span or `{attrs}` counts.
   An emphasis delimiter run in text escapes where CommonMark can open **or** close with it, so
   `*not emphasis*` is `\*not emphasis\*` — no delimiter the emitter did not write reaches the
   matching below, which is what lets the emitter decide its own pairings.

src/markdown/emit/line-escaping.ts

@@ -3,6 +3,7 @@ import { delimiterFlags, isWordCharacter, matchEmphasis, runLength } from '../em
 import { backslashEscape, escapesLineClaim, inlineHtmlConstruct, opensBracketedAutolink, opensEmailAutolink, type LinePosition } from '../commonmark-grammar.ts'
 import { isBareDelimiterRow } from '../pipe-table-syntax.ts'
 import { opensInlineDirective } from '../directive-syntax.ts'
+import { opensLinkDefinition } from '../link-reference-definitions.ts'
 import { readEntityReference } from '../entity-references.ts'
 
 export type EmphasisRole = 'close' | 'open'
@@ -27,8 +28,7 @@ type EmittedRun = { canClose: boolean; canOpen: boolean; character: string; deli
 
 const delimiters = ['*', '_', '`', '~']
 
-// The `:` keeps a `[label]: url` line escaped: unescaped, the parser swallows it as a link reference definition.
-const followsLinkText = /[([:]/
+const followsLinkText = /[([]/
 
 export function assembleInlineLine(segments: readonly InlineSegment[], container: LineContainer): AssembledLine {
   return escape(resolveEmphasis(segments), container)
@@ -90,7 +90,8 @@ function escape(segments: readonly InlineSegment[], container: LineContainer): A
     placements.push(output.length)
     output += scan.charAt(index)
   }
-  return { line: output, unspellableRun: unspellableRun(segments, output, placements) }
+  const spelled = container === 'paragraph' && opensLinkDefinition(output) ? `\\${output}` : output
+  return { line: spelled, unspellableRun: unspellableRun(segments, output, placements) }
 }
 
 function unspellableRun(segments: readonly InlineSegment[], output: string, placements: readonly number[]): NodeRange | undefined {

src/markdown/link-reference-definitions.ts

@@ -1,10 +1,14 @@
-import type { LinkDefinition, LinkPart } from '../link-syntax.ts'
-import { normalizeLabel, readDestination, readLabel, readTitle, skipLinkWhitespace } from '../link-syntax.ts'
+import type { LinkDefinition, LinkPart } from './link-syntax.ts'
+import { normalizeLabel, readDestination, readLabel, readTitle, skipLinkWhitespace } from './link-syntax.ts'
 
 type ReadDefinition = { definition: LinkDefinition; label: string; length: number }
 
 const restOfLine = /^[ \t]*(?:\n|$)/
 
+export function opensLinkDefinition(text: string): boolean {
+  return readDefinition(text) !== undefined
+}
+
 export function readLinkDefinitions(definitions: Map<string, LinkDefinition>, text: string): string {
   let rest = text
   let read = readDefinition(rest)

src/markdown/parse/blocks.ts

@@ -18,7 +18,7 @@ import {
 } from '../commonmark-grammar.ts'
 import { directiveLineEscape, malformedDirective, readDirectiveLine } from '../directive-syntax.ts'
 import { barePipeCells, isDelimiterRow, isPipeAlignment, isPipeDelimiter, malformedPipeTable, pipeCells } from '../pipe-table-syntax.ts'
-import { readLinkDefinitions } from './link-reference-definitions.ts'
+import { readLinkDefinitions } from '../link-reference-definitions.ts'
 
 export type Block = { position: SourcePosition } & (
   | { argument: string | undefined; attributes: DirectiveAttributes; blocks: Block[] | undefined; kind: 'directive'; name: string }