feat: Parse an ATX heading followed by --- or === as a heading (for IJPL-94630)

CHANGELOG.md

@@ -3,6 +3,7 @@
 
 ## [Unreleased]
 - Decode character references above U+FFFF, such as `😀` or `𝔄`, to the right character instead of a truncated one, and replace `�`, surrogates and out of range numeric references with U+FFFD as required by CommonMark
+- Parse an ATX heading followed by `---` or `===` as a heading, not as setext content (IJPL-94630)
 
 ## [0.7.18]
 

src/commonMain/kotlin/org/intellij/markdown/parser/markerblocks/providers/SetextHeaderProvider.kt

@@ -22,6 +22,10 @@ class SetextHeaderProvider : MarkerBlockProvider<MarkerProcessor.StateInfo> {
         if (stateInfo.nextConstraints != currentConstaints) {
             return emptyList()
         }
+        // An ATX heading can't be the content of a setext heading
+        if (ATX_HEADER_PROVIDER.interruptsParagraph(pos, currentConstaints)) {
+            return emptyList()
+        }
         if (MarkerBlockProvider.isStartOfLineWithConstraints(pos, currentConstaints)
                 && getNextLineFromConstraints(pos, currentConstaints)?.let { REGEX.matches(it) } == true) {
             return listOf(SetextHeaderMarkerBlock(currentConstaints, productionHolder))
@@ -50,5 +54,7 @@ class SetextHeaderProvider : MarkerBlockProvider<MarkerProcessor.StateInfo> {
 
     companion object {
         val REGEX: Regex = Regex("^ {0,3}(-+|=+) *$")
+
+        private val ATX_HEADER_PROVIDER = AtxHeaderProvider()
     }
 }

src/fileBasedTest/resources/data/parser/hruleAndSetext.md

@@ -1,3 +1,7 @@
 ***
 ---
 ___
+# atx
+---
+# atx
+===

src/fileBasedTest/resources/data/parser/hruleAndSetext.txt

@@ -4,4 +4,21 @@ Markdown:MARKDOWN_FILE
   Markdown:HORIZONTAL_RULE('---')
   Markdown:EOL('\n')
   Markdown:HORIZONTAL_RULE('___')
+  Markdown:EOL('\n')
+  Markdown:ATX_1
+    Markdown:ATX_HEADER('#')
+    Markdown:ATX_CONTENT
+      WHITE_SPACE(' ')
+      Markdown:TEXT('atx')
+  Markdown:EOL('\n')
+  Markdown:HORIZONTAL_RULE('---')
+  Markdown:EOL('\n')
+  Markdown:ATX_1
+    Markdown:ATX_HEADER('#')
+    Markdown:ATX_CONTENT
+      WHITE_SPACE(' ')
+      Markdown:TEXT('atx')
+  Markdown:EOL('\n')
+  Markdown:PARAGRAPH
+    Markdown:TEXT('===')
   Markdown:EOL('\n')