Preserve EOF token when probing arrow EOL

Author: som-snytt

compiler/src/dotty/tools/dotc/parsing/Scanners.scala

@@ -682,13 +682,16 @@ object Scanners {
         reset()
         if atEOL then token = COLONeol
 
-    // consume => and insert <indent> if applicable
+    // consume => and insert <indent> if applicable. Used to detect colon arrow: x =>
     def observeArrowIndented(): Unit =
       if isArrow && indentSyntax then
         peekAhead()
-        val atEOL = isAfterLineEnd || token == EOF
+        val atEOL = isAfterLineEnd
+        val atEOF = token == EOF
         reset()
-        if atEOL then
+        if atEOF then
+          token = EOF
+        else if atEOL then
           val nextWidth = indentWidth(next.offset)
           val lastWidth = currentRegion.indentWidth
           if lastWidth < nextWidth then

compiler/test/dotty/tools/repl/ReplCompilerTests.scala

@@ -518,6 +518,9 @@ class ReplCompilerTests extends ReplTest:
          |""".stripMargin // outdent, but this test does not exercise the bug
     assertEquals(List("hello, world"), lines())
 
+  @Test def `i22844b regression colon arrow eol`: Unit = contextually:
+    assertTrue(ParseResult.isIncomplete("List(42).map: x =>"))
+
 object ReplCompilerTests:
 
   private val pattern = Pattern.compile("\\r[\\n]?|\\n");