Single line case lambda gets a region

Author: som-snytt

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

@@ -1137,7 +1137,11 @@ object Parsers {
           lookahead.skipParens()
           isArrowIndent()
         else if lookahead.token == CASE && in.featureEnabled(Feature.relaxedLambdaSyntax) then
-          Some(() => singleCaseMatch())
+          Some: () =>
+            inSepRegion(SingleLineLambda(_)):
+              singleCaseMatch()
+            .tap: _ =>
+              accept(ENDlambda)
         else
           None
       isParamsAndArrow()

tests/neg/i24496.scala

@@ -0,0 +1,12 @@
+import scala.language.experimental.relaxedLambdaSyntax
+
+@main def Test =
+  val list = List(1, 2, 3)
+
+  val three = list
+        .collect: case x =>
+          (x, x + 1) // error not a member of tuple
+        .toMap
+
+  val huh = list
+        .collect: x => case y => (y, y + 1) // error expecting case at x

tests/pos/i24496.scala

@@ -0,0 +1,15 @@
+import scala.language.experimental.relaxedLambdaSyntax
+
+@main def Test =
+  val list = List(1, 2, 3)
+
+  val two = list
+        .collect: x => (x, x + 1)
+        .toMap
+
+  val one = list
+        .collect: case x => (x, x + 1)
+        .toMap
+
+  //val huh = list
+  //      .collect: x => case y => (y, y + 1) correctly errors expecting case at x