Single line case lambda gets a region (#24497)

Fixes #24496 

Relaxed case lambda uses `SingleLineLambda` region for EOL handling. If
`ExprCaseClause` sees `ENDlambda` after `=>`, permit an indented region.
`SingleLineLambda` preserves indent width.

Author: noti0na1

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

@@ -1139,7 +1139,11 @@ 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()
@@ -3230,6 +3234,9 @@ object Parsers {
       val body = tok match
         case ARROW => atSpan(in.skipToken()):
           if exprOnly then
+            if in.token == ENDlambda then
+              in.token = NEWLINE
+              in.observeIndented()
             if in.indentSyntax && in.isAfterLineEnd && in.token != INDENT then
               warning(em"""Misleading indentation: this expression forms part of the preceding case.
                           |If this is intended, it should be indented for clarity.

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

@@ -596,7 +596,7 @@ object Scanners {
           lastWidth = r.width
           newlineIsSeparating = lastWidth <= nextWidth || r.isOutermost
           indentPrefix = r.prefix
-        case _: InString => ()
+        case _: InString | _: SingleLineLambda => ()
         case r =>
           indentIsSignificant = indentSyntax
           r.proposeKnownWidth(nextWidth, lastToken)

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)
+        .toMap // error value toMap is not a member of (Int, Int)
+
+  val huh = list
+        .collect: x => case y => (y, y + 1) // error expecting case at x

tests/pos/i24496.scala

@@ -0,0 +1,18 @@
+import scala.language.experimental.relaxedLambdaSyntax
+
+@main def Test =
+  val list = List(1, 2, 3)
+
+  val three = list
+        .collect: case x =>
+          val y = x + 1
+          (x, y)
+        .toMap
+
+  val two = list
+        .collect: x => (x, x + 1)
+        .toMap
+
+  val one = list
+        .collect: case x => (x, x + 1)
+        .toMap