Permit indented RHS of single line case

Author: som-snytt

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

@@ -3230,15 +3230,20 @@ object Parsers {
           atSpan(self.span)(Block(Nil, EmptyTree))
 
       val body = tok match
-        case ARROW => atSpan(in.skipToken()):
-          if exprOnly then
-            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.
-                          |Otherwise, if the handler is intended to be empty, use a multi-line match or catch with
-                          |an indented case.""")
-            expr()
-          else block()
+        case ARROW =>
+          val arrowAt = in.offset
+          atSpan(in.skipToken()):
+            if exprOnly then
+              if in.token == ENDlambda && source.offsetToLine(arrowAt) != source.offsetToLine(in.offset) 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.
+                            |Otherwise, if the handler is intended to be empty, use a multi-line match or catch with
+                            |an indented case.""")
+              expr()
+            else block()
         case IF => atSpan(in.skipToken()):
           // a sub match after a guard is parsed the same as one without
           val t = inSepRegion(InCase)(postfixExpr(Location.InGuard))

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

@@ -5,8 +5,8 @@ import scala.language.experimental.relaxedLambdaSyntax
 
   val three = list
         .collect: case x =>
-          (x, x + 1) // error not a member of tuple
-        .toMap
+        (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

@@ -3,13 +3,16 @@ 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
-
-  //val huh = list
-  //      .collect: x => case y => (y, y + 1) correctly errors expecting case at x