Allow single sub case without new line

Author: EugeneFlesselle

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

@@ -2595,8 +2595,10 @@ object Parsers {
     /** SubMatchClause ::= `match' `{' CaseClauses `}'
      */
     def subMatchClause(t: Tree): SubMatch = atSpan(startOffset(t), accept(MATCH)):
-      val cases = inBracesOrIndented(caseClauses(() => caseClause()))
-      if !in.isNestedEnd then acceptStatSep() // else is sub sub match
+      val cases =
+        if in.token == CASE
+        then caseClause(exprOnly = true) :: Nil // single sub case without new line
+        else inBracesOrIndented(caseClauses(() => caseClause()))
       SubMatch(t, cases)
 
     /**    `match' <<< TypeCaseClauses >>>
@@ -3109,7 +3111,9 @@ object Parsers {
       }
       val body =
         if in.token == WITH && in.featureEnabled(Feature.subCases) then atSpan(in.skipToken()):
-          subMatchClause(simpleExpr(Location.ElseWhere))
+          val t = subMatchClause(simpleExpr(Location.ElseWhere))
+          if in.isStatSep then in.nextToken() // else may have been consumed by sub sub match
+          t
         else atSpan(accept(ARROW)):
           if exprOnly then
             if in.indentSyntax && in.isAfterLineEnd && in.token != INDENT then

tests/pos/match-single-sub-case.scala

@@ -0,0 +1,17 @@
+import scala.language.experimental.subCases
+
+// single sub case can be one the same line as outer case
+object Test:
+  val x: Option[Option[Int]] = ???
+  x match
+    case Some(x2) with x2 match case Some(x3) => "aa"
+    case Some(x2) if false with x2 match case Some(x3) if true => "aa"
+    case Some(x2) with x2 match case Some(x3) with x2 match case Some(x3) => "bb"
+    case Some(y2) with y2 match
+      case Some(y3) with y3 match
+        case 1 => "a"
+        case 2 => "b"
+    case Some(x2) with x2 match case Some(x3) with x3 match
+      case 1 => "a"
+      case 2 => "b"
+    case None => "d"