Add parsing of match sub cases

Author: EugeneFlesselle

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

@@ -2592,6 +2592,13 @@ object Parsers {
         Match(t, inBracesOrIndented(caseClauses(() => caseClause())))
       }
 
+    /** 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
+      SubMatch(t, cases)
+
     /**    `match' <<< TypeCaseClauses >>>
      */
     def matchType(t: Tree): MatchTypeTree =
@@ -3092,24 +3099,27 @@ object Parsers {
       buf.toList
     }
 
-    /** CaseClause         ::= ‘case’ Pattern [Guard] `=>' Block
-     *  ExprCaseClause    ::=  ‘case’ Pattern [Guard] ‘=>’ Expr
+    /** CaseClause        ::= ‘case’ Pattern [Guard] (‘with’ SimpleExpr SubMatchClause | `=>' Block)
+     *  ExprCaseClause    ::= ‘case’ Pattern [Guard] (‘with’ SimpleExpr SubMatchClause | `=>' Expr)
      */
     def caseClause(exprOnly: Boolean = false): CaseDef = atSpan(in.offset) {
       val (pat, grd) = inSepRegion(InCase) {
         accept(CASE)
         (withinMatchPattern(pattern()), guard())
       }
-      CaseDef(pat, grd, atSpan(accept(ARROW)) {
-        if exprOnly then
-          if in.indentSyntax && in.isAfterLineEnd && in.token != INDENT then
-            warning(em"""Misleading indentation: this expression forms part of the preceding catch case.
-                        |If this is intended, it should be indented for clarity.
-                        |Otherwise, if the handler is intended to be empty, use a multi-line catch with
-                        |an indented case.""")
-          expr()
-        else block()
-      })
+      val body =
+        if in.token == WITH && in.featureEnabled(Feature.matchWithSubCases) then atSpan(in.skipToken()):
+          subMatchClause(simpleExpr(Location.ElseWhere))
+        else atSpan(accept(ARROW)):
+          if exprOnly then
+            if in.indentSyntax && in.isAfterLineEnd && in.token != INDENT then
+              warning(em"""Misleading indentation: this expression forms part of the preceding catch case.
+                          |If this is intended, it should be indented for clarity.
+                          |Otherwise, if the handler is intended to be empty, use a multi-line catch with
+                          |an indented case.""")
+            expr()
+          else block()
+      CaseDef(pat, grd, body)
     }
 
     /** TypeCaseClause     ::= ‘case’ (InfixType | ‘_’) ‘=>’ Type [semi]

tests/run/sub-cases.scala

@@ -0,0 +1,46 @@
+import scala.language.experimental.matchWithSubCases
+
+enum E:
+  case A(e: E)
+  case B(e: E)
+  case C
+
+  def f: Option[E] = this match
+    case A(e) => Some(e)
+    case B(e) => Some(e)
+    case C => None
+
+end E
+import E.*
+
+
+@main def Test =
+
+  def test(e: E): Int = e match
+    case A(B(e1)) if true with e1.f match
+      case Some(x) with x match
+        case A(_) => 11
+        case C => 12
+    case B(A(e1)) with e1.f match
+      case Some(C) => 21
+      case None => 22
+    case _ => 3
+  end test
+
+  def check(e: E, r: Int): Unit = assert(test(e) == r)
+
+  check(A(A(C)), 3)
+
+  val x1 = B(A(C))
+  check(A(B(x1)), 11)
+
+  val x2 = B(C)
+  check(A(B(x2)), 12)
+
+  check(A(B(C)), 3)
+  check(A(B(B(B(C)))), 3)
+
+  check(B(A(x2)), 21)
+  check(B(A(C)), 22)
+
+  check(A(A(C)), 3)