Handle partial functions with sub cases

Author: EugeneFlesselle

compiler/src/dotty/tools/dotc/transform/ExpandSAMs.scala

@@ -180,7 +180,11 @@ class ExpandSAMs extends MiniPhase:
 
       def isDefinedAtRhs(paramRefss: List[List[Tree]])(using Context) =
         val tru = Literal(Constant(true))
-        def translateCase(cdef: CaseDef) = cpy.CaseDef(cdef)(body = tru)
+        def translateCase(cdef: CaseDef): CaseDef =
+          val body1 = cdef.body match
+            case b: SubMatch => cpy.Match(b)(b.selector, b.cases.map(translateCase))
+            case _ => tru
+          cpy.CaseDef(cdef)(body = body1)
         val paramRef = paramRefss.head.head
         val defaultValue = Literal(Constant(false))
         translateMatch(isDefinedAtFn)(paramRef.symbol, pfRHS.cases.map(translateCase), defaultValue)

tests/run/pf-sub-cases.scala

@@ -0,0 +1,14 @@
+import scala.language.experimental.matchWithSubCases
+
+val pf: PartialFunction[Option[Option[Int]], String] =
+  case Some(x2) with x2 match
+    case Some(x3) with x3 match
+      case 1 => "a"
+      case 2 => "b"
+  case Some(None) => "c"
+
+@main def Test =
+  assert(pf(Some(Some(2))) == "b")
+  assert(pf(Some(None)) == "c")
+  assert(!pf.isDefinedAt(None))
+  assert(!pf.isDefinedAt(Some(Some(3))))