Fix match type reduction with avoided types (#18043)

Author: dwijnand

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -1007,7 +1007,18 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
       case tp1: MatchType =>
         def compareMatch = tp2 match {
           case tp2: MatchType =>
-            isSameType(tp1.scrutinee, tp2.scrutinee) &&
+            // we allow a small number of scrutinee types to be widened:
+            // * skolems, which may appear from type avoidance, but are widened in the inferred result type
+            // * inline proxies, which is inlining's solution to the same problem
+            def widenScrutinee(scrutinee1: Type) = scrutinee1 match
+                case tp: TermRef if tp.symbol.is(InlineProxy) => tp.info
+                case tp                                       => tp.widenSkolem
+            def checkScrutinee(scrutinee1: Type): Boolean =
+              isSameType(scrutinee1, tp2.scrutinee) || {
+                val widenScrutinee1 = widenScrutinee(scrutinee1)
+                (widenScrutinee1 ne scrutinee1) && checkScrutinee(widenScrutinee1)
+              }
+            checkScrutinee(tp1.scrutinee) &&
             tp1.cases.corresponds(tp2.cases)(isSubType)
           case _ => false
         }

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -1373,7 +1373,7 @@ object Types {
           Atoms.Range(set, set)
         else Atoms.Unknown
 
-      dealias match
+      dealias.normalized match
         case tp: SingletonType =>
           tp.underlying.atoms match
             case as @ Atoms.Range(lo, hi) =>

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

@@ -763,23 +763,16 @@ object TreeChecker {
     override def adapt(tree: Tree, pt: Type, locked: TypeVars)(using Context): Tree = {
       def isPrimaryConstructorReturn =
         ctx.owner.isPrimaryConstructor && pt.isRef(ctx.owner.owner) && tree.tpe.isRef(defn.UnitClass)
-      def infoStr(tp: Type) = tp match {
-        case tp: TypeRef =>
-          val sym = tp.symbol
-          i"${sym.showLocated} with ${tp.designator}, flags = ${sym.flagsString}, underlying = ${tp.underlyingIterator.toList}%, %"
-        case _ =>
-          "??"
-      }
-      if (ctx.mode.isExpr &&
-          !tree.isEmpty &&
-          !isPrimaryConstructorReturn &&
-          !pt.isInstanceOf[FunOrPolyProto])
+      if ctx.mode.isExpr
+        && !tree.isEmpty
+        && !isPrimaryConstructorReturn
+        && !pt.isInstanceOf[FunOrPolyProto]
+      then
         assert(tree.tpe <:< pt, {
           val mismatch = TypeMismatch(tree.tpe, pt, Some(tree))
-          i"""|${mismatch.msg}
-              |found: ${infoStr(tree.tpe)}
-              |expected: ${infoStr(pt)}
-              |tree = $tree""".stripMargin
+          i"""|Type Mismatch:
+              |${mismatch.message}
+              |tree = $tree ${tree.className}""".stripMargin
         })
       tree
     }

tests/neg/mt-scrutinee-widen.scala

@@ -0,0 +1,24 @@
+// We widen scrutinee's that are inline proxies
+// But make sure that term refs in scrutinees are not widened in general
+
+val x: Int = 42
+val y: Int = 43
+val z: Int = 44
+
+type IsX[T] =
+    T match
+        case x.type => true
+        case _ => false
+def test = summon[IsX[y.type] =:= IsX[z.type]] // error
+
+def test2 = summon[
+  (
+    y.type match
+      case x.type => true
+      case _ => false
+  ) =:= (
+    z.type match
+          case x.type => true
+          case _ => false
+  )
+] // error

tests/neg/mt-scrutinee-widen2.scala

@@ -0,0 +1,13 @@
+// A test case showing how we shouldn't widen
+// both IsX scrutinees and make "def test" typecheck
+import scala.util.Random
+val x = 42
+
+type IsX[T] =
+    T match
+        case x.type => true
+        case _ => false
+
+def bothXOrNot(a: Int, b: Int)(using IsX[a.type] =:= IsX[b.type]) = ???
+
+def test = bothXOrNot(Random.nextInt(), Random.nextInt()) // error

tests/neg/mt-subtyping-transitivity.scala

@@ -0,0 +1,14 @@
+final class A
+final class B
+
+type MT[X] = X match
+  case A => String
+  case B => Int
+
+def test: MT[A | B] = ??? : MT[A] // error
+// testing that
+//    MT[A]  !<: MT[A | B]
+// otherwise
+//    String <: MT[A] <: MT[A | B]
+// but
+//    String !<: MT[A | B]

tests/pos/16583.scala

@@ -0,0 +1,19 @@
+import scala.compiletime.constValueTuple
+
+val ll0: Tuple3["one", "two", "three"] = constValueTuple[("one", "two", "three")]
+val ll1 = constValueTuple[("one", "two", "three")].toList
+val ll3: List["one" | ("two" | ("three" | Nothing))] = constValueTuple[("one", "two", "three")].toList
+val ll4: List["one" | ("two" | "three")] = constValueTuple[("one", "two", "three")].toList
+
+inline def labels[Labels <: Tuple](using ev: Tuple.Union[Labels] <:< String): List[String] =
+  val tmp = constValueTuple[Labels].toList
+  ev.substituteCo(tmp)
+
+def test = labels[("one", "two", "three")]
+
+def toList(x: Tuple): List[Tuple.Union[x.type]] = ???
+def test2[Labels <: Tuple] = toList((???): Labels)
+
+def i16654 =
+  def t1: Tuple = EmptyTuple
+  val t2 = t1.toList

tests/pos/16654.scala

@@ -0,0 +1,7 @@
+def toCsvFlat[A <: Product](a: A)(using m: scala.deriving.Mirror.ProductOf[A]) = {
+  def flatTuple(any: Any): Tuple = any match
+    case p: Product => p.productIterator.map(flatTuple).foldLeft(EmptyTuple: Tuple)(_ ++ _)
+    case a          => Tuple1(a)
+
+  val tuple = flatTuple(Tuple.fromProductTyped(a)).toList
+}

tests/pos/mt-scrutinee-widen3.scala

@@ -0,0 +1,12 @@
+// Like widen2, but using a.type only, meaning it should typecheck
+import scala.util.Random
+val x = 42
+
+type IsX[T] =
+    T match
+        case x.type => true
+        case _ => false
+
+def bothXOrNot(a: Int, b: Int)(using IsX[a.type] =:= IsX[a.type]) = ???
+
+def test = bothXOrNot(Random.nextInt(), Random.nextInt())