Normalize match types before computing implicit scopes

compiler/src/dotty/tools/dotc/typer/Implicits.scala

@@ -615,9 +615,9 @@ trait ImplicitRunInfo:
     if migrateTo3 then false else sym.is(Package) || sym.isPackageObject
 
   /** Is `sym` an anchor type for which givens may exist? Anchor types are classes,
-    *  opaque type aliases, match aliases and abstract types, but not type parameters
-    *  or package objects.
-    */
+   *  abstract types, opaque type aliases, and unreducible match aliases, but not type parameters
+   *  or package objects.
+   */
   private def isAnchor(sym: Symbol) =
     sym.isClass && !isExcluded(sym)
     || sym.isOpaqueAlias
@@ -636,7 +636,7 @@ trait ImplicitRunInfo:
         else if implicitScopeCache.contains(t) then parts += t
         else
           partSeen += t
-          t.dealias match
+          t.dealias.normalized match
             case t: TypeRef =>
               if isAnchor(t.symbol) then
                 parts += t
@@ -663,7 +663,6 @@ trait ImplicitRunInfo:
               traverseChildren(t)
             case t =>
               traverseChildren(t)
-              traverse(t.normalized)
       catch case ex: Throwable => handleRecursive("collectParts of", t.show, ex)
 
       def apply(tp: Type): collection.Set[Type] =

compiler/src/dotty/tools/dotc/typer/ImportInfo.scala

@@ -180,8 +180,6 @@ class ImportInfo(symf: Context ?=> Symbol,
       assert(myUnimported != null)
     myUnimported.uncheckedNN
 
-  private val isLanguageImport: Boolean = untpd.languageImport(qualifier).isDefined
-
   private var myUnimported: Symbol | Null = uninitialized
 
   private var featureCache: SimpleIdentityMap[TermName, java.lang.Boolean] = SimpleIdentityMap.empty

tests/neg/i15183.check

@@ -0,0 +1,96 @@
+
+-- [E172] Type Error: tests/neg/i15183/test_2.scala:2:17 ---------------------------------------------------------------
+2 |enum Env derives Decoder:  // error
+  |                 ^
+  |No given instance of type Tuple.Map[m.MirroredElemTypes, Decoder] was found for parameter d of given instance derived in object Decoder.
+  |I found:
+  |
+  |    Decoder.summonTuple[(Env.Local : Env), ((Env.Sit : Env), (Env.Prod : Env))](
+  |      Decoder.derived[(Env.Local : Env)](
+  |        Env.Local.$asInstanceOf[
+  |          scala.deriving.Mirror.Singleton{
+  |            type MirroredMonoType = (Env.Local : Env); type MirroredType = (Env.Local : Env);
+  |              type MirroredLabel = ("Local" : String)
+  |          }
+  |        ],
+  |      /* missing */summon[Tuple.Map[?1.MirroredElemTypes, Decoder]]),
+  |    ???)
+  |
+  |But no implicit values were found that match type Tuple.Map[?1.MirroredElemTypes, Decoder]
+  |
+  |where:    ?1 is an unknown value of type scala.deriving.Mirror.Singleton{
+  |  type MirroredMonoType = (Env.Local : Env); type MirroredType = (Env.Local : Env);
+  |    type MirroredLabel = ("Local" : String)
+  |}
+  |.
+  |
+  |Note: a match type could not be fully reduced:
+  |
+  |  trying to reduce  Tuple.Map[m.MirroredElemTypes, Decoder]
+  |  failed since selector m.MirroredElemTypes
+  |  does not match  case EmptyTuple => EmptyTuple
+  |  and cannot be shown to be disjoint from it either.
+  |  Therefore, reduction cannot advance to the remaining case
+  |
+  |    case h *: t => Decoder[h] *: Tuple.Map[t, Decoder]
+-- [E172] Type Error: tests/neg/i15183/test_2.scala:5:18 ---------------------------------------------------------------
+5 |enum Env2 derives Decoder: // error
+  |                  ^
+  |No given instance of type Tuple.Map[m.MirroredElemTypes, Decoder] was found for parameter d of given instance derived in object Decoder.
+  |I found:
+  |
+  |    Decoder.summonTuple[Env2.Local, (Env2.Sit, Env2.Prod)](
+  |      Decoder.derived[Env2.Local](
+  |        Env2.Local.$asInstanceOf[
+  |          scala.deriving.Mirror.Product{
+  |            type MirroredMonoType = Env2.Local; type MirroredType = Env2.Local; type MirroredLabel = ("Local" : String);
+  |              type MirroredElemTypes = EmptyTuple.type; type MirroredElemLabels = EmptyTuple.type
+  |          }
+  |        ],
+  |      /* missing */summon[Tuple.Map[EmptyTuple.type, Decoder]]),
+  |    ???)
+  |
+  |But no implicit values were found that match type Tuple.Map[EmptyTuple.type, Decoder].
+  |
+  |Note: a match type could not be fully reduced:
+  |
+  |  trying to reduce  Tuple.Map[m.MirroredElemTypes, Decoder]
+  |  failed since selector m.MirroredElemTypes
+  |  does not match  case EmptyTuple => EmptyTuple
+  |  and cannot be shown to be disjoint from it either.
+  |  Therefore, reduction cannot advance to the remaining case
+  |
+  |    case h *: t => Decoder[h] *: Tuple.Map[t, Decoder]
+-- [E172] Type Error: tests/neg/i15183/test_2.scala:10:19 --------------------------------------------------------------
+10 |enum Shape derives Decoder: // error
+   |                   ^
+   |No given instance of type Tuple.Map[m.MirroredElemTypes, Decoder] was found for parameter d of given instance derived in object Decoder.
+   |I found:
+   |
+   |    Decoder.summonTuple[Shape.Rectangle, Shape.Circle *: EmptyTuple.type](
+   |      Decoder.derived[Shape.Rectangle](
+   |        Shape.Rectangle.$asInstanceOf[
+   |          scala.deriving.Mirror.Product{
+   |            type MirroredMonoType = Shape.Rectangle; type MirroredType = Shape.Rectangle;
+   |              type MirroredLabel = ("Rectangle" : String); type MirroredElemTypes = (Double, Double);
+   |              type MirroredElemLabels = (("width" : String), ("height" : String))
+   |          }
+   |        ],
+   |        Decoder.summonTuple[Double, Double *: EmptyTuple.type](Decoder.given_Decoder_Double,
+   |          Decoder.summonTuple[Double, EmptyTuple.type](Decoder.given_Decoder_Double,
+   |            /* missing */summon[Tuple.Map[EmptyTuple.type, Decoder]])
+   |        )
+   |      ),
+   |    ???)
+   |
+   |But no implicit values were found that match type Tuple.Map[EmptyTuple.type, Decoder].
+   |
+   |Note: a match type could not be fully reduced:
+   |
+   |  trying to reduce  Tuple.Map[m.MirroredElemTypes, Decoder]
+   |  failed since selector m.MirroredElemTypes
+   |  does not match  case EmptyTuple => EmptyTuple
+   |  and cannot be shown to be disjoint from it either.
+   |  Therefore, reduction cannot advance to the remaining case
+   |
+   |    case h *: t => Decoder[h] *: Tuple.Map[t, Decoder]

tests/pos/i15183/test_2.scala → tests/neg/i15183/test_2.scala

@@ -1,12 +1,12 @@
 // Fails in each cases below
-enum Env derives Decoder:
+enum Env derives Decoder:  // error
   case Local,Sit,Prod
 
-enum Env2 derives Decoder:
+enum Env2 derives Decoder: // error
   case Local()
   case Sit()
   case Prod()
 
-enum Shape derives Decoder:
+enum Shape derives Decoder: // error
   case Rectangle(width: Double, height: Double)
   case Circle(radius: Double)

tests/neg/i20071.check

@@ -0,0 +1,5 @@
+
+-- [E172] Type Error: tests/neg/i20071/test_2.scala:3:20 ---------------------------------------------------------------
+3 |def test: Unit = bar // error
+  |                    ^
+  |                    No given instance of type M[Decoder] was found for parameter d of method bar

tests/neg/i20071/decoder_1.scala

@@ -0,0 +1,9 @@
+//  decoder_1.scala
+trait Decoder
+object Decoder:
+  given foo: Int = ???
+
+def bar(using d: M[Decoder]): Any = ???
+
+type M[Y] = Y match
+  case Decoder => Int

tests/neg/i20071/test_2.scala

@@ -0,0 +1,3 @@
+// test_2.scala
+// should fail, does when compiling at the same time as decoder_2.scala
+def test: Unit = bar // error

tests/pos/i20071.scala

@@ -0,0 +1,8 @@
+trait Decoder
+object Decoder:
+  given foo: Int = ???
+
+type DecoderToInt[Why] >: Int <: Int
+
+def bar[T](using d: DecoderToInt[T]): Any = ???
+def test: Unit = bar[Decoder]