Run all MatchType reduction under Mode.Type

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

@@ -4943,7 +4943,7 @@ object Types {
         record("MatchType.reduce computed")
         if (myReduced != null) record("MatchType.reduce cache miss")
         myReduced =
-          trace(i"reduce match type $this $hashCode", matchTypes, show = true) {
+          trace(i"reduce match type $this $hashCode", matchTypes, show = true)(inMode(Mode.Type) {
             def matchCases(cmp: TrackingTypeComparer): Type =
               val saved = ctx.typerState.snapshot()
               try cmp.matchCases(scrutinee.normalized, cases)
@@ -4956,7 +4956,7 @@ object Types {
                   // instantiations during matchtype reduction
 
             TypeComparer.tracked(matchCases)
-          }
+          })
       myReduced.nn
     }
 

tests/pos/i16408.min1.scala

@@ -0,0 +1,26 @@
+object Helpers:
+  type NodeFun[R] = Matchable // compiles without [R] parameter
+
+  type URIFun[R] = R match
+    case GetURI[u] => u & NodeFun[R]
+
+  private type GetURI[U] = RDF { type URI = U }
+end Helpers
+
+trait RDF:
+  type URI
+
+trait ROps[R <: RDF]:
+  def auth(uri: Helpers.URIFun[R]): String
+
+object TraitRDF extends RDF:
+  override type URI = TraitTypes.UriImpl
+
+  val rops = new ROps[TraitRDF.type] {
+    override def auth(uri: Helpers.URIFun[TraitRDF.type]): String = ???
+  }
+end TraitRDF
+
+object TraitTypes:
+  trait UriImpl // doesn't compile
+  // class UriImpl // compiles

tests/pos/i16408.min2.scala

@@ -0,0 +1,22 @@
+object Helpers:
+  type NodeFun[R] = Matchable // compiles without [R] parameter
+
+  type URIFun[R] = R match
+    case RDF[u] => u & NodeFun[R]
+end Helpers
+
+trait RDF[URIParam]
+
+trait ROps[R <: RDF[?]]:
+  def auth(uri: Helpers.URIFun[R]): String
+
+object TraitRDF extends RDF[TraitTypes.UriImpl]:
+
+  val rops = new ROps[TraitRDF.type] {
+    override def auth(uri: Helpers.URIFun[TraitRDF.type]): String = ???
+  }
+end TraitRDF
+
+object TraitTypes:
+  trait UriImpl // doesn't compile
+  // class UriImpl // compiles

tests/pos/i16408.scala

@@ -0,0 +1,57 @@
+import scala.util.Try
+
+trait RDF:
+  rdf =>
+
+  type R = rdf.type
+  type Node <: Matchable
+  type URI <: Node
+
+  given rops: ROps[R]
+end RDF
+
+object RDF:
+  type Node[R <: RDF] = R match
+    case GetNode[n] => Matchable //n & rNode[R]
+
+  type URI[R <: RDF] <: Node[R] = R match
+    case GetURI[u] => u & Node[R]
+
+  private type GetNode[N] = RDF { type Node = N }
+  private type GetURI[U] = RDF { type URI = U }
+end RDF
+
+trait ROps[R <: RDF]:
+  def mkUri(str: String): Try[RDF.URI[R]]
+  def auth(uri: RDF.URI[R]): Try[String]
+
+object TraitTypes:
+  trait Node:
+    def value: String
+
+  trait Uri extends Node
+
+  def mkUri(u: String): Uri =
+    new Uri { def value = u }
+
+object TraitRDF extends RDF:
+  import TraitTypes as tz
+
+  override opaque type Node <: Matchable = tz.Node
+  override opaque type URI <: Node = tz.Uri
+
+  given rops: ROps[R] with
+    override def mkUri(str: String): Try[RDF.URI[R]] = Try(tz.mkUri(str))
+    override def auth(uri: RDF.URI[R]): Try[String] =
+      Try(java.net.URI.create(uri.value).getAuthority())
+
+end TraitRDF
+
+class Test[R <: RDF](using rops: ROps[R]):
+  import rops.given
+  lazy val uriT: Try[RDF.URI[R]] = rops.mkUri("https://bblfish.net/#i")
+  lazy val x: String = "uri authority=" + uriT.map(u => rops.auth(u))
+
+@main def run =
+  val test = Test[TraitRDF.type]
+  println(test.x)