From 5bafff7cc96f1f31f6e77620ca509dfa55d816b4 Mon Sep 17 00:00:00 2001
From: Dale Wijnand <dale.wijnand@gmail.com>
Date: Wed, 10 May 2023 16:39:12 +0100
Subject: [PATCH] Normalize match type usage during implicit lookup

---
 .../dotty/tools/dotc/core/TypeComparer.scala  |  2 +-
 .../dotty/tools/dotc/core/TypeErrors.scala    |  3 +++
 .../dotty/tools/dotc/typer/Implicits.scala    |  7 ++++++
 tests/pos/i17395.scala                        | 25 +++++++++++++++++++
 4 files changed, 36 insertions(+), 1 deletion(-)
 create mode 100644 tests/pos/i17395.scala

diff --git a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
index 465978d329e6..ee8b8c3cdfcc 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -3180,7 +3180,7 @@ class TrackingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
             tp
       case Nil =>
         val casesText = MatchTypeTrace.noMatchesText(scrut, cases)
-        throw TypeError(em"Match type reduction $casesText")
+        throw MatchTypeReductionError(em"Match type reduction $casesText")
 
     inFrozenConstraint {
       // Empty types break the basic assumption that if a scrutinee and a
diff --git a/compiler/src/dotty/tools/dotc/core/TypeErrors.scala b/compiler/src/dotty/tools/dotc/core/TypeErrors.scala
index 24a207da6836..f59bd08da779 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeErrors.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeErrors.scala
@@ -46,6 +46,9 @@ object TypeError:
     def toMessage(using Context) = msg
 end TypeError
 
+class MatchTypeReductionError(msg: Message)(using Context) extends TypeError:
+  def toMessage(using Context) = msg
+
 class MalformedType(pre: Type, denot: Denotation, absMembers: Set[Name])(using Context) extends TypeError:
   def toMessage(using Context) = em"malformed type: $pre is not a legal prefix for $denot because it contains abstract type member${if (absMembers.size == 1) "" else "s"} ${absMembers.mkString(", ")}"
 
diff --git a/compiler/src/dotty/tools/dotc/typer/Implicits.scala b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
index a9631ad45e28..b5a20b6838bf 100644
--- a/compiler/src/dotty/tools/dotc/typer/Implicits.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
@@ -634,6 +634,13 @@ trait ImplicitRunInfo:
             case t: TypeLambda =>
               for p <- t.paramRefs do partSeen += p
               traverseChildren(t)
+            case t: MatchType =>
+              traverseChildren(t)
+              traverse(try t.normalized catch case _: MatchTypeReductionError => t)
+            case MatchType.InDisguise(mt)
+                if !t.isInstanceOf[LazyRef] // skip recursive applications (eg. Tuple.Map)
+            =>
+              traverse(mt)
             case t =>
               traverseChildren(t)
 
diff --git a/tests/pos/i17395.scala b/tests/pos/i17395.scala
new file mode 100644
index 000000000000..87c0a45a9ff5
--- /dev/null
+++ b/tests/pos/i17395.scala
@@ -0,0 +1,25 @@
+trait TC[T]
+
+object TC {
+  def optionTCForPart[T](implicit tc: TC[ExtractPart[T]]): TC[Option[ExtractPart[T]]] = new TC[Option[ExtractPart[T]]] {}
+}
+
+type ExtractPart[T] = T match {
+  case PartField[t] => t
+}
+type PartField[T] = Any { type Part = T }
+
+class ValuePartHolder {
+  type Part = Value
+}
+
+class Value
+object Value {
+  implicit val tcValue: TC[Value] = new {}
+}
+
+@main def main(): Unit = {
+//  import Value.tcValue // explicit import works around the issue, but shouldn't be necessary
+  val tc = TC.optionTCForPart[ValuePartHolder]
+  println(tc)
+}