diff --git a/compiler/src/dotty/tools/dotc/typer/Implicits.scala b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
index 5162b3fed1b9..949e791d0496 100644
--- a/compiler/src/dotty/tools/dotc/typer/Implicits.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
@@ -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] =
@@ -775,6 +774,7 @@ trait ImplicitRunInfo:
    *     if `T` is of the form `(P#x).type`, the anchors of `P`.
    *   - If `T` is the this-type of a static object, the anchors of a term reference to that object.
    *   - If `T` is some other this-type `P.this.type`, the anchors of `P`.
+   *   - If `T` is match type or an applied match alias, the anchors of the normalization of `T`.
    *   - If `T` is some other type, the union of the anchors of each constituent type of `T`.
    *
    *  The _implicit scope_ of a type `tp` is the smallest set S of term references (i.e. TermRefs)
@@ -787,7 +787,7 @@ trait ImplicitRunInfo:
    *   - If `T` is a reference to an opaque type alias named `A`, S includes
    *     a reference to an object `A` defined in the same scope as the type, if it exists,
    *     as well as the implicit scope of `T`'s underlying type or bounds.
-   *   - If `T` is a reference to an an abstract type or match type alias named `A`,
+   *   - If `T` is a reference to an an abstract type or unreducible match type alias named `A`,
    *     S includes a reference to an object `A` defined in the same scope as the type,
    *     if it exists, as well as the implicit scopes of `T`'s lower and upper bound,
    *     if present.
diff --git a/tests/neg/i20071.scala b/tests/neg/i20071.scala
new file mode 100644
index 000000000000..2d3dd5fe17d1
--- /dev/null
+++ b/tests/neg/i20071.scala
@@ -0,0 +1,28 @@
+
+trait Scope
+object Scope:
+  given i: Int = ???
+
+type ReferencesScope[S] >: Int <: Int
+
+type ScopeToInt[Why] = Why match
+  case Scope => Int
+
+def foo[T](using d: ReferencesScope[T]): Any = ???
+
+def bar[T](using d: ScopeToInt[T]): Any = ???
+
+def test: Unit =
+  foo[Scope] // ok
+  bar[Scope] // error
+
+  import Scope.i
+  bar[Scope] // ok
+
+  /*
+  Before the changes:
+  `ScopeToInt[Scope]` may or may not be reduced before implicit search,
+  thereby impacting the scope considered for the search. `Scope.i` is included
+  iff `Scope` still appears in the type, which is the case only before reduction.
+  In contrast, `ReferencesScope[Scope]` is ok since it will never lose the anchor.
+  */
diff --git a/tests/pos/i15183/test_2.scala b/tests/pos/i15183/test_2.scala
index 2069d5637734..eeb3848449be 100644
--- a/tests/pos/i15183/test_2.scala
+++ b/tests/pos/i15183/test_2.scala
@@ -1,4 +1,8 @@
 // Fails in each cases below
+import Decoder.{derived as _, given}
+// NOTE Decoder.derived is already in the implicit scope
+//  but the others require an import as they depend on match type reduction
+
 enum Env derives Decoder:
   case Local,Sit,Prod