diff --git a/compiler/src/dotty/tools/dotc/core/Types.scala b/compiler/src/dotty/tools/dotc/core/Types.scala
index d403aa1dd183..3bb56a77e7da 100644
--- a/compiler/src/dotty/tools/dotc/core/Types.scala
+++ b/compiler/src/dotty/tools/dotc/core/Types.scala
@@ -4966,34 +4966,51 @@ object Types {
   }
 
   class TypeSizeAccumulator(implicit ctx: Context) extends TypeAccumulator[Int] {
-    def apply(n: Int, tp: Type): Int = tp match {
-      case tp: AppliedType =>
-        foldOver(n + 1, tp)
-      case tp: RefinedType =>
-        foldOver(n + 1, tp)
-      case tp: TypeRef if tp.info.isTypeAlias =>
-        apply(n, tp.superType)
-      case _ =>
-        foldOver(n, tp)
+    val seen = new java.util.IdentityHashMap[Type, Type]
+    def apply(n: Int, tp: Type): Int =
+      if (seen.get(tp) != null) n
+      else {
+        seen.put(tp, tp)
+        tp match {
+        case tp: AppliedType =>
+          foldOver(n + 1, tp)
+        case tp: RefinedType =>
+          foldOver(n + 1, tp)
+        case tp: TypeRef if tp.info.isTypeAlias =>
+          apply(n, tp.superType)
+        case tp: TypeParamRef =>
+          apply(n, ctx.typeComparer.bounds(tp))
+        case _ =>
+          foldOver(n, tp)
+      }
     }
   }
 
   class CoveringSetAccumulator(implicit ctx: Context) extends TypeAccumulator[Set[Symbol]] {
+    val seen = new java.util.IdentityHashMap[Type, Type]
     def apply(cs: Set[Symbol], tp: Type): Set[Symbol] = {
-      val sym = tp.typeSymbol
-      tp match {
-        case tp if tp.isTopType || tp.isBottomType =>
-          cs
-        case tp: AppliedType =>
-          foldOver(cs + sym, tp)
-        case tp: RefinedType =>
-          foldOver(cs + sym, tp)
-        case tp: TypeRef if tp.info.isTypeAlias =>
-          apply(cs, tp.superType)
-        case tp: TypeBounds =>
-          foldOver(cs, tp)
-        case other =>
-          foldOver(cs + sym, tp)
+      if (seen.get(tp) != null) cs
+      else {
+        seen.put(tp, tp)
+        tp match {
+          case tp if tp.isTopType || tp.isBottomType =>
+            cs
+          case tp: AppliedType =>
+            foldOver(cs + tp.typeSymbol, tp)
+          case tp: RefinedType =>
+            foldOver(cs + tp.typeSymbol, tp)
+          case tp: TypeRef if tp.info.isTypeAlias =>
+            apply(cs, tp.superType)
+          case tp: TypeRef if tp.typeSymbol.isClass =>
+            foldOver(cs + tp.typeSymbol, tp)
+          case tp: TermRef =>
+            val tsym = if (tp.termSymbol.is(Param)) tp.underlying.typeSymbol else tp.termSymbol
+            foldOver(cs + tsym, tp)
+          case tp: TypeParamRef =>
+            apply(cs, ctx.typeComparer.bounds(tp))
+          case other =>
+            foldOver(cs, tp)
+        }
       }
     }
   }
diff --git a/compiler/src/dotty/tools/dotc/typer/Implicits.scala b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
index c488f915013b..3eb8becb2d41 100644
--- a/compiler/src/dotty/tools/dotc/typer/Implicits.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
@@ -1413,9 +1413,10 @@ abstract class SearchHistory { outer =>
           if (cand1.ref == cand.ref) {
             val wideTp = tp.widenExpr
             lazy val wildTp = wildApprox(wideTp)
+            lazy val tpSize = wideTp.typeSize
             if (belowByname && (wildTp <:< wildPt)) false
-            else if ((wideTp.typeSize < ptSize && wideTp.coveringSet == ptCoveringSet) || (wildTp =:= wildPt)) true
-            else loop(tl, isByname(tp) || belowByname)
+            else if (tpSize > ptSize || wideTp.coveringSet != ptCoveringSet) loop(tl, isByname(tp) || belowByname)
+            else tpSize < ptSize || wildTp =:= wildPt || loop(tl, isByname(tp) || belowByname)
           }
           else loop(tl, isByname(tp) || belowByname)
       }
diff --git a/compiler/test/dotty/tools/dotc/CompilationTests.scala b/compiler/test/dotty/tools/dotc/CompilationTests.scala
index ce81eabc4f0c..b275459485c4 100644
--- a/compiler/test/dotty/tools/dotc/CompilationTests.scala
+++ b/compiler/test/dotty/tools/dotc/CompilationTests.scala
@@ -171,7 +171,8 @@ class CompilationTests extends ParallelTesting {
     compileList("duplicate source", List(
       "tests/neg-custom-args/toplevel-samesource/S.scala",
       "tests/neg-custom-args/toplevel-samesource/nested/S.scala"),
-      defaultOptions)
+      defaultOptions) +
+    compileFile("tests/neg-custom-args/i6300.scala", allowDeepSubtypes)
   }.checkExpectedErrors()
 
   @Test def fuzzyAll: Unit = {
diff --git a/tests/neg-custom-args/i6300.scala b/tests/neg-custom-args/i6300.scala
new file mode 100644
index 000000000000..f69766b06b01
--- /dev/null
+++ b/tests/neg-custom-args/i6300.scala
@@ -0,0 +1,8 @@
+object Test {
+  class Foo[T <: Foo[T]]
+  class Bar extends Foo[Bar]
+
+  implicit def i[T <: Foo[T]](implicit t: Foo[Foo[T]]): Foo[T] = ???
+
+  implicitly[Foo[Bar]] // error
+}