diff --git a/compiler/src/dotty/tools/dotc/ast/Desugar.scala b/compiler/src/dotty/tools/dotc/ast/Desugar.scala
index b1b771bc7512..f72a9590a077 100644
--- a/compiler/src/dotty/tools/dotc/ast/Desugar.scala
+++ b/compiler/src/dotty/tools/dotc/ast/Desugar.scala
@@ -92,7 +92,7 @@ object desugar {
     override def ensureCompletions(using Context): Unit = {
       def completeConstructor(sym: Symbol) =
         sym.infoOrCompleter match {
-          case completer: Namer#ClassCompleter =>
+          case completer: Namer#ClassCompleter if !sym.isCompleting =>
             completer.completeConstructor(sym)
           case _ =>
         }
diff --git a/compiler/src/dotty/tools/dotc/core/NamerOps.scala b/compiler/src/dotty/tools/dotc/core/NamerOps.scala
index 07cb9292baa4..363a01665564 100644
--- a/compiler/src/dotty/tools/dotc/core/NamerOps.scala
+++ b/compiler/src/dotty/tools/dotc/core/NamerOps.scala
@@ -272,7 +272,7 @@ object NamerOps:
    *  where
    *
    *      <context-bound-companion> is the CBCompanion type created in Definitions
-   *      withnessRefK is a refence to the K'th witness.
+   *      withnessRefK is a reference to the K'th witness.
    *
    *  The companion has the same access flags as the original type.
    */
diff --git a/compiler/src/dotty/tools/dotc/core/TypeApplications.scala b/compiler/src/dotty/tools/dotc/core/TypeApplications.scala
index 54636ff4ad58..85013992278b 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeApplications.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeApplications.scala
@@ -267,7 +267,9 @@ class TypeApplications(val self: Type) extends AnyVal {
    */
   def hkResult(using Context): Type = self.dealias match {
     case self: TypeRef =>
-      if (self.symbol == defn.AnyKindClass) self else self.info.hkResult
+      if (self.symbol == defn.AnyKindClass) self
+      else if self.symbol.isClass then NoType // avoid forcing symbol if it's a class, not an alias to a HK type lambda
+      else self.info.hkResult
     case self: AppliedType =>
       if (self.tycon.typeSymbol.isClass) NoType else self.superType.hkResult
     case self: HKTypeLambda => self.resultType
diff --git a/compiler/src/dotty/tools/dotc/core/Types.scala b/compiler/src/dotty/tools/dotc/core/Types.scala
index eeffc41d4159..ef99c051ab68 100644
--- a/compiler/src/dotty/tools/dotc/core/Types.scala
+++ b/compiler/src/dotty/tools/dotc/core/Types.scala
@@ -196,7 +196,9 @@ object Types extends TypeUtils {
      */
     def isRef(sym: Symbol, skipRefined: Boolean = true)(using Context): Boolean = this match {
       case this1: TypeRef =>
-        this1.info match { // see comment in Namer#TypeDefCompleter#typeSig
+        // avoid forcing symbol if it's a class, not a type alias
+        if this1.symbol.isClass then this1.symbol eq sym
+        else this1.info match { // see comment in Namer#TypeDefCompleter#typeSig
           case TypeAlias(tp) => tp.isRef(sym, skipRefined)
           case _ => this1.symbol eq sym
         }
diff --git a/compiler/src/dotty/tools/dotc/typer/Namer.scala b/compiler/src/dotty/tools/dotc/typer/Namer.scala
index 83964417a6f1..c87106ab2c5a 100644
--- a/compiler/src/dotty/tools/dotc/typer/Namer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Namer.scala
@@ -822,8 +822,7 @@ class Namer { typer: Typer =>
         if (sym.is(Module)) moduleValSig(sym)
         else valOrDefDefSig(original, sym, Nil, identity)(using localContext(sym).setNewScope)
       case original: DefDef =>
-        val typer1 = ctx.typer.newLikeThis(ctx.nestingLevel + 1)
-        nestedTyper(sym) = typer1
+        val typer1 = newNestedTyper(sym)
         typer1.defDefSig(original, sym, this)(using localContext(sym).setTyper(typer1))
       case imp: Import =>
         try
@@ -833,6 +832,12 @@ class Namer { typer: Typer =>
           typr.println(s"error while completing ${imp.expr}")
           throw ex
 
+    def newNestedTyper(sym: Symbol) = nestedTyper.getOrElseUpdate(sym, ctx.typer.newLikeThis(ctx.nestingLevel + 1))
+
+    def indexConstructor(constr: DefDef, sym: Symbol): Unit =
+      val typer1 = newNestedTyper(sym)
+      typer1.indexConstructor(constr, sym)(using localContext(sym).setTyper(typer1))
+
     final override def complete(denot: SymDenotation)(using Context): Unit = {
       if (Config.showCompletions && ctx.typerState != creationContext.typerState) {
         def levels(c: Context): Int =
@@ -986,7 +991,7 @@ class Namer { typer: Typer =>
 
     /** If completion of the owner of the to be completed symbol has not yet started,
      *  complete the owner first and check again. This prevents cyclic references
-     *  where we need to copmplete a type parameter that has an owner that is not
+     *  where we need to complete a type parameter that has an owner that is not
      *  yet completed. Test case is pos/i10967.scala.
      */
     override def needsCompletion(symd: SymDenotation)(using Context): Boolean =
@@ -994,7 +999,8 @@ class Namer { typer: Typer =>
       !owner.exists
       || owner.is(Touched)
       || {
-        owner.ensureCompleted()
+        if owner.isType then
+          owner.ensureCompleted()
         !symd.isCompleted
       }
 
@@ -1519,12 +1525,9 @@ class Namer { typer: Typer =>
       index(constr)
       index(rest)(using localCtx)
 
-      symbolOfTree(constr).info.stripPoly match // Completes constr symbol as a side effect
-        case mt: MethodType if cls.is(Case) && mt.isParamDependent =>
-          // See issue #8073 for background
-          report.error(
-              em"""Implementation restriction: case classes cannot have dependencies between parameters""",
-              cls.srcPos)
+      val constrSym = symbolOfTree(constr)
+      constrSym.infoOrCompleter match
+        case completer: Completer => completer.indexConstructor(constr, constrSym)
         case _ =>
 
       tempInfo = denot.asClass.classInfo.integrateOpaqueMembers.asInstanceOf[TempClassInfo]
@@ -1853,31 +1856,6 @@ class Namer { typer: Typer =>
     // Beware: ddef.name need not match sym.name if sym was freshened!
     val isConstructor = sym.name == nme.CONSTRUCTOR
 
-    // A map from context-bounded type parameters to associated evidence parameter names
-    val witnessNamesOfParam = mutable.Map[TypeDef, List[TermName]]()
-    if !ddef.name.is(DefaultGetterName) && !sym.is(Synthetic) then
-      for params <- ddef.paramss; case tdef: TypeDef <- params do
-        for case WitnessNamesAnnot(ws) <- tdef.mods.annotations do
-          witnessNamesOfParam(tdef) = ws
-
-    /** Is each name in `wnames` defined somewhere in the longest prefix of all `params`
-     *  that have been typed ahead (i.e. that carry the TypedAhead attachment)?
-     */
-    def allParamsSeen(wnames: List[TermName], params: List[MemberDef]) =
-      (wnames.toSet[Name] -- params.takeWhile(_.hasAttachment(TypedAhead)).map(_.name)).isEmpty
-
-    /** Enter and typecheck parameter list.
-     *  Once all witness parameters for a context bound are seen, create a
-     *  context bound companion for it.
-     */
-    def completeParams(params: List[MemberDef])(using Context): Unit =
-      index(params)
-      for param <- params do
-        typedAheadExpr(param)
-        for (tdef, wnames) <- witnessNamesOfParam do
-          if wnames.contains(param.name) && allParamsSeen(wnames, params) then
-            addContextBoundCompanionFor(symbolOfTree(tdef), wnames, params.map(symbolOfTree))
-
     // The following 3 lines replace what was previously just completeParams(tparams).
     // But that can cause bad bounds being computed, as witnessed by
     // tests/pos/paramcycle.scala. The problematic sequence is this:
@@ -1901,39 +1879,15 @@ class Namer { typer: Typer =>
     //   3. Info of CP is computed (to be copied to DP).
     //   4. CP is completed.
     //   5. Info of CP is copied to DP and DP is completed.
-    index(ddef.leadingTypeParams)
-    if (isConstructor) sym.owner.typeParams.foreach(_.ensureCompleted())
+    if !sym.isPrimaryConstructor then index(ddef.leadingTypeParams)
     val completedTypeParams =
       for tparam <- ddef.leadingTypeParams yield typedAheadExpr(tparam).symbol
     if completedTypeParams.forall(_.isType) then
       completer.setCompletedTypeParams(completedTypeParams.asInstanceOf[List[TypeSymbol]])
-    ddef.trailingParamss.foreach(completeParams)
+    completeTrailingParamss(ddef, sym)
     val paramSymss = normalizeIfConstructor(ddef.paramss.nestedMap(symbolOfTree), isConstructor)
     sym.setParamss(paramSymss)
 
-    /** Under x.modularity, we add `tracked` to context bound witnesses
-     *  that have abstract type members
-     */
-    def needsTracked(sym: Symbol, param: ValDef)(using Context) =
-      !sym.is(Tracked)
-      && param.hasAttachment(ContextBoundParam)
-      && sym.info.memberNames(abstractTypeNameFilter).nonEmpty
-
-    /** Under x.modularity, set every context bound evidence parameter of a class to be tracked,
-     *  provided it has a type that has an abstract type member. Reset private and local flags
-     *  so that the parameter becomes a `val`.
-     */
-    def setTracked(param: ValDef): Unit =
-      val sym = symbolOfTree(param)
-      sym.maybeOwner.maybeOwner.infoOrCompleter match
-        case info: TempClassInfo if needsTracked(sym, param) =>
-          typr.println(i"set tracked $param, $sym: ${sym.info} containing ${sym.info.memberNames(abstractTypeNameFilter).toList}")
-          for acc <- info.decls.lookupAll(sym.name) if acc.is(ParamAccessor) do
-            acc.resetFlag(PrivateLocal)
-            acc.setFlag(Tracked)
-            sym.setFlag(Tracked)
-        case _ =>
-
     def wrapMethType(restpe: Type): Type =
       instantiateDependent(restpe, paramSymss)
       methodType(paramSymss, restpe, ddef.mods.is(JavaDefined))
@@ -1942,11 +1896,18 @@ class Namer { typer: Typer =>
       wrapMethType(addParamRefinements(restpe, paramSymss))
 
     if isConstructor then
-      if sym.isPrimaryConstructor && Feature.enabled(modularity) then
-        ddef.termParamss.foreach(_.foreach(setTracked))
       // set result type tree to unit, but take the current class as result type of the symbol
       typedAheadType(ddef.tpt, defn.UnitType)
-      wrapMethType(effectiveResultType(sym, paramSymss))
+      val mt = wrapMethType(effectiveResultType(sym, paramSymss))
+      if sym.isPrimaryConstructor then
+        mt.stripPoly match
+          case mt: MethodType if sym.owner.is(Case) && mt.isParamDependent =>
+            // See issue #8073 for background
+            report.error(
+              em"""Implementation restriction: case classes cannot have dependencies between parameters""",
+              sym.owner.srcPos)
+          case _ =>
+      mt
     else if sym.isAllOf(Given | Method) && Feature.enabled(modularity) then
       // set every context bound evidence parameter of a given companion method
       // to be tracked, provided it has a type that has an abstract type member.
@@ -1959,6 +1920,74 @@ class Namer { typer: Typer =>
       valOrDefDefSig(ddef, sym, paramSymss, wrapMethType)
   end defDefSig
 
+  def indexConstructor(constr: DefDef, sym: Symbol)(using Context): Unit =
+    index(constr.leadingTypeParams)
+    sym.owner.typeParams.foreach(_.ensureCompleted())
+    completeTrailingParamss(constr, sym, indexingCtor = true)
+    if Feature.enabled(modularity) then
+      constr.termParamss.foreach(_.foreach(setTracked))
+
+  def completeTrailingParamss(ddef: DefDef, sym: Symbol, indexingCtor: Boolean = false)(using Context): Unit =
+    // A map from context-bounded type parameters to associated evidence parameter names
+    val witnessNamesOfParam = mutable.Map[TypeDef, List[TermName]]()
+    if !ddef.name.is(DefaultGetterName) && !sym.is(Synthetic) && (indexingCtor || !sym.isPrimaryConstructor) then
+      for params <- ddef.paramss; case tdef: TypeDef <- params do
+        for case WitnessNamesAnnot(ws) <- tdef.mods.annotations do
+          witnessNamesOfParam(tdef) = ws
+
+    /** Is each name in `wnames` defined somewhere in the previous parameters? */
+    def allParamsSeen(wnames: List[TermName], prevParams1: List[Name]) =
+      (wnames.toSet[Name] -- prevParams1).isEmpty
+
+    /** Enter and typecheck parameter list.
+     *  Once all witness parameters for a context bound are seen, create a
+     *  context bound companion for it.
+     */
+    def completeParams(params: List[MemberDef])(using Context): Unit =
+      if indexingCtor || !sym.isPrimaryConstructor then index(params)
+      val paramSyms = params.map(symbolOfTree)
+
+      def loop(nextParams: List[MemberDef], prevParams: List[Name]): Unit = nextParams match
+        case param :: nextParams1 =>
+          if !indexingCtor then
+            typedAheadExpr(param)
+
+          val prevParams1 = param.name :: prevParams
+          for (tdef, wnames) <- witnessNamesOfParam do
+            if wnames.contains(param.name) && allParamsSeen(wnames, prevParams1) then
+              addContextBoundCompanionFor(symbolOfTree(tdef), wnames, paramSyms)
+
+          loop(nextParams1, prevParams1)
+        case _ =>
+      loop(params, Nil)
+    end completeParams
+
+    ddef.trailingParamss.foreach(completeParams)
+  end completeTrailingParamss
+
+  /** Under x.modularity, we add `tracked` to context bound witnesses
+   *  that have abstract type members
+   */
+  def needsTracked(sym: Symbol, param: ValDef)(using Context) =
+    !sym.is(Tracked)
+    && param.hasAttachment(ContextBoundParam)
+    && sym.info.memberNames(abstractTypeNameFilter).nonEmpty
+
+  /** Under x.modularity, set every context bound evidence parameter of a class to be tracked,
+   *  provided it has a type that has an abstract type member. Reset private and local flags
+   *  so that the parameter becomes a `val`.
+   */
+  def setTracked(param: ValDef)(using Context): Unit =
+    val sym = symbolOfTree(param)
+    sym.maybeOwner.maybeOwner.infoOrCompleter match
+      case info: ClassInfo if needsTracked(sym, param) =>
+        typr.println(i"set tracked $param, $sym: ${sym.info} containing ${sym.info.memberNames(abstractTypeNameFilter).toList}")
+        for acc <- info.decls.lookupAll(sym.name) if acc.is(ParamAccessor) do
+          acc.resetFlag(PrivateLocal)
+          acc.setFlag(Tracked)
+          sym.setFlag(Tracked)
+      case _ =>
+
   def inferredResultType(
       mdef: ValOrDefDef,
       sym: Symbol,
diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index e44da20814dd..dd6e0ca7fb24 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -2472,12 +2472,6 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
               (arg, tparamBounds)
             else
               (arg, WildcardType)
-          if (tpt1.symbol.isClass)
-            tparam match {
-              case tparam: Symbol =>
-                tparam.ensureCompleted() // This is needed to get the test `compileParSetSubset` to work
-              case _ =>
-            }
           if (desugaredArg.isType)
             arg match {
               case untpd.WildcardTypeBoundsTree()
diff --git a/tests/neg/i15177.FakeEnum.min.alt1.scala b/tests/neg/i15177.FakeEnum.min.alt1.scala
new file mode 100644
index 000000000000..ad92f3cddcd0
--- /dev/null
+++ b/tests/neg/i15177.FakeEnum.min.alt1.scala
@@ -0,0 +1,4 @@
+trait Foo
+trait X[T <: Foo] { trait Id }
+object A extends X[B] // error: Type argument B does not conform to upper bound Foo
+class B extends A.Id
diff --git a/tests/neg/i15177.constr-dep.scala b/tests/neg/i15177.constr-dep.scala
new file mode 100644
index 000000000000..85608a23668e
--- /dev/null
+++ b/tests/neg/i15177.constr-dep.scala
@@ -0,0 +1,5 @@
+class Foo[A]
+class Foo1(val x: Int)
+  extends Foo[ // error: The type of a class parent cannot refer to constructor parameters, but Foo[(Foo1.this.x : Int)] refers to x
+    x.type
+  ]
diff --git a/tests/neg/i15177.ub.scala b/tests/neg/i15177.ub.scala
new file mode 100644
index 000000000000..d504528572ed
--- /dev/null
+++ b/tests/neg/i15177.ub.scala
@@ -0,0 +1,13 @@
+// like tests/pos/i15177.scala
+// but with T having an upper bound
+// that B doesn't conform to
+// just to be sure that not forcing B
+// doesn't backdoor an illegal X[B]
+class X[T <: C] {
+  type Id
+}
+object A
+  extends X[ // error
+    B] // error
+class B(id: A.Id)
+class C
diff --git a/tests/pos/i15177.FakeEnum.min.alt2.scala b/tests/pos/i15177.FakeEnum.min.alt2.scala
new file mode 100644
index 000000000000..fd5f87c649ef
--- /dev/null
+++ b/tests/pos/i15177.FakeEnum.min.alt2.scala
@@ -0,0 +1,4 @@
+trait Foo
+trait X[T <: Foo] { trait Id }
+object A extends X[B]
+class B extends A.Id with Foo
diff --git a/tests/pos/i15177.FakeEnum.min.alt3.scala b/tests/pos/i15177.FakeEnum.min.alt3.scala
new file mode 100644
index 000000000000..b0a16f634bd4
--- /dev/null
+++ b/tests/pos/i15177.FakeEnum.min.alt3.scala
@@ -0,0 +1,4 @@
+trait Foo
+trait X[T <: Foo] { trait Id extends Foo }
+object A extends X[B]
+class B extends A.Id
diff --git a/tests/pos/i15177.FakeEnum.min.scala b/tests/pos/i15177.FakeEnum.min.scala
new file mode 100644
index 000000000000..0c426d159b1e
--- /dev/null
+++ b/tests/pos/i15177.FakeEnum.min.scala
@@ -0,0 +1,3 @@
+trait X[T] { trait Id }
+object A extends X[B]
+class B extends A.Id
diff --git a/tests/pos/i15177.FakeEnum.scala b/tests/pos/i15177.FakeEnum.scala
new file mode 100644
index 000000000000..e154866fe513
--- /dev/null
+++ b/tests/pos/i15177.FakeEnum.scala
@@ -0,0 +1,20 @@
+trait FakeEnum[A, @specialized(Byte, Short, Int, Long) B]
+{
+  trait Value {
+    self: A =>
+      def name: String
+      def id: B
+  }
+}
+
+object FakeEnumType
+  extends FakeEnum[FakeEnumType, Short]
+{
+  val MEMBER1 = new FakeEnumType((0: Short), "MEMBER1") {}
+  val MEMBER2 = new FakeEnumType((1: Short), "MEMBER2") {}
+}
+
+sealed abstract
+class FakeEnumType(val id: Short, val name: String)
+  extends FakeEnumType.Value
+{}
diff --git a/tests/pos/i15177.app.scala b/tests/pos/i15177.app.scala
new file mode 100644
index 000000000000..5f602e7c0593
--- /dev/null
+++ b/tests/pos/i15177.app.scala
@@ -0,0 +1,10 @@
+// like tests/pos/i15177.scala
+// but with an applied type B[D]
+class X[T] { type Id }
+object A extends X[B[D]]
+class B[
+  C]( // error: Something's wrong: missing original symbol for type tree
+    id:
+      A
+        .Id) // should-be-error: type Id is not a member of object A
+class D
diff --git a/tests/pos/i15177.constr-dep.scala b/tests/pos/i15177.constr-dep.scala
new file mode 100644
index 000000000000..f225d14c78ec
--- /dev/null
+++ b/tests/pos/i15177.constr-dep.scala
@@ -0,0 +1,2 @@
+class Bar(val y: Long)
+class Bar1(val z: Long) extends Bar(z)
diff --git a/tests/pos/i15177.hk.scala b/tests/pos/i15177.hk.scala
new file mode 100644
index 000000000000..4530e6c960e2
--- /dev/null
+++ b/tests/pos/i15177.hk.scala
@@ -0,0 +1,5 @@
+// like tests/pos/i15177.scala
+// but with B being higher kinded
+class X[T[_]] { type Id }
+object A extends X[B]
+class B[C](id: A.Id)
diff --git a/tests/pos/i15177.hk2.scala b/tests/pos/i15177.hk2.scala
new file mode 100644
index 000000000000..f5dce9020dc6
--- /dev/null
+++ b/tests/pos/i15177.hk2.scala
@@ -0,0 +1,5 @@
+// like tests/pos/i15177.scala
+// but with B being higher kinded
+class X[T[_]] { type Id }
+class A extends X[B]
+class B[C]
\ No newline at end of file
diff --git a/tests/pos/i15177.hylolib.scala b/tests/pos/i15177.hylolib.scala
new file mode 100644
index 000000000000..96cf87680a1c
--- /dev/null
+++ b/tests/pos/i15177.hylolib.scala
@@ -0,0 +1,11 @@
+//> using options -language:experimental.modularity -source future
+// A minimisation of pos/hylolib-cb that broke while fixing i15177
+trait Value[Self]
+trait Coll[Self]:
+  type Pos: Value
+  extension (self: Self) def pos: Pos
+extension [Self: Coll](self: Self) def trigger = self.pos
+class Slice[Base]
+given SliceIsColl[T: Coll as c]: Coll[Slice[T]] with
+  type Pos = c.Pos
+  extension (self: Slice[T]) def pos: Pos = ???
diff --git a/tests/pos/i15177.scala b/tests/pos/i15177.scala
new file mode 100644
index 000000000000..278994961810
--- /dev/null
+++ b/tests/pos/i15177.scala
@@ -0,0 +1,3 @@
+class X[T] { trait Id }
+object A extends X[B]
+class B(id: A.Id)
diff --git a/tests/pos/i15177.without.scala b/tests/pos/i15177.without.scala
new file mode 100644
index 000000000000..ca40bc096597
--- /dev/null
+++ b/tests/pos/i15177.without.scala
@@ -0,0 +1,3 @@
+class X[T] { trait Id }
+class A extends X[B]
+class B
diff --git a/tests/pos/parsercombinators-pc.scala b/tests/pos/parsercombinators-pc.scala
new file mode 100644
index 000000000000..e18fbb2d1c33
--- /dev/null
+++ b/tests/pos/parsercombinators-pc.scala
@@ -0,0 +1,9 @@
+//> using options -language:experimental.modularity -source future
+
+trait Foo:
+  type Self
+  type Bar
+
+given inst[A: Foo, B: Foo { type Bar = A.Bar }]: Foo with
+  type Self = String
+  type Bar  = A.Bar