diff --git a/compiler/src/dotty/tools/dotc/ast/Desugar.scala b/compiler/src/dotty/tools/dotc/ast/Desugar.scala
index d88b0df2bede..a55434c54bd0 100644
--- a/compiler/src/dotty/tools/dotc/ast/Desugar.scala
+++ b/compiler/src/dotty/tools/dotc/ast/Desugar.scala
@@ -341,8 +341,21 @@ object desugar {
       (if (args.isEmpty) tycon else AppliedTypeTree(tycon, args))
         .withPos(cdef.pos.startPos)
 
-    def appliedRef(tycon: Tree, tparams: List[TypeDef] = constrTparams) =
-      appliedTypeTree(tycon, tparams map refOfDef)
+    def appliedRef(tycon: Tree, tparams: List[TypeDef] = constrTparams, widenHK: Boolean = false) = {
+      val targs = for (tparam <- tparams) yield {
+        val targ = refOfDef(tparam)
+        def fullyApplied(tparam: Tree): Tree = tparam match {
+          case TypeDef(_, LambdaTypeTree(tparams, body)) =>
+            AppliedTypeTree(targ, tparams.map(_ => TypeBoundsTree(EmptyTree, EmptyTree)))
+          case TypeDef(_, rhs: DerivedTypeTree) =>
+            fullyApplied(rhs.watched)
+          case _ =>
+            targ
+        }
+        if (widenHK) fullyApplied(tparam) else targ
+      }
+      appliedTypeTree(tycon, targs)
+    }
 
     // a reference to the class type bound by `cdef`, with type parameters coming from the constructor
     val classTypeRef = appliedRef(classTycon)
@@ -431,12 +444,16 @@ object desugar {
     //
     //    implicit def eqInstance[T1$1, ..., Tn$1, T1$2, ..., Tn$2](implicit
     //      ev1: Eq[T1$1, T1$2], ..., evn: Eq[Tn$1, Tn$2]])
-    //      : Eq[C[T1$1, ..., Tn$1], C[T1$2, ..., Tn$2]] = Eq
+    //      : Eq[C[T1$, ..., Tn$1], C[T1$2, ..., Tn$2]] = Eq
+    //
+    // If any of the T_i are higher-kinded, say `Ti[X1 >: L1 <: U1, ..., Xm >: Lm <: Um]`,
+    // the corresponding type parameters for $ev_i are `Ti$1[_, ..., _], Ti$2[_, ..., _]`
+    // (with m underscores `_`).
     def eqInstance = {
       val leftParams = constrTparams.map(derivedTypeParam(_, "$1"))
       val rightParams = constrTparams.map(derivedTypeParam(_, "$2"))
       val subInstances = (leftParams, rightParams).zipped.map((param1, param2) =>
-        appliedRef(ref(defn.EqType), List(param1, param2)))
+        appliedRef(ref(defn.EqType), List(param1, param2), widenHK = true))
       DefDef(
           name = nme.eqInstance,
           tparams = leftParams ++ rightParams,
diff --git a/compiler/src/dotty/tools/dotc/core/Types.scala b/compiler/src/dotty/tools/dotc/core/Types.scala
index e7f3acf4d033..4238cbfc1359 100644
--- a/compiler/src/dotty/tools/dotc/core/Types.scala
+++ b/compiler/src/dotty/tools/dotc/core/Types.scala
@@ -1754,21 +1754,25 @@ object Types {
 
       val idx = typeParams.indexOf(param)
 
-      assert(args.nonEmpty,
-      	i"""bad parameter reference $this at ${ctx.phase}
-      	   |the parameter is ${param.showLocated} but the prefix $prefix
-      	   |does not define any corresponding arguments.""")
-
-      val argInfo = args(idx) match {
-        case arg: TypeBounds =>
-          val v = param.paramVariance
-          val pbounds = param.paramInfo
-          if (v > 0 && pbounds.loBound.dealias.isBottomType) TypeAlias(arg.hiBound & rebase(pbounds.hiBound))
-          else if (v < 0 && pbounds.hiBound.dealias.isTopType) TypeAlias(arg.loBound | rebase(pbounds.loBound))
-          else arg recoverable_& rebase(pbounds)
-        case arg => TypeAlias(arg)
+      if (idx < args.length) {
+        val argInfo = args(idx) match {
+          case arg: TypeBounds =>
+            val v = param.paramVariance
+            val pbounds = param.paramInfo
+            if (v > 0 && pbounds.loBound.dealias.isBottomType) TypeAlias(arg.hiBound & rebase(pbounds.hiBound))
+            else if (v < 0 && pbounds.hiBound.dealias.isTopType) TypeAlias(arg.loBound | rebase(pbounds.loBound))
+            else arg recoverable_& rebase(pbounds)
+          case arg => TypeAlias(arg)
+        }
+        param.derivedSingleDenotation(param, argInfo)
+      }
+      else {
+        assert(ctx.reporter.errorsReported,
+          i"""bad parameter reference $this at ${ctx.phase}
+            |the parameter is ${param.showLocated} but the prefix $prefix
+            |does not define any corresponding arguments.""")
+        NoDenotation
       }
-      param.derivedSingleDenotation(param, argInfo)
     }
 
     /** Reload denotation by computing the member with the reference's name as seen
diff --git a/tests/neg/i3976.scala b/tests/neg/i3976.scala
new file mode 100644
index 000000000000..9462ce583fa6
--- /dev/null
+++ b/tests/neg/i3976.scala
@@ -0,0 +1,58 @@
+object Test {
+  enum Hoge[F[_]] {
+    case A extends Hoge[List]
+    case B extends Hoge[[X] => String]
+  }
+  import Hoge._
+
+  A == A
+  A == (B: Hoge[_])
+
+  A == B // error: cannot be compared
+
+  class C
+
+  A == "" // error: cannot be compared
+  A == new C // error: cannot be compared
+
+}
+
+object Test2 {
+  enum Hoge[F[G[_]]] {
+    case A extends Hoge[[F[_]] => F[Int]]
+    case B extends Hoge[[F[_]] => F[String]]
+  }
+  import Hoge._
+
+  A == A
+  A == (B: Hoge[_])
+
+  A == B
+
+  class C
+
+  A == "" // error: cannot be compared
+  A == new C // error: cannot be compared
+
+}
+
+object Test3 {
+  enum Hoge[F[G[_]]] {
+    case A extends Hoge[[X] => List]   // error: wrong kind
+    case B extends Hoge[[X] => [Y] => String]  // error: wrong kind
+  }
+  import Hoge._
+
+  A == A
+  A == (B: Hoge[_])
+
+  A == B
+
+  class C
+
+  A == ""
+  A == new C
+
+}
+
+
diff --git a/tests/pos/i3976.scala b/tests/pos/i3976.scala
new file mode 100644
index 000000000000..03ae4bb146cf
--- /dev/null
+++ b/tests/pos/i3976.scala
@@ -0,0 +1,10 @@
+object Test {
+  enum Hoge[F[_]] {
+    case A extends Hoge[List]
+    case B extends Hoge[[X] => String]
+  }
+  import Hoge._
+
+  A == A
+  A == (B: Hoge[_])
+}