diff --git a/compiler/src/dotty/tools/dotc/typer/Inferencing.scala b/compiler/src/dotty/tools/dotc/typer/Inferencing.scala
index af4174423cbc..c31477d0270e 100644
--- a/compiler/src/dotty/tools/dotc/typer/Inferencing.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Inferencing.scala
@@ -307,16 +307,17 @@ object Inferencing {
   }
 
   /** If `tree` has a type lambda type, infer its type parameters by comparing with expected type `pt` */
-  def inferTypeParams(tree: Tree, pt: Type)(using Context): Tree = tree.tpe match {
+  def inferTypeParams(tree: Tree, pt: Type)(using Context): Tree = tree.tpe match
     case tl: TypeLambda =>
       val (tl1, tvars) = constrained(tl, tree)
       var tree1 = AppliedTypeTree(tree.withType(tl1), tvars)
       tree1.tpe <:< pt
-      fullyDefinedType(tree1.tpe, "template parent", tree.srcPos)
-      tree1
+      if isFullyDefined(tree1.tpe, force = ForceDegree.failBottom) then
+        tree1
+      else
+        EmptyTree
     case _ =>
       tree
-  }
 
   def isSkolemFree(tp: Type)(using Context): Boolean =
     !tp.existsPart(_.isInstanceOf[SkolemType])
diff --git a/compiler/src/dotty/tools/dotc/typer/Namer.scala b/compiler/src/dotty/tools/dotc/typer/Namer.scala
index 6f85efb0fc8a..af6673086339 100644
--- a/compiler/src/dotty/tools/dotc/typer/Namer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Namer.scala
@@ -1453,27 +1453,41 @@ class Namer { typer: Typer =>
        * only if parent type contains uninstantiated type parameters.
        */
       def parentType(parent: untpd.Tree)(using Context): Type =
-        if (parent.isType)
-          typedAheadType(parent, AnyTypeConstructorProto).tpe
-        else {
-          val (core, targs) = stripApply(parent) match {
+
+        def typedParentApplication(parent: untpd.Tree): Type =
+          val (core, targs) = stripApply(parent) match
             case TypeApply(core, targs) => (core, targs)
             case core => (core, Nil)
-          }
-          core match {
+          core match
             case Select(New(tpt), nme.CONSTRUCTOR) =>
               val targs1 = targs map (typedAheadType(_))
               val ptype = typedAheadType(tpt).tpe appliedTo targs1.tpes
               if (ptype.typeParams.isEmpty) ptype
-              else {
+              else
                 if (denot.is(ModuleClass) && denot.sourceModule.isOneOf(GivenOrImplicit))
                   missingType(denot.symbol, "parent ")(using creationContext)
                 fullyDefinedType(typedAheadExpr(parent).tpe, "class parent", parent.srcPos)
-              }
             case _ =>
               UnspecifiedErrorType.assertingErrorsReported
-          }
-        }
+
+        def typedParentType(tree: untpd.Tree): tpd.Tree =
+          val parentTpt = typer.typedType(parent, AnyTypeConstructorProto)
+          val ptpe = parentTpt.tpe
+          if ptpe.typeParams.nonEmpty
+              && ptpe.underlyingClassRef(refinementOK = false).exists
+          then
+            // Try to infer type parameters from a synthetic application.
+            // This might yield new info if implicit parameters are resolved.
+            // A test case is i16778.scala.
+            val app = untpd.Apply(untpd.Select(untpd.New(parentTpt), nme.CONSTRUCTOR), Nil)
+            typedParentApplication(app)
+            app.getAttachment(TypedAhead).getOrElse(parentTpt)
+          else
+            parentTpt
+
+        if parent.isType then typedAhead(parent, typedParentType).tpe
+        else typedParentApplication(parent)
+      end parentType
 
       /** Check parent type tree `parent` for the following well-formedness conditions:
        *  (1) It must be a class type with a stable prefix (@see checkClassTypeWithStablePrefix)
@@ -1607,7 +1621,7 @@ class Namer { typer: Typer =>
       case Some(ttree) => ttree
       case none =>
         val ttree = typed(tree)
-        xtree.putAttachment(TypedAhead, ttree)
+        if !ttree.isEmpty then xtree.putAttachment(TypedAhead, ttree)
         ttree
     }
   }
diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index a87d6dd7e703..5752409c51c1 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -843,14 +843,11 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
             isSkolemFree(pt) &&
             isEligible(pt.underlyingClassRef(refinementOK = false)))
           templ1 = cpy.Template(templ)(parents = untpd.TypeTree(pt) :: Nil)
-        templ1.parents foreach {
-          case parent: RefTree =>
-            typedAhead(parent, tree => inferTypeParams(typedType(tree), pt))
-          case _ =>
-        }
-        val x = tpnme.ANON_CLASS
-        val clsDef = TypeDef(x, templ1).withFlags(Final | Synthetic)
-        typed(cpy.Block(tree)(clsDef :: Nil, New(Ident(x), Nil)), pt)
+        for case parent: RefTree <- templ1.parents do
+          typedAhead(parent, tree => inferTypeParams(typedType(tree), pt))
+        val anon = tpnme.ANON_CLASS
+        val clsDef = TypeDef(anon, templ1).withFlags(Final | Synthetic)
+        typed(cpy.Block(tree)(clsDef :: Nil, New(Ident(anon), Nil)), pt)
       case _ =>
         var tpt1 = typedType(tree.tpt)
         val tsym = tpt1.tpe.underlyingClassRef(refinementOK = false).typeSymbol
diff --git a/compiler/test-resources/repl/i7644 b/compiler/test-resources/repl/i7644
index 8ceaf8b00804..786823073470 100644
--- a/compiler/test-resources/repl/i7644
+++ b/compiler/test-resources/repl/i7644
@@ -5,11 +5,7 @@ scala> class T extends CanEqual
   |       Cannot extend sealed trait CanEqual in a different source file
   |
   | longer explanation available when compiling with `-explain`
--- [E056] Syntax Error: --------------------------------------------------------
-1 | class T extends CanEqual
-  |                 ^^^^^^^^
-  |                 Missing type parameter for CanEqual
-2 errors found
+1 error found
 scala> class T extends CanEqual
 -- [E112] Syntax Error: --------------------------------------------------------
 1 | class T extends CanEqual
@@ -17,8 +13,5 @@ scala> class T extends CanEqual
   |       Cannot extend sealed trait CanEqual in a different source file
   |
   | longer explanation available when compiling with `-explain`
--- [E056] Syntax Error: --------------------------------------------------------
-1 | class T extends CanEqual
-  |                 ^^^^^^^^
-  |                 Missing type parameter for CanEqual
-2 errors found
+1 error found
+
diff --git a/tests/neg/i1643.scala b/tests/neg/i1643.scala
index a10422de6eab..1745539d73f5 100644
--- a/tests/neg/i1643.scala
+++ b/tests/neg/i1643.scala
@@ -1,4 +1,4 @@
-trait T extends Array { // error // error
+trait T extends Array { // error
   def t1(as: String*): Array[String] = { varargs1(as*) } // error
   def t2(as: String*): Array[String] = { super.varargs1(as*) } // error
 }
@@ -7,7 +7,7 @@ class C extends Base_1 { // error
   def c2(as: String*): Array[String] = { super.varargs1(as*) } // error
 }
 object Test extends App {
-  val t = new T {}  // error
+  val t = new T {}
   println(t.t1("a", "b").mkString(","))
   println(t.t2("a", "b").mkString(","))
   val c = new C {}
diff --git a/tests/neg/i4820.scala b/tests/neg/i4820.scala
deleted file mode 100644
index e19183b17b14..000000000000
--- a/tests/neg/i4820.scala
+++ /dev/null
@@ -1,2 +0,0 @@
-class Foo[A]
-class Bar[A] extends Foo  // error
diff --git a/tests/neg/i4820b.scala b/tests/neg/i4820b.scala
deleted file mode 100644
index 4a7b3da3fb1b..000000000000
--- a/tests/neg/i4820b.scala
+++ /dev/null
@@ -1,5 +0,0 @@
-trait SetOps[A, +C <: SetOps[A, C]]  {
-  def concat(that: Iterable[A]): C = ???
-}
-
-class Set1[A] extends SetOps // error: should be SetOps[A, Set1[A]]
diff --git a/tests/neg/i4820c.scala b/tests/neg/i4820c.scala
deleted file mode 100644
index 6956b23363b5..000000000000
--- a/tests/neg/i4820c.scala
+++ /dev/null
@@ -1,2 +0,0 @@
-trait Foo[A]
-class Bar[A] extends Foo // error
\ No newline at end of file
diff --git a/tests/pos/i16778.scala b/tests/pos/i16778.scala
new file mode 100644
index 000000000000..426f3c86c0bd
--- /dev/null
+++ b/tests/pos/i16778.scala
@@ -0,0 +1,22 @@
+final abstract class ForcedRecompilationToken[T]
+
+object ForcedRecompilationToken {
+  implicit def materialize: ForcedRecompilationToken["x"] = null.asInstanceOf[ForcedRecompilationToken["x"]]
+}
+
+class PluginDef[T](implicit val recompilationToken: ForcedRecompilationToken[T])
+
+object X {
+  val no = {
+    final class anon extends PluginDef {} // was: missing type parameters
+    new anon
+  }
+
+  val bad = new PluginDef {} // was: No given instance
+  val good = new PluginDef() {} // ok
+}
+
+object DependingPlugin {
+  class NestedDoublePlugin extends PluginDef
+  object NestedDoublePlugin extends PluginDef
+}
diff --git a/tests/pos/i4820.scala b/tests/pos/i4820.scala
new file mode 100644
index 000000000000..8d368d150a00
--- /dev/null
+++ b/tests/pos/i4820.scala
@@ -0,0 +1,2 @@
+class Foo[A]
+class Bar[A] extends Foo  // was error, now expanded to Foo[Nothing]
diff --git a/tests/pos/i4820b.scala b/tests/pos/i4820b.scala
new file mode 100644
index 000000000000..a1c7d54f0c76
--- /dev/null
+++ b/tests/pos/i4820b.scala
@@ -0,0 +1,5 @@
+trait SetOps[A, +C <: SetOps[A, C]]  {
+  def concat(that: Iterable[A]): C = ???
+}
+
+class Set1[A] extends SetOps // ideally should be SetOps[A, Set1[A]], but SetOps[Nothing, Nothin] is inferred