diff --git a/compiler/src/dotty/tools/dotc/ast/TreeTypeMap.scala b/compiler/src/dotty/tools/dotc/ast/TreeTypeMap.scala
index a3c1484d4e62..1d98d643ad89 100644
--- a/compiler/src/dotty/tools/dotc/ast/TreeTypeMap.scala
+++ b/compiler/src/dotty/tools/dotc/ast/TreeTypeMap.scala
@@ -3,7 +3,7 @@ package dotc
 package ast
 
 import core._
-import Types._, Contexts._
+import Types._, Contexts._, Flags._
 import Symbols._, Annotations._, Trees._, Symbols._, Constants.Constant
 import Decorators._
 import dotty.tools.dotc.transform.SymUtils._
@@ -178,25 +178,25 @@ class TreeTypeMap(
    *  and return a treemap that contains the substitution
    *  between original and mapped symbols.
    */
-  def withMappedSyms(syms: List[Symbol], mapAlways: Boolean = false): TreeTypeMap =
-    withMappedSyms(syms, mapSymbols(syms, this, mapAlways))
+  def withMappedSyms(syms: List[Symbol]): TreeTypeMap =
+    withMappedSyms(syms, mapSymbols(syms, this))
 
   /** The tree map with the substitution between originals `syms`
    *  and mapped symbols `mapped`. Also goes into mapped classes
    *  and substitutes their declarations.
    */
-  def withMappedSyms(syms: List[Symbol], mapped: List[Symbol]): TreeTypeMap = {
-    val symsChanged = syms ne mapped
-    val substMap = withSubstitution(syms, mapped)
-    val fullMap = mapped.filter(_.isClass).foldLeft(substMap) { (tmap, cls) =>
-      val origDcls = cls.info.decls.toList
-      val mappedDcls = mapSymbols(origDcls, tmap)
-      val tmap1 = tmap.withMappedSyms(origDcls, mappedDcls)
-      if (symsChanged)
+  def withMappedSyms(syms: List[Symbol], mapped: List[Symbol]): TreeTypeMap =
+    if syms eq mapped then this
+    else
+      val substMap = withSubstitution(syms, mapped)
+      lazy val origCls = mapped.zip(syms).filter(_._1.isClass).toMap
+      mapped.filter(_.isClass).foldLeft(substMap) { (tmap, cls) =>
+        val origDcls = cls.info.decls.toList.filterNot(_.is(TypeParam))
+        val mappedDcls = mapSymbols(origDcls, tmap, mapAlways = true)
+        val tmap1 = tmap.withMappedSyms(
+          origCls(cls).typeParams ::: origDcls,
+          cls.typeParams ::: mappedDcls)
         origDcls.lazyZip(mappedDcls).foreach(cls.asClass.replace)
-      tmap1
-    }
-    if (symsChanged || (fullMap eq substMap)) fullMap
-    else withMappedSyms(syms, mapAlways = true)
-  }
+        tmap1
+      }
 }
diff --git a/compiler/src/dotty/tools/dotc/core/Symbols.scala b/compiler/src/dotty/tools/dotc/core/Symbols.scala
index b1939261084f..016a8cabaf59 100644
--- a/compiler/src/dotty/tools/dotc/core/Symbols.scala
+++ b/compiler/src/dotty/tools/dotc/core/Symbols.scala
@@ -811,22 +811,38 @@ object Symbols {
       val ttmap1 = ttmap.withSubstitution(originals, copies)
       originals.lazyZip(copies) foreach { (original, copy) =>
         val odenot = original.denot
-        val oinfo = original.info match {
-          case ClassInfo(pre, _, parents, decls, selfInfo) =>
-            assert(original.isClass)
-            ClassInfo(pre, copy.asClass, parents, decls.cloneScope, selfInfo)
-          case oinfo => oinfo
-        }
+        val completer = new LazyType:
+
+          def complete(denot: SymDenotation)(using Context): Unit =
+
+            val oinfo = original.info match
+              case ClassInfo(pre, _, parents, decls, selfInfo) =>
+                assert(original.isClass)
+                val otypeParams = original.typeParams
+                if otypeParams.isEmpty then
+                  ClassInfo(pre, copy.asClass, parents, decls.cloneScope, selfInfo)
+                else
+                  // copy type params, enter other definitions unchanged
+                  // type parameters need to be copied early, since other type
+                  // computations depend on them.
+                  val decls1 = newScope
+                  val newTypeParams = mapSymbols(original.typeParams, ttmap1, mapAlways = true)
+                  newTypeParams.foreach(decls1.enter)
+                  for sym <- decls do if !sym.is(TypeParam) then decls1.enter(sym)
+                  val parents1 = parents.map(_.substSym(otypeParams, newTypeParams))
+                  val selfInfo1 = selfInfo match
+                    case selfInfo: Type => selfInfo.substSym(otypeParams, newTypeParams)
+                    case _ => selfInfo
+                  ClassInfo(pre, copy.asClass, parents1, decls1, selfInfo1)
+              case oinfo => oinfo
 
-        val completer = new LazyType {
-          def complete(denot: SymDenotation)(using Context): Unit = {
             denot.info = oinfo // needed as otherwise we won't be able to go from Sym -> parents & etc
                                // Note that this is a hack, but hack commonly used in Dotty
                                // The same thing is done by other completers all the time
             denot.info = ttmap1.mapType(oinfo)
             denot.annotations = odenot.annotations.mapConserve(ttmap1.apply)
-          }
-        }
+
+        end completer
 
         copy.denot = odenot.copySymDenotation(
           symbol = copy,
diff --git a/tests/pos/i9965.scala b/tests/pos/i9965.scala
new file mode 100644
index 000000000000..8e2ceb414613
--- /dev/null
+++ b/tests/pos/i9965.scala
@@ -0,0 +1,19 @@
+class D[T]
+
+class C {
+  def f() = {
+    locally {
+      class dd[U] extends D[U] {
+        val xx = 1
+      }
+      class ee[V] extends dd[(V, V)]
+      def d[V]: dd[V] = new dd[V]
+      g[D[Int]](d[Int])
+      g[D[(Int, Int)]](new ee[Int])
+    }
+  }
+
+  inline def locally[T](inline body: T): T = body
+
+  def g[T](x: T): T = x
+}
\ No newline at end of file