diff --git a/compiler/src/dotty/tools/dotc/ast/tpd.scala b/compiler/src/dotty/tools/dotc/ast/tpd.scala
index dd83a52ef40b..5adbef96ec89 100644
--- a/compiler/src/dotty/tools/dotc/ast/tpd.scala
+++ b/compiler/src/dotty/tools/dotc/ast/tpd.scala
@@ -344,24 +344,27 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
 
   /** An anonymous class
    *
-   *      new parents { forwarders }
+   *      new parents { termForwarders; typeAliases }
    *
-   *  where `forwarders` contains forwarders for all functions in `fns`.
-   *  @param parents    a non-empty list of class types
-   *  @param fns        a non-empty of functions for which forwarders should be defined in the class.
-   *  The class has the same owner as the first function in `fns`.
-   *  Its position is the union of all functions in `fns`.
+   *  @param parents        a non-empty list of class types
+   *  @param termForwarders a non-empty list of forwarding definitions specified by their name and the definition they forward to.
+   *  @param typeMembers    a possibly-empty list of type members specified by their name and their right hand side.
+   *
+   *  The class has the same owner as the first function in `termForwarders`.
+   *  Its position is the union of all symbols in `termForwarders`.
    */
-  def AnonClass(parents: List[Type], fns: List[TermSymbol], methNames: List[TermName])(using Context): Block = {
-    AnonClass(fns.head.owner, parents, fns.map(_.span).reduceLeft(_ union _)) { cls =>
-      def forwarder(fn: TermSymbol, name: TermName) = {
+  def AnonClass(parents: List[Type], termForwarders: List[(TermName, TermSymbol)],
+      typeMembers: List[(TypeName, TypeBounds)] = Nil)(using Context): Block = {
+    AnonClass(termForwarders.head._2.owner, parents, termForwarders.map(_._2.span).reduceLeft(_ union _)) { cls =>
+      def forwarder(name: TermName, fn: TermSymbol) = {
         val fwdMeth = fn.copy(cls, name, Synthetic | Method | Final).entered.asTerm
         for overridden <- fwdMeth.allOverriddenSymbols do
           if overridden.is(Extension) then fwdMeth.setFlag(Extension)
           if !overridden.is(Deferred) then fwdMeth.setFlag(Override)
         DefDef(fwdMeth, ref(fn).appliedToArgss(_))
       }
-      fns.lazyZip(methNames).map(forwarder)
+      termForwarders.map((name, sym) => forwarder(name, sym)) ++
+      typeMembers.map((name, info) => TypeDef(newSymbol(cls, name, Synthetic, info).entered))
     }
   }
 
diff --git a/compiler/src/dotty/tools/dotc/core/Types.scala b/compiler/src/dotty/tools/dotc/core/Types.scala
index 9d0b3d839619..3c887bfc076d 100644
--- a/compiler/src/dotty/tools/dotc/core/Types.scala
+++ b/compiler/src/dotty/tools/dotc/core/Types.scala
@@ -5534,13 +5534,16 @@ object Types extends TypeUtils {
    *     and PolyType not allowed!) according to `possibleSamMethods`.
    *   - can be instantiated without arguments or with just () as argument.
    *
+   *  Additionally, a SAM type may contain type aliases refinements if they refine
+   *  an existing type member.
+   *
    *  The pattern `SAMType(samMethod, samParent)` matches a SAM type, where `samMethod` is the
    *  type of the single abstract method and `samParent` is a subtype of the matched
    *  SAM type which has been stripped of wildcards to turn it into a valid parent
    *  type.
    */
   object SAMType {
-    /** If possible, return a type which is both a subtype of `origTp` and a type
+    /** If possible, return a type which is both a subtype of `origTp` and a (possibly refined) type
      *  application of `samClass` where none of the type arguments are
      *  wildcards (thus making it a valid parent type), otherwise return
      *  NoType.
@@ -5570,27 +5573,41 @@ object Types extends TypeUtils {
      *  we arbitrarily pick the upper-bound.
      */
     def samParent(origTp: Type, samClass: Symbol, samMeth: Symbol)(using Context): Type =
-      val tp = origTp.baseType(samClass)
+      val tp0 = origTp.baseType(samClass)
+
+      /** Copy type aliases refinements to `toTp` from `fromTp` */
+      def withRefinements(toType: Type, fromTp: Type): Type = fromTp.dealias match
+        case RefinedType(fromParent, name, info: TypeAlias) if tp0.member(name).exists =>
+          val parent1 = withRefinements(toType, fromParent)
+          RefinedType(toType, name, info)
+        case _ => toType
+      val tp = withRefinements(tp0, origTp)
+
       if !(tp <:< origTp) then NoType
-      else tp match
-        case tp @ AppliedType(tycon, args) if tp.hasWildcardArg =>
-          val accu = new TypeAccumulator[VarianceMap[Symbol]]:
-            def apply(vmap: VarianceMap[Symbol], t: Type): VarianceMap[Symbol] = t match
-              case tp: TypeRef if tp.symbol.isAllOf(ClassTypeParam) =>
-                vmap.recordLocalVariance(tp.symbol, variance)
-              case _ =>
-                foldOver(vmap, t)
-          val vmap = accu(VarianceMap.empty, samMeth.info)
-          val tparams = tycon.typeParamSymbols
-          val args1 = args.zipWithConserve(tparams):
-            case (arg @ TypeBounds(lo, hi), tparam) =>
-              val v = vmap.computedVariance(tparam)
-              if v.uncheckedNN < 0 then lo
-              else hi
-            case (arg, _) => arg
-          tp.derivedAppliedType(tycon, args1)
-        case _ =>
-          tp
+      else
+        def approxWildcardArgs(tp: Type): Type = tp match
+          case tp @ AppliedType(tycon, args) if tp.hasWildcardArg =>
+            val accu = new TypeAccumulator[VarianceMap[Symbol]]:
+              def apply(vmap: VarianceMap[Symbol], t: Type): VarianceMap[Symbol] = t match
+                case tp: TypeRef if tp.symbol.isAllOf(ClassTypeParam) =>
+                  vmap.recordLocalVariance(tp.symbol, variance)
+                case _ =>
+                  foldOver(vmap, t)
+            val vmap = accu(VarianceMap.empty, samMeth.info)
+            val tparams = tycon.typeParamSymbols
+            val args1 = args.zipWithConserve(tparams):
+              case (arg @ TypeBounds(lo, hi), tparam) =>
+                val v = vmap.computedVariance(tparam)
+                if v.uncheckedNN < 0 then lo
+                else hi
+              case (arg, _) => arg
+            tp.derivedAppliedType(tycon, args1)
+          case tp @ RefinedType(parent, name, info) =>
+            tp.derivedRefinedType(approxWildcardArgs(parent), name, info)
+          case _ =>
+            tp
+        approxWildcardArgs(tp)
+    end samParent
 
     def samClass(tp: Type)(using Context): Symbol = tp match
       case tp: ClassInfo =>
diff --git a/compiler/src/dotty/tools/dotc/transform/ExpandSAMs.scala b/compiler/src/dotty/tools/dotc/transform/ExpandSAMs.scala
index 47c2dad1d61b..4347cca7f9d9 100644
--- a/compiler/src/dotty/tools/dotc/transform/ExpandSAMs.scala
+++ b/compiler/src/dotty/tools/dotc/transform/ExpandSAMs.scala
@@ -6,6 +6,8 @@ import core.*
 import Scopes.newScope
 import Contexts.*, Symbols.*, Types.*, Flags.*, Decorators.*, StdNames.*, Constants.*
 import MegaPhase.*
+import Names.TypeName
+import SymUtils.*
 import NullOpsDecorator.*
 import ast.untpd
 
@@ -50,16 +52,28 @@ class ExpandSAMs extends MiniPhase:
         case tpe if defn.isContextFunctionType(tpe) =>
           tree
         case SAMType(_, tpe) if tpe.isRef(defn.PartialFunctionClass) =>
-          val tpe1 = checkRefinements(tpe, fn)
-          toPartialFunction(tree, tpe1)
+          toPartialFunction(tree, tpe)
         case SAMType(_, tpe) if ExpandSAMs.isPlatformSam(tpe.classSymbol.asClass) =>
-          checkRefinements(tpe, fn)
           tree
         case tpe =>
-          val tpe1 = checkRefinements(tpe.stripNull, fn)
+          // A SAM type is allowed to have type aliases refinements (see
+          // SAMType#samParent) which must be converted into type members if
+          // the closure is desugared into a class.
+          val refinements = collection.mutable.ListBuffer[(TypeName, TypeAlias)]()
+          def collectAndStripRefinements(tp: Type): Type = tp match
+            case RefinedType(parent, name, info: TypeAlias) =>
+              val res = collectAndStripRefinements(parent)
+              refinements += ((name.asTypeName, info))
+              res
+            case _ => tp
+          val tpe1 = collectAndStripRefinements(tpe)
           val Seq(samDenot) = tpe1.possibleSamMethods
           cpy.Block(tree)(stats,
-              AnonClass(tpe1 :: Nil, fn.symbol.asTerm :: Nil, samDenot.symbol.asTerm.name :: Nil))
+            AnonClass(List(tpe1),
+              List(samDenot.symbol.asTerm.name -> fn.symbol.asTerm),
+              refinements.toList
+            )
+          )
       }
     case _ =>
       tree
@@ -170,13 +184,4 @@ class ExpandSAMs extends MiniPhase:
       List(isDefinedAtDef, applyOrElseDef)
     }
   }
-
-  private def checkRefinements(tpe: Type, tree: Tree)(using Context): Type = tpe.dealias match {
-    case RefinedType(parent, name, _) =>
-      if (name.isTermName && tpe.member(name).symbol.ownersIterator.isEmpty) // if member defined in the refinement
-        report.error(em"Lambda does not define $name", tree.srcPos)
-      checkRefinements(parent, tree)
-    case tpe =>
-      tpe
-  }
 end ExpandSAMs
diff --git a/tests/run/i18315.scala b/tests/run/i18315.scala
new file mode 100644
index 000000000000..85824920efbd
--- /dev/null
+++ b/tests/run/i18315.scala
@@ -0,0 +1,15 @@
+trait Sam1:
+  type T
+  def apply(x: T): T
+
+trait Sam2:
+  var x: Int = 1 // To force anonymous class generation
+  type T
+  def apply(x: T): T
+
+object Test:
+  def main(args: Array[String]): Unit =
+    val s1: Sam1 { type T = String } = x => x.trim
+    s1.apply("foo")
+    val s2: Sam2 { type T = Int } = x => x + 1
+    s2.apply(1)