diff --git a/compiler/src/dotty/tools/dotc/parsing/Parsers.scala b/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
index 9dbb2a7efbb9..70e28a176c06 100644
--- a/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
+++ b/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
@@ -3278,6 +3278,7 @@ object Parsers {
             leadingVparamss ::: rparamss
         var tpt = fromWithinReturnType {
           if in.token == SUBTYPE && mods.is(Inline) && AllowOldWhiteboxSyntax then
+            deprecationWarning("`<:` return type will no longer be supported. Use transparent modifier instead.")
             in.nextToken()
             mods1 = addMod(mods1, Mod.Transparent())
             toplevelTyp()
@@ -3547,6 +3548,7 @@ object Parsers {
           mods1 |= Final
           DefDef(name, tparams, vparamss, tpt, subExpr())
         if in.token == USCORE && AllowOldWhiteboxSyntax then
+          deprecationWarning("`<:` return type will no longer be supported. Use transparent modifier instead.")
           if !mods.is(Inline) then
             syntaxError("`_ <:` is only allowed for given with `inline` modifier")
           in.nextToken()
diff --git a/library/src/dotty/DottyPredef.scala b/library/src/dotty/DottyPredef.scala
index 8e334dfd99a5..c58408ec3434 100644
--- a/library/src/dotty/DottyPredef.scala
+++ b/library/src/dotty/DottyPredef.scala
@@ -8,7 +8,7 @@ object DottyPredef {
       assertFail(message)
   }
 
-  inline final def assert(inline assertion: Boolean) <: Unit = {
+  transparent inline final def assert(inline assertion: Boolean): Unit = {
     if (!assertion)
       assertFail()
   }
diff --git a/library/src/scala/compiletime/package.scala b/library/src/scala/compiletime/package.scala
index f5a069a9424e..550ea677b6be 100644
--- a/library/src/scala/compiletime/package.scala
+++ b/library/src/scala/compiletime/package.scala
@@ -52,7 +52,7 @@ package object compiletime {
    *
    *  the returned value would be `2`.
    */
-  inline def summonFrom[T](f: Nothing => T) <: T = ???
+  transparent inline def summonFrom[T](f: Nothing => T): T = ???
 
 
   /** Summon a given value of type `T`. Usually, the argument is not passed explicitly.
@@ -61,7 +61,7 @@ package object compiletime {
    *  @tparam T the type of the value to be summoned
    *  @return the given value typed as the provided type parameter
    */
-  inline def summonInline[T] <: T = summonFrom {
+  transparent inline def summonInline[T]: T = summonFrom {
     case t: T => t
   }
 
diff --git a/tests/invalid/run/typelevel-patmat.scala b/tests/invalid/run/typelevel-patmat.scala
index 9f463638c125..6d85692955c2 100644
--- a/tests/invalid/run/typelevel-patmat.scala
+++ b/tests/invalid/run/typelevel-patmat.scala
@@ -45,7 +45,7 @@ object Test extends App {
   inline val i2 = toInt(y2)
   val j2: 2 = i2
 
-  inline def concat(xs: HList, ys: HList) <: HList = inline xs match {
+  transparent inline def concat(xs: HList, ys: HList): HList = inline xs match {
     case HNil => ys
     case HCons(x, xs1) => HCons(x, concat(xs1, ys))
   }
@@ -68,7 +68,7 @@ object Test extends App {
   val r6 = concat(HCons(1, HCons("a", HNil)), HCons(true, HCons(1.0, HNil)))
   val c6: HCons[Int, HCons[String, HCons[Boolean, HCons[Double, HNil]]]] = r6
 
-  inline def nth(xs: HList, n: Int) <: Any = inline xs match {
+  transparent inline def nth(xs: HList, n: Int): Any = inline xs match {
     case HCons(x, _)   if n == 0 => x
     case HCons(_, xs1) if n > 0  => nth(xs1, n - 1)
   }
diff --git a/tests/invalid/run/typelevel1.scala b/tests/invalid/run/typelevel1.scala
index 8324c5c58df9..a4f7f6ce0f41 100644
--- a/tests/invalid/run/typelevel1.scala
+++ b/tests/invalid/run/typelevel1.scala
@@ -4,17 +4,17 @@ trait HList {
   def head: Any
   def tail: HList
 
-  inline def isEmpty <: Boolean = length == 0
+  transparent inline def isEmpty: Boolean = length == 0
 }
 
 case object HNil extends HList {
-  inline override def length <: Int = 0
+  transparent inline override def length: Int = 0
   def head: Nothing = ???
   def tail: Nothing = ???
 }
 
 case class :: [H, T <: HList] (hd: H, tl: T) extends HList {
-  inline override def length <: Int = 1 + tl.length
+  transparent inline override def length: Int = 1 + tl.length
   inline def head: H = this.hd
   inline def tail: T = this.tl
 }
@@ -32,7 +32,7 @@ object Test extends App {
 
   // Does not work since it infers `Any` as a type argument for `::`
   // and we cannot undo that without a typing from untyped.
-  inline def concat[T1, T2](xs: HList, ys: HList) <: HList =
+  transparent inline def concat[T1, T2](xs: HList, ys: HList): HList =
     inline if xs.isEmpty then ys
     else new ::(xs.head, concat(xs.tail, ys))
 
diff --git a/tests/neg-macros/quote-whitebox/Macro_1.scala b/tests/neg-macros/quote-whitebox/Macro_1.scala
index be0024a0d8b9..8b34d3346394 100644
--- a/tests/neg-macros/quote-whitebox/Macro_1.scala
+++ b/tests/neg-macros/quote-whitebox/Macro_1.scala
@@ -1,7 +1,7 @@
 import scala.quoted._
 
 object Macros {
-  inline def defaultOf(inline str: String) <: Any = ${ defaultOfImpl('str) }
+  transparent inline def defaultOf(inline str: String): Any = ${ defaultOfImpl('str) }
   def defaultOfImpl(str: Expr[String]) (using QuoteContext): Expr[Any] = str.unliftOrError match {
     case "int" => '{1}
     case "string" => '{"a"}
diff --git a/tests/neg/specializing-inline.scala b/tests/neg/specializing-inline.scala
index 424ff500b6c2..57b56b99bccc 100644
--- a/tests/neg/specializing-inline.scala
+++ b/tests/neg/specializing-inline.scala
@@ -4,7 +4,7 @@ object Test {
   val z = h(true)
   val zc: Int = z // error
 
-  inline def g <: Any = 1
+  transparent inline def g: Any = 1
   val y = g
   val yc: Int = y // OK
 
diff --git a/tests/pending/pos/summonFrom.scala b/tests/pending/pos/summonFrom.scala
index 0669609337a1..e245ca687ec3 100644
--- a/tests/pending/pos/summonFrom.scala
+++ b/tests/pending/pos/summonFrom.scala
@@ -4,7 +4,7 @@ object summonFroms {
   object invariant {
     case class Box[T](value: T)
     implicit val box: Box[Int] = Box(0)
-    inline def unbox <: Any = summonInline[Box[t]].value
+    transparent inline def unbox: Any = summonInline[Box[t]].value
     val i: Int = unbox
     val i2 = unbox
     val i3: Int = i2
@@ -13,7 +13,7 @@ object summonFroms {
   object covariant {
     case class Box[+T](value: T)
     implicit val box: Box[Int] = Box(0)
-    inline def unbox <: Any = summonInline[Box[t]].value
+    transparent inline def unbox: Any = summonInline[Box[t]].value
     val i: Int = unbox
     val i2 = unbox
     val i3: Int = i2
@@ -22,7 +22,7 @@ object summonFroms {
   object contravariant {
     case class TrashCan[-T](trash: T => Unit)
     implicit val trashCan: TrashCan[Int] = TrashCan { i => ; }
-    inline def trash <: Nothing => Unit = summonInline[TrashCan[t]].trash
+    transparent inline def trash: Nothing => Unit = summonInline[TrashCan[t]].trash
     val t1: Int => Unit = trash
     val t2 = trash
     val t3: Int => Unit = t2
diff --git a/tests/pos-macros/quote-whitebox-2/Macro_1.scala b/tests/pos-macros/quote-whitebox-2/Macro_1.scala
index e84716716018..94a5c9e5e688 100644
--- a/tests/pos-macros/quote-whitebox-2/Macro_1.scala
+++ b/tests/pos-macros/quote-whitebox-2/Macro_1.scala
@@ -3,7 +3,7 @@ import scala.quoted._
 
 object Macro {
 
-  inline def charOrString(inline str: String) <: Any = ${ impl('str) }
+  transparent inline def charOrString(inline str: String): Any = ${ impl('str) }
 
   def impl(strExpr: Expr[String]) (using QuoteContext)=
     val str = strExpr.unliftOrError
diff --git a/tests/pos/given-pattern.scala b/tests/pos/given-pattern.scala
index 7986dc56402f..db655d093482 100644
--- a/tests/pos/given-pattern.scala
+++ b/tests/pos/given-pattern.scala
@@ -3,7 +3,7 @@
 class Test {
   import scala.collection.immutable.{TreeSet, HashSet}
 
-  inline def trySummon[S, T](f: PartialFunction[S, T]) <: T = ???
+  transparent inline def trySummon[S, T](f: PartialFunction[S, T]): T = ???
 
   inline def setFor[T]: Set[T] = trySummon {
     case given ord: Ordering[T] => new TreeSet[T]
diff --git a/tests/pos/i4006.scala b/tests/pos/i4006.scala
index 328020381f37..c8f6418460c1 100644
--- a/tests/pos/i4006.scala
+++ b/tests/pos/i4006.scala
@@ -1,4 +1,4 @@
 class Foo {
-  inline def foo <: Int = try { 1 } finally println("Hello")
+  transparent inline def foo: Int = try { 1 } finally println("Hello")
   foo
 }
diff --git a/tests/pos/i5574.scala b/tests/pos/i5574.scala
index 9c360f8b81c8..0f212f5ffe1a 100644
--- a/tests/pos/i5574.scala
+++ b/tests/pos/i5574.scala
@@ -3,7 +3,7 @@ import scala.compiletime._
 object i5574 {
   class Box[F[_]]
 
-  inline def foo[T] <: Any =
+  transparent inline def foo[T]: Any =
     inline erasedValue[T] match {
       case _: Box[f] =>
         type t = f
diff --git a/tests/pos/i6213.scala b/tests/pos/i6213.scala
index 6feda0ef6cf9..f0fc532fd2d3 100644
--- a/tests/pos/i6213.scala
+++ b/tests/pos/i6213.scala
@@ -1,6 +1,6 @@
 object Test {
   class C { type T }
-  inline def foo[U] <: Any = (??? : C { type T = U })
+  transparent inline def foo[U]: Any = (??? : C { type T = U })
 
   foo[Int]
 }
\ No newline at end of file
diff --git a/tests/pos/i7078.scala b/tests/pos/i7078.scala
index 70106e2e0356..ab49673b3ca6 100644
--- a/tests/pos/i7078.scala
+++ b/tests/pos/i7078.scala
@@ -1,7 +1,7 @@
 trait A
 class B extends A
 
-inline given tc as _ <: A = B()
+transparent inline given tc as A = B()
 
 val x: B = summon[A]
 
diff --git a/tests/pos/i7358.scala b/tests/pos/i7358.scala
index a8f6e3aaf99c..37032e60d52b 100644
--- a/tests/pos/i7358.scala
+++ b/tests/pos/i7358.scala
@@ -3,7 +3,7 @@ package test
 import scala.quoted._
 import scala.compiletime._
 
-inline def summonT[Tp <: Tuple](using QuoteContext) <: Tuple = inline erasedValue[Tp] match {
+transparent inline def summonT[Tp <: Tuple](using QuoteContext): Tuple = inline erasedValue[Tp] match {
   case _ : Unit => ()
   case _ : (hd *: tl) => {
     type H = hd
diff --git a/tests/pos/inline-caseclass.scala b/tests/pos/inline-caseclass.scala
index e7dd05559654..42d0df6fc857 100644
--- a/tests/pos/inline-caseclass.scala
+++ b/tests/pos/inline-caseclass.scala
@@ -5,7 +5,7 @@ case class S[N <: Nat](n: N) extends Nat
 object Test {
   type Z = Z.type
 
-  inline def add(x: Nat, y: Int) <: Int = inline x match {
+  transparent inline def add(x: Nat, y: Int): Int = inline x match {
     case Z => y
     case S(x1) => add(x1, y) + 1
   }
diff --git a/tests/pos/inline-constfold.scala b/tests/pos/inline-constfold.scala
index 528a8a2bf295..6e083646424c 100644
--- a/tests/pos/inline-constfold.scala
+++ b/tests/pos/inline-constfold.scala
@@ -1,12 +1,12 @@
 object Test {
-  inline def not(x: Boolean) <: Boolean = {
+  transparent inline def not(x: Boolean): Boolean = {
     !x
   }
 
   final val a = not(true)
   val b: false = a
 
-  inline def add(x: Int, y: Int) <: Int = {
+  transparent inline def add(x: Int, y: Int): Int = {
     x + y
   }
 
diff --git a/tests/pos/inline-match-separate/inline-match-separate_1.scala b/tests/pos/inline-match-separate/inline-match-separate_1.scala
index 667ac73edcfd..755cb34eb6b4 100644
--- a/tests/pos/inline-match-separate/inline-match-separate_1.scala
+++ b/tests/pos/inline-match-separate/inline-match-separate_1.scala
@@ -1,6 +1,6 @@
 object Utils {
   class Box[T]
-  inline def foo[T](t: T) <: Any = inline t match {
+  transparent inline def foo[T](t: T): Any = inline t match {
     case _: Box[a] => scala.compiletime.constValue[a]
   }
 }
diff --git a/tests/pos/reference/compile-time.scala b/tests/pos/reference/compile-time.scala
index 14b3d08b2a7c..ff400133ad01 100644
--- a/tests/pos/reference/compile-time.scala
+++ b/tests/pos/reference/compile-time.scala
@@ -7,7 +7,7 @@ class Test:
   case object Zero extends Nat
   case class Succ[N <: Nat](n: N) extends Nat
 
-  inline def toIntC[N] <: Int =
+  transparent inline def toIntC[N]: Int =
     inline constValue[N] match
       case 0 => 0
       case _: S[n1] => 1 + toIntC[n1]
@@ -31,10 +31,10 @@ class Test:
   val dBoolean: Some[Boolean] = defaultValue[Boolean]
   val dAny: None.type = defaultValue[Any]
 
-  inline def toIntT[N <: Nat] <: Int = inline scala.compiletime.erasedValue[N] match
+  transparent inline def toIntT[N <: Nat]: Int = inline scala.compiletime.erasedValue[N] match
     case _: Zero.type => 0
     case _: Succ[n] => toIntT[n] + 1
 
-  inline def summonFrom(f: Nothing => Any) <: Any = ???
+  transparent inline def summonFrom(f: Nothing => Any): Any = ???
 
   final val two = toIntT[Succ[Succ[Zero.type]]]
diff --git a/tests/pos/reference/inline-match.scala b/tests/pos/reference/inline-match.scala
index 173d0615a9f4..d539763af351 100644
--- a/tests/pos/reference/inline-match.scala
+++ b/tests/pos/reference/inline-match.scala
@@ -2,7 +2,7 @@ package inlinematch
 
 class Test {
 
-  inline def g(x: Any) <: Any = inline x match {
+  transparent inline def g(x: Any): Any = inline x match {
     case x: String => (x, x) // Tuple2[String, String](x, x)
     case x: Double => x
   }
@@ -14,7 +14,7 @@ class Test {
   case object Zero extends Nat
   case class Succ[N <: Nat](n: N) extends Nat
 
-  inline def toInt(n: Nat) <: Int = inline n match {
+  transparent inline def toInt(n: Nat): Int = inline n match {
     case Zero => 0
     case Succ(n1) => toInt(n1) + 1
   }
diff --git a/tests/pos/reference/inline-specializing.scala b/tests/pos/reference/inline-specializing.scala
index 839c5929aa08..f119f96f0fcb 100644
--- a/tests/pos/reference/inline-specializing.scala
+++ b/tests/pos/reference/inline-specializing.scala
@@ -6,7 +6,7 @@ object Test{
   def meth() = true
   }
 
-  inline def choose(b: Boolean) <: A = {
+  transparent inline def choose(b: Boolean): A = {
   if (b) new A()
   else new B()
   }
diff --git a/tests/pos/typelevel0.scala b/tests/pos/typelevel0.scala
index 1b5dc643cc6e..a2da98241025 100644
--- a/tests/pos/typelevel0.scala
+++ b/tests/pos/typelevel0.scala
@@ -4,17 +4,17 @@ trait HList {
   def head: Any
   def tail: HList
 
-  inline def isEmpty <: Boolean = length == 0
+  transparent inline def isEmpty: Boolean = length == 0
 }
 
 case object HNil extends HList {
-  inline override def length <: Int = 0
+  transparent inline override def length: Int = 0
   def head: Nothing = ???
   def tail: Nothing = ???
 }
 
 case class :: [+H, +T <: HList] (hd: H, tl: T) extends HList {
-  inline override def length <: Int = 1 + tl.length
+  transparent inline override def length: Int = 1 + tl.length
   def head: H = this.hd
   def tail: T = this.tl
 }
diff --git a/tests/run-custom-args/companion-loading.scala b/tests/run-custom-args/companion-loading.scala
index b34f67750f4b..640658b0003c 100644
--- a/tests/run-custom-args/companion-loading.scala
+++ b/tests/run-custom-args/companion-loading.scala
@@ -21,7 +21,7 @@ implicit object FooAssoc extends Assoc[Foo] {
 
 import compiletime.summonFrom
 
-inline def link[T] <: Any =
+transparent inline def link[T]: Any =
   summonFrom {
     case _: Link[T, s] =>
       summonFrom {
diff --git a/tests/run-macros/i7898/Macro_1.scala b/tests/run-macros/i7898/Macro_1.scala
index 5eabdb272f52..efdff11d4afb 100644
--- a/tests/run-macros/i7898/Macro_1.scala
+++ b/tests/run-macros/i7898/Macro_1.scala
@@ -12,7 +12,7 @@ object Main {
     }
   }
 
-  inline def myMacro(body: => Any) <: Any = ${
+  transparent inline def myMacro(body: => Any): Any = ${
     myMacroImpl('body)
   }
 }
diff --git a/tests/run-macros/quote-whitebox/Macro_1.scala b/tests/run-macros/quote-whitebox/Macro_1.scala
index be0024a0d8b9..8b34d3346394 100644
--- a/tests/run-macros/quote-whitebox/Macro_1.scala
+++ b/tests/run-macros/quote-whitebox/Macro_1.scala
@@ -1,7 +1,7 @@
 import scala.quoted._
 
 object Macros {
-  inline def defaultOf(inline str: String) <: Any = ${ defaultOfImpl('str) }
+  transparent inline def defaultOf(inline str: String): Any = ${ defaultOfImpl('str) }
   def defaultOfImpl(str: Expr[String]) (using QuoteContext): Expr[Any] = str.unliftOrError match {
     case "int" => '{1}
     case "string" => '{"a"}
diff --git a/tests/run-macros/refined-selectable-macro/Macro_1.scala b/tests/run-macros/refined-selectable-macro/Macro_1.scala
index 6bc5936a283b..d73004b445f8 100644
--- a/tests/run-macros/refined-selectable-macro/Macro_1.scala
+++ b/tests/run-macros/refined-selectable-macro/Macro_1.scala
@@ -3,12 +3,12 @@ import scala.quoted._
 object Macro {
 
   trait SelectableRecord extends Selectable {
-    inline def toTuple <: Tuple = ${ toTupleImpl('this)}
+    transparent inline def toTuple: Tuple = ${ toTupleImpl('this)}
   }
 
   trait SelectableRecordCompanion[T] {
     protected def fromUntypedTuple(elems: (String, Any)*): T
-    inline def fromTuple[T <: Tuple](inline s: T) <: Any = ${ fromTupleImpl('s, '{ (x: Array[(String, Any)]) => fromUntypedTuple(x: _*) } ) }
+    transparent inline def fromTuple[T <: Tuple](inline s: T): Any = ${ fromTupleImpl('s, '{ (x: Array[(String, Any)]) => fromUntypedTuple(x: _*) } ) }
   }
 
   private def toTupleImpl(s: Expr[Selectable])(using qctx:QuoteContext) : Expr[Tuple] = {
diff --git a/tests/run-staging/i3876-d.scala b/tests/run-staging/i3876-d.scala
index 853d41c52854..c34c6ae26d38 100644
--- a/tests/run-staging/i3876-d.scala
+++ b/tests/run-staging/i3876-d.scala
@@ -13,5 +13,5 @@ object Test {
     println(withQuoteContext(Expr.betaReduce(f4)(x).show))
   }
 
-  inline def inlineLambda <: Int => Int = x => x + x
+  transparent inline def inlineLambda: Int => Int = x => x + x
 }
\ No newline at end of file
diff --git a/tests/run-staging/i3876-e.scala b/tests/run-staging/i3876-e.scala
index f51e67127484..7acbf8c71886 100644
--- a/tests/run-staging/i3876-e.scala
+++ b/tests/run-staging/i3876-e.scala
@@ -13,5 +13,5 @@ object Test {
     println(withQuoteContext(Expr.betaReduce(f4)(x).show))
   }
 
-  inline def inlineLambda <: Int => Int = x => x + x
+  transparent inline def inlineLambda: Int => Int = x => x + x
 }
\ No newline at end of file