Use transparent inline in the library

And add deprecation warnings for old syntax

Author: nicolasstucki

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 suported. Use transparent modifier insead.")
             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 suported. Use transparent modifier insead.")
           if !mods.is(Inline) then
             syntaxError("`_ <:` is only allowed for given with `inline` modifier")
           in.nextToken()

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()
   }

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
   }