Support inline overrides in value classes (#16523)

Fixes #15725

Three fixes:

 - Don't make extension method copies of inline methods
- Make retainedBody methods discoverable by entering them in their
parent scope, so that they get an extension method
 - But strip the retainedBody in the extension method

compiler/src/dotty/tools/dotc/inlines/Inlines.scala

@@ -229,8 +229,8 @@ object Inlines:
 
     val retainer = meth.copy(
       name = BodyRetainerName(meth.name),
-      flags = meth.flags &~ (Inline | Macro | Override) | Private,
-      coord = mdef.rhs.span.startPos).asTerm
+      flags = (meth.flags &~ (Inline | Macro | Override | AbsOverride)) | Private,
+      coord = mdef.rhs.span.startPos).asTerm.entered
     retainer.deriveTargetNameAnnotation(meth, name => BodyRetainerName(name.asTermName))
     DefDef(retainer, prefss =>
       inlineCall(

compiler/src/dotty/tools/dotc/transform/ExtensionMethods.scala

@@ -13,7 +13,7 @@ import core._
 import Types._, Contexts._, Names._, Flags._, DenotTransformers._, Phases._
 import SymDenotations._, Symbols._, StdNames._, Denotations._
 import TypeErasure.{ valueErasure, ErasedValueType }
-import NameKinds.ExtMethName
+import NameKinds.{ExtMethName, BodyRetainerName}
 import Decorators._
 import TypeUtils._
 
@@ -79,7 +79,7 @@ class ExtensionMethods extends MiniPhase with DenotTransformer with FullParamete
           // because it adds extension methods before pickling.
           if (!(valueClass.is(Scala2x)))
             for (decl <- valueClass.classInfo.decls)
-              if (isMethodWithExtension(decl))
+              if isMethodWithExtension(decl) then
                 enterInModuleClass(createExtensionMethod(decl, moduleClassSym.symbol))
 
           // Create synthetic methods to cast values between the underlying type
@@ -179,7 +179,10 @@ object ExtensionMethods {
 
   /** Name of the extension method that corresponds to given instance method `meth`. */
   def extensionName(imeth: Symbol)(using Context): TermName =
-    ExtMethName(imeth.name.asTermName)
+    ExtMethName(
+      imeth.name.asTermName match
+        case BodyRetainerName(name) => name
+        case name => name)
 
   /** Return the extension method that corresponds to given instance method `meth`. */
   def extensionMethod(imeth: Symbol)(using Context): TermSymbol =

compiler/src/dotty/tools/dotc/transform/ValueClasses.scala

@@ -29,6 +29,7 @@ object ValueClasses {
       isDerivedValueClass(d.owner) &&
       !d.isConstructor &&
       !d.symbol.isSuperAccessor &&
+      !d.isInlineMethod &&
       !d.is(Macro)
     }
 

tests/run/i15725.check

@@ -0,0 +1 @@
+2 meters

tests/run/i15725.scala

@@ -0,0 +1,8 @@
+class ToString(d: Int) extends AnyVal :
+  inline override def toString():String = s"$d meters"
+
+@main def Test =
+  val ts = ToString(2)
+  val a: Any = ts
+  println(a)
+