Specialized retained inline FunctionN apply methods

Fixes #19724

Author: nicolasstucki

Diff for: compiler/src/dotty/tools/dotc/transform/Erasure.scala

@@ -1000,21 +1000,28 @@ object Erasure {
      *  which is then dropped in `typedStats`.
      */
     private def addRetainedInlineBodies(stats: List[untpd.Tree])(using Context): List[untpd.Tree] =
-      lazy val retainerDef: Map[Symbol, DefDef] = stats.collect {
-        case stat: DefDef @unchecked if stat.symbol.name.is(BodyRetainerName) =>
-          val retainer = stat.symbol
-          val origName = retainer.name.asTermName.exclude(BodyRetainerName)
-          val targetName =
-            if retainer.hasAnnotation(defn.TargetNameAnnot) then
-              retainer.targetName.unmangle(BodyRetainerName).exclude(BodyRetainerName)
-            else origName
-          val inlineMeth = atPhase(typerPhase) {
-            retainer.owner.info.decl(origName)
-              .matchingDenotation(retainer.owner.thisType, stat.symbol.info, targetName)
-              .symbol
-          }
-          (inlineMeth, stat)
-      }.toMap
+      lazy val (rSyms, originalSyms, retainerDef): (List[Symbol], List[Symbol], Map[Symbol, DefDef]) =
+        val rSyms = List.newBuilder[Symbol]
+        val originalSyms = List.newBuilder[Symbol]
+        val retainerDef = stats.collect {
+          case stat: DefDef @unchecked if stat.symbol.name.is(BodyRetainerName) =>
+            val retainer = stat.symbol
+            val origName = retainer.name.asTermName.exclude(BodyRetainerName)
+            val targetName =
+              if retainer.hasAnnotation(defn.TargetNameAnnot) then
+                retainer.targetName.unmangle(BodyRetainerName).exclude(BodyRetainerName)
+              else origName
+            val inlineMeth = atPhase(typerPhase) {
+              retainer.owner.info.decl(origName)
+                .matchingDenotation(retainer.owner.thisType, stat.symbol.info, targetName)
+                .symbol
+            }
+            if inlineMeth.name.endsWith(nme.SPECIALIZED_SUFFIX.toSimpleName) then
+              rSyms += retainer
+              originalSyms += inlineMeth
+            (inlineMeth, stat)
+        }.toMap
+        (rSyms.result(), originalSyms.result(), retainerDef)
       stats.mapConserve {
         case stat: DefDef @unchecked if stat.symbol.isRetainedInlineMethod =>
           val rdef = retainerDef(stat.symbol)
@@ -1024,8 +1031,8 @@ object Erasure {
           val mapBody = TreeTypeMap(
             oldOwners = rdef.symbol :: Nil,
             newOwners = stat.symbol :: Nil,
-            substFrom = fromParams,
-            substTo   = toParams)
+            substFrom = rSyms ::: fromParams,
+            substTo   = originalSyms ::: toParams)
           cpy.DefDef(stat)(rhs = mapBody.transform(rdef.rhs))
         case stat => stat
       }

Diff for: compiler/src/dotty/tools/dotc/transform/SpecializeFunctions.scala

@@ -3,7 +3,7 @@ package transform
 
 import ast.Trees.*, ast.tpd, core.*
 import Contexts.*, Types.*, Decorators.*, Symbols.*, DenotTransformers.*
-import SymDenotations.*, Scopes.*, StdNames.*, NameOps.*, Names.*
+import SymDenotations.*, Scopes.*, StdNames.*, NameOps.*, Names.*, NameKinds.*
 import MegaPhase.MiniPhase
 
 
@@ -25,7 +25,7 @@ class SpecializeFunctions extends MiniPhase {
   /** Create forwarders from the generic applys to the specialized ones.
    */
   override def transformDefDef(ddef: DefDef)(using Context) = {
-    if ddef.name != nme.apply
+    if ddef.name.asTermName.exclude(BodyRetainerName) != nme.apply
        || ddef.termParamss.length != 1
        || ddef.termParamss.head.length > 2
        || !ctx.owner.isClass
@@ -41,6 +41,8 @@ class SpecializeFunctions extends MiniPhase {
       val paramTypes = ddef.termParamss.head.map(_.symbol.info)
       val retType = sym.info.finalResultType
       specName = nme.apply.specializedFunction(retType, paramTypes)
+      if ddef.name.is(BodyRetainerName) then
+        specName = BodyRetainerName(specName.nn.asTermName)
       defn.isSpecializableFunction(cls, paramTypes, retType)
     }
 

Diff for: tests/run/i19724.scala

@@ -0,0 +1,6 @@
+class MyMapper extends (Int => Double):
+  inline def apply(v: Int): Double = v.toDouble
+
+@main def Test =
+  val f: (Int => Double) = new MyMapper
+  assert(f(3) == 3.0)