Fix inlining when outer select is needed from inline call prefix

compiler/src/dotty/tools/dotc/core/TypeOps.scala

@@ -60,6 +60,9 @@ object TypeOps:
 
       /** Map a `C.this` type to the right prefix. If the prefix is unstable, and
        *  the current variance is <= 0, return a range.
+       *  @param  pre     The prefix
+       *  @param  cls     The class in which the `C.this` type occurs
+       *  @param  thiscls The prefix `C` of the `C.this` type.
        */
       def toPrefix(pre: Type, cls: Symbol, thiscls: ClassSymbol): Type = /*>|>*/ trace.conditionally(track, s"toPrefix($pre, $cls, $thiscls)", show = true) /*<|<*/ {
         if ((pre eq NoType) || (pre eq NoPrefix) || (cls is PackageClass))

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

@@ -27,7 +27,10 @@ class ExplicitSelf extends MiniPhase {
   override def description: String = ExplicitSelf.description
 
   private def needsCast(tree: RefTree, cls: ClassSymbol)(using Context) =
-    !cls.is(Package) && cls.givenSelfType.exists && !cls.derivesFrom(tree.symbol.owner)
+    !cls.is(Package)
+    && cls.givenSelfType.exists
+    && tree.symbol.exists
+    && !cls.derivesFrom(tree.symbol.owner)
 
   private def castQualifier(tree: RefTree, cls: ClassSymbol, thiz: Tree)(using Context) =
     val selfType = cls.classInfo.selfType

compiler/src/dotty/tools/dotc/typer/Inliner.scala

@@ -573,9 +573,13 @@ class Inliner(call: tpd.Tree, rhsToInline: tpd.Tree)(using Context) {
           else if lastSelf.exists then
             ref(lastSelf).outerSelect(lastLevel - level, selfSym.info)
           else
-            inlineCallPrefix match
-              case Super(_, _) => This(rhsClsSym.asClass)
-              case _ => inlineCallPrefix
+            val pre = inlineCallPrefix match
+              case Super(qual, _) => qual
+              case pre => pre
+            val preLevel = inlinedMethod.owner.ownersIterator.length
+            if preLevel > level then pre.outerSelect(preLevel - level, selfSym.info)
+            else pre
+
       val binding = accountForOpaques(
         ValDef(selfSym.asTerm, QuoteUtils.changeOwnerOfTree(rhs, selfSym)).withSpan(selfSym.span))
       bindingsBuf += binding

tests/run/i15317.scala

@@ -0,0 +1,15 @@
+class Outer {
+  val outer = 1
+  object Inner {
+    def forwarder: Int = inlined
+    transparent inline def inlined: Int = outer
+  }
+  object Inner2 {
+    def forwarder: Int = inlined
+    inline def inlined: Int = outer
+  }
+}
+
+@main def Test =
+  assert((new Outer).Inner.forwarder == 1)
+  assert((new Outer).Inner2.forwarder == 1)