add proper error for illegal access for extension methods

Author: rochala

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

@@ -208,7 +208,7 @@ object ErrorReporting {
         i""".
            |Extension methods were tried, but the search failed with:
            |
-           |    ${nested.head.explanation}"""
+           |${nested.head.explanation.indented(4)}"""
       else if tree.hasAttachment(desugar.MultiLineInfix) then
         i""".
            |Note that `${tree.name}` is treated as an infix operator in Scala 3.

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

@@ -1108,7 +1108,9 @@ trait Implicits:
         ctx.reporter.removeBufferedMessages
         adapted.tpe match {
           case _: SearchFailureType => SearchFailure(adapted)
-          case _ =>
+          case error: PreviousErrorType if !adapted.symbol.isAccessibleFrom(cand.ref.prefix) => 
+            SearchFailure(adapted.withType(new NestedFailure(error.msg, pt)))
+          case _ => 
             // Special case for `$conforms` and `<:<.refl`. Showing them to the users brings
             // no value, so we instead report a `NoMatchingImplicitsFailure`
             if (adapted.symbol == defn.Predef_conforms || adapted.symbol == defn.SubType_refl)

tests/neg/i12573.check

@@ -0,0 +1,9 @@
+-- [E008] Not Found Error: tests/neg/i12573.scala:23:38 ----------------------------------------------------------------
+23 |val w: Value[8] = DFBits(Value[8](8)).getDFType.width // error
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |         value getDFType is not a member of DFBits[(8 : Int)].
+   |         Extension methods were tried, but the search failed with:
+   |
+   |             method getDFType cannot be accessed as a member of DFType.type from module class i12573$package$.
+   |              Access to protected method getDFType not permitted because enclosing package object i12573$package
+   |              is not a subclass of object DFType where target is defined

tests/neg/i12573.scala

@@ -0,0 +1,23 @@
+class Value[T <: Int](val value: T)
+
+sealed trait DFType:
+  type Width <: Int
+  val width: Value[Width]
+
+object DFType:
+  trait TC[T]:
+    type Type <: DFType
+    def apply(t: T): Type
+  type Aux[T, Type0 <: DFType] = TC[T] { type Type = Type0 }
+  transparent inline given ofDFType[T <: DFType]: TC[T] =
+    new TC[T]:
+      type Type = T
+      def apply(t: T): Type = t
+
+  extension [T, Type <: DFType](t: T)(using tc: Aux[T, Type])
+    protected def getDFType: Type = tc(t)
+
+final case class DFBits[W <: Int](width: Value[W]) extends DFType:
+  type Width = W
+
+val w: Value[8] = DFBits(Value[8](8)).getDFType.width // error