From 689a69e2f3829bd0f45b7da298e184c1508e1e63 Mon Sep 17 00:00:00 2001
From: rochala <rochala@tutanota.com>
Date: Mon, 21 Mar 2022 13:34:24 +0100
Subject: [PATCH] add proper error for illegal access for extension methods

---
 .../tools/dotc/typer/ErrorReporting.scala     |  2 +-
 .../dotty/tools/dotc/typer/Implicits.scala    |  4 +++-
 tests/neg/i12573.check                        |  9 ++++++++
 tests/neg/i12573.scala                        | 23 +++++++++++++++++++
 4 files changed, 36 insertions(+), 2 deletions(-)
 create mode 100644 tests/neg/i12573.check
 create mode 100644 tests/neg/i12573.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala b/compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala
index 73ceda9223fd..97b5f9bb6680 100644
--- a/compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala
+++ b/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 `${tree.name}` is treated as an infix operator in Scala 3.
diff --git a/compiler/src/dotty/tools/dotc/typer/Implicits.scala b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
index fc1fd57c7bd8..4fae9264939d 100644
--- a/compiler/src/dotty/tools/dotc/typer/Implicits.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Implicits.scala
@@ -1108,7 +1108,9 @@ trait Implicits:
         ctx.reporter.removeBufferedMessages
         adapted.tpe match {
           case _: SearchFailureType => SearchFailure(adapted)
-          case _ =>
+          case error: PreviousErrorType if !adapted.symbol.isAccessibleFrom(cand.ref.prefix) => 
+            SearchFailure(adapted.withType(new NestedFailure(error.msg, pt)))
+          case _ => 
             // Special case for `$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)
diff --git a/tests/neg/i12573.check b/tests/neg/i12573.check
new file mode 100644
index 000000000000..d250f4beabbe
--- /dev/null
+++ b/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
\ No newline at end of file
diff --git a/tests/neg/i12573.scala b/tests/neg/i12573.scala
new file mode 100644
index 000000000000..c45ef6974a84
--- /dev/null
+++ b/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
\ No newline at end of file