Explain accessible scope of private members in error message

Fixes #18686

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -2987,12 +2987,18 @@ extends ReferenceMsg(CannotBeAccessedID):
     val where = if (ctx.owner.exists) i" from ${ctx.owner.enclosingClass}" else ""
     val whyNot = new StringBuffer
     for alt <- alts do
-      if alt.is(Protected) then
-        val cls = alt.owner.enclosingSubClass
-        val owner = if cls.exists then cls else alt.owner
-        val location = if owner.is(Final) then owner.showLocated else owner.showLocated + " or one of its subclasses"
-        whyNot.append(i"""
-            | Protected $alt can only be accessed from $location.""")
+      val cls = alt.owner.enclosingSubClass
+      val owner = if cls.exists then cls else alt.owner
+      val location =
+        if alt.is(Protected) && !owner.is(Final) then owner.showLocated  + " or one of its subclasses"
+        else if alt.privateWithin.exists then alt.privateWithin.showLocated
+        else owner.showLocated
+      val accessMod =
+        if alt.is(Protected) then "protected"
+        else if alt.is(Private) then "private"
+        else i"private[${alt.privateWithin.name}]"
+      whyNot.append(i"""
+          |  $accessMod $alt can only be accessed from $location.""")
     i"$whatCanNot be accessed as a member of $pre$where.$whyNot"
   def explain(using Context) = ""
 

tests/neg/i12573.check

@@ -5,4 +5,4 @@
    |Extension methods were tried, but the search failed with:
    |
    |    method getDFType cannot be accessed as a member of DFType.type from the top-level definitions in package <empty>.
-   |     Protected method getDFType can only be accessed from object DFType.
+   |      protected method getDFType can only be accessed from object DFType.

tests/neg/i14432c.check

@@ -2,6 +2,7 @@
 12 |class Bar extends example.Foo(23) { // error: cant access private[example] ctor
    |                  ^^^^^^^^^^^
    |                  constructor Foo cannot be accessed as a member of example.Foo from class Bar.
+   |                    private[example] constructor Foo can only be accessed from package example.
 -- [E172] Type Error: tests/neg/i14432c.scala:16:43 --------------------------------------------------------------------
 16 |  val mFoo = summon[Mirror.Of[example.Foo]] // error: no mirror
    |                                           ^

tests/neg/i18686.check

@@ -1,13 +1,20 @@
--- [E173] Reference Error: tests/neg/i18686.scala:9:16 -----------------------------------------------------------------
-9 |    println(Foo.Bar1) // error
-  |            ^^^^^^^^
-  |            value Bar1 cannot be accessed as a member of Foo.type from object Main.
--- [E173] Reference Error: tests/neg/i18686.scala:10:16 ----------------------------------------------------------------
-10 |    println(Foo.Bar2) // error
+-- [E173] Reference Error: tests/neg/i18686.scala:11:16 ----------------------------------------------------------------
+11 |    println(Foo.Bar1) // error
+   |            ^^^^^^^^
+   |            value Bar1 cannot be accessed as a member of Foo.type from object Main.
+   |              private value Bar1 can only be accessed from object Foo.
+-- [E173] Reference Error: tests/neg/i18686.scala:12:16 ----------------------------------------------------------------
+12 |    println(Foo.Bar2) // error
    |            ^^^^^^^^
    |            value Bar2 cannot be accessed as a member of Foo.type from object Main.
--- [E173] Reference Error: tests/neg/i18686.scala:11:16 ----------------------------------------------------------------
-11 |    println(Foo.Bar3) // error
+   |              private[Foo] value Bar2 can only be accessed from object Foo.
+-- [E173] Reference Error: tests/neg/i18686.scala:13:16 ----------------------------------------------------------------
+13 |    println(Foo.Bar3) // error
    |            ^^^^^^^^
    |            value Bar3 cannot be accessed as a member of Foo.type from object Main.
-   |             Protected value Bar3 can only be accessed from object Foo.
+   |              protected value Bar3 can only be accessed from object Foo.
+-- [E173] Reference Error: tests/neg/i18686.scala:14:20 ----------------------------------------------------------------
+14 |    println(Foo.Baz.Bar4) // error
+   |            ^^^^^^^^^^^^
+   |            value Bar4 cannot be accessed as a member of Foo.Baz.type from object Main.
+   |              private[Foo] value Bar4 can only be accessed from object Foo.

tests/neg/i18686.scala

@@ -1,13 +1,16 @@
 object Foo:
-  private val Bar1: Int = 3
-  private[Foo] val Bar2: Int = 3
+  private val Bar1: Int = 1
+  private[Foo] val Bar2: Int = 2
   protected val Bar3: Int = 3
+  object Baz:
+    private[Foo] val Bar4: Int = 4
 end Foo
 
 object Main:
   def main(args: Array[String]): Unit =
     println(Foo.Bar1) // error
     println(Foo.Bar2) // error
     println(Foo.Bar3) // error
+    println(Foo.Baz.Bar4) // error
   end main
 end Main

tests/neg/i7709.check

@@ -2,39 +2,39 @@
 5 |  class B extends X.Y // error
   |                  ^^^
   |                  class Y cannot be accessed as a member of X.type from class B.
-  |                   Protected class Y can only be accessed from object X.
+  |                    protected class Y can only be accessed from object X.
 -- [E173] Reference Error: tests/neg/i7709.scala:6:21 ------------------------------------------------------------------
 6 |  class B2 extends X.Y: // error
   |                   ^^^
   |                   class Y cannot be accessed as a member of X.type from class B2.
-  |                    Protected class Y can only be accessed from object X.
+  |                     protected class Y can only be accessed from object X.
 -- [E173] Reference Error: tests/neg/i7709.scala:9:28 ------------------------------------------------------------------
 9 |  class B4 extends B3(new X.Y) // error
   |                          ^^^
   |                          class Y cannot be accessed as a member of X.type from class B4.
-  |                           Protected class Y can only be accessed from object X.
+  |                            protected class Y can only be accessed from object X.
 -- [E173] Reference Error: tests/neg/i7709.scala:11:34 -----------------------------------------------------------------
 11 |    def this(n: Int) = this(new X.Y().toString) // error
    |                                ^^^
    |                                class Y cannot be accessed as a member of X.type from class B5.
-   |                                 Protected class Y can only be accessed from object X.
+   |                                  protected class Y can only be accessed from object X.
 -- [E173] Reference Error: tests/neg/i7709.scala:13:20 -----------------------------------------------------------------
 13 |  class B extends X.Y // error
    |                  ^^^
    |                  class Y cannot be accessed as a member of X.type from class B.
-   |                   Protected class Y can only be accessed from object X.
+   |                    protected class Y can only be accessed from object X.
 -- [E173] Reference Error: tests/neg/i7709.scala:18:18 -----------------------------------------------------------------
 18 |  def y  = new xx.Y  // error
    |               ^^^^
    |               class Y cannot be accessed as a member of XX from class C.
-   |                Protected class Y can only be accessed from class XX or one of its subclasses.
+   |                 protected class Y can only be accessed from class XX or one of its subclasses.
 -- [E173] Reference Error: tests/neg/i7709.scala:23:20 -----------------------------------------------------------------
 23 |    def y  = new xx.Y  // error
    |                 ^^^^
    |                 class Y cannot be accessed as a member of XX from class D.
-   |                  Protected class Y can only be accessed from class XX or one of its subclasses.
+   |                   protected class Y can only be accessed from class XX or one of its subclasses.
 -- [E173] Reference Error: tests/neg/i7709.scala:31:20 -----------------------------------------------------------------
 31 |  class Q extends X.Y // error
    |                  ^^^
    |                  class Y cannot be accessed as a member of p.X.type from class Q.
-   |                   Protected class Y can only be accessed from object X in package p.
+   |                    protected class Y can only be accessed from object X in package p.

tests/neg/not-accessible.check

@@ -2,19 +2,24 @@
 8 |    def test(a: A) = a.x // error
   |                     ^^^
   |                     value x cannot be accessed as a member of (a : foo.A) from class B.
+  |                       private[A] value x can only be accessed from class A in package foo.
 -- [E173] Reference Error: tests/neg/not-accessible.scala:10:23 --------------------------------------------------------
 10 |    def test(a: A) = a.x // error
    |                     ^^^
    |                     value x cannot be accessed as a member of (a : foo.A) from object B.
+   |                       private[A] value x can only be accessed from class A in package foo.
 -- [E173] Reference Error: tests/neg/not-accessible.scala:13:23 --------------------------------------------------------
 13 |    def test(a: A) = a.x // error
    |                     ^^^
    |            value x cannot be accessed as a member of (a : foo.A) from the top-level definitions in package bar.
+   |              private[A] value x can only be accessed from class A in package foo.
 -- [E173] Reference Error: tests/neg/not-accessible.scala:5:21 ---------------------------------------------------------
 5 |  def test(a: A) = a.x // error
   |                   ^^^
   |              value x cannot be accessed as a member of (a : foo.A) from the top-level definitions in package foo.
+  |                private[A] value x can only be accessed from class A in package foo.
 -- [E173] Reference Error: tests/neg/not-accessible.scala:15:23 --------------------------------------------------------
 15 |def test(a: foo.A) = a.x // error
    |                     ^^^
    |        value x cannot be accessed as a member of (a : foo.A) from the top-level definitions in package <empty>.
+   |          private[A] value x can only be accessed from class A in package foo.