Explain accessible scope of private members in error message

Fixes #18686

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

@@ -2987,12 +2987,23 @@ 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 $location.""")
+      val cls = alt.owner.enclosingSubClass
+      val owner = if cls.exists then cls else alt.owner
+      val location: String =
+        if alt.is(Protected) then
+          if alt.privateWithin.exists && alt.privateWithin != owner then
+            if owner.is(Final) then alt.privateWithin.showLocated
+            else alt.privateWithin.showLocated + ", or " + owner.showLocated + " or one of its subclasses"
+          else
+            if owner.is(Final) then owner.showLocated
+            else owner.showLocated + " or one of its subclasses"
+        else
+          alt.privateWithin.orElse(owner).showLocated
+      val accessMod = if alt.is(Protected) then "protected" else "private"
+      val within = if alt.privateWithin.exists then i"[${alt.privateWithin.name}]"
+        else ""
+      whyNot.append(i"""
+          |  $accessMod$within $alt can only be accessed from $location.""")
     i"$whatCanNot be accessed as a member of $pre$where.$whyNot"
   def explain(using Context) = ""
 

compiler/test-resources/repl/i1370

@@ -3,4 +3,5 @@ scala> object Lives { class Private { def foo1: Any = new Private.C1; def foo2:
 1 | object Lives { class Private { def foo1: Any = new Private.C1; def foo2: Any = new Private.C2 };  object Private { class C1 private {}; private class C2 {} } }
   |                                                    ^^^^^^^^^^
   |constructor C1 cannot be accessed as a member of Lives.Private.C1 from class Private.
+  |  private constructor C1 can only be accessed from class C1 in object Private.
 1 error found

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 +1,30 @@
--- [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:13:16 ----------------------------------------------------------------
+13 |    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:14:16 ----------------------------------------------------------------
+14 |    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:15:16 ----------------------------------------------------------------
+15 |    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:16:16 ----------------------------------------------------------------
+16 |    println(Foo.Bar4) // error
+   |            ^^^^^^^^
+   |            value Bar4 cannot be accessed as a member of Foo.type from object Main.
+   |              protected[Foo] value Bar4 can only be accessed from object Foo.
+-- [E173] Reference Error: tests/neg/i18686.scala:17:20 ----------------------------------------------------------------
+17 |    println(Foo.Baz.Bar5) // error
+   |            ^^^^^^^^^^^^
+   |            value Bar5 cannot be accessed as a member of Foo.Baz.type from object Main.
+   |              private[Foo] value Bar5 can only be accessed from object Foo.
+-- [E173] Reference Error: tests/neg/i18686.scala:18:20 ----------------------------------------------------------------
+18 |    println(Foo.Baz.Bar6) // error
+   |            ^^^^^^^^^^^^
+   |            value Bar6 cannot be accessed as a member of Foo.Baz.type from object Main.
+   |              protected[Foo] value Bar6 can only be accessed from object Foo.

tests/neg/i18686.scala

@@ -1,13 +1,20 @@
 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
+  protected[Foo] val Bar4: Int = 5
+  object Baz:
+    private[Foo] val Bar5: Int = 5
+    protected[Foo] val Bar6: Int = 6
 end Foo
 
 object Main:
   def main(args: Array[String]): Unit =
     println(Foo.Bar1) // error
     println(Foo.Bar2) // error
     println(Foo.Bar3) // error
+    println(Foo.Bar4) // error
+    println(Foo.Baz.Bar5) // error
+    println(Foo.Baz.Bar6) // error
   end main
 end Main

tests/neg/i18686b.check

@@ -0,0 +1,28 @@
+-- [E173] Reference Error: tests/neg/i18686b.scala:15:16 ---------------------------------------------------------------
+15 |    println(foo.Bar1) // error
+   |            ^^^^^^^^
+   |            value Bar1 cannot be accessed as a member of Foo from object Main.
+   |              private value Bar1 can only be accessed from class Foo.
+-- [E173] Reference Error: tests/neg/i18686b.scala:16:16 ---------------------------------------------------------------
+16 |    println(foo.Bar2) // error
+   |            ^^^^^^^^
+   |            value Bar2 cannot be accessed as a member of Foo from object Main.
+   |              private[Foo] value Bar2 can only be accessed from class Foo.
+-- [E173] Reference Error: tests/neg/i18686b.scala:17:16 ---------------------------------------------------------------
+17 |    println(foo.Bar3) // error
+   |            ^^^^^^^^
+   |            value Bar3 cannot be accessed as a member of Foo from object Main.
+   |              protected value Bar3 can only be accessed from class Foo or one of its subclasses.
+-- [E173] Reference Error: tests/neg/i18686b.scala:18:16 ---------------------------------------------------------------
+18 |    println(foo.Bar4) // error
+   |            ^^^^^^^^
+   |            value Bar4 cannot be accessed as a member of Foo from object Main.
+   |              protected[Foo] value Bar4 can only be accessed from class Foo or one of its subclasses.
+-- [E008] Not Found Error: tests/neg/i18686b.scala:19:20 ---------------------------------------------------------------
+19 |    println(foo.Baz.Bar5) // error
+   |            ^^^^^^^^^^^^
+   |            value Bar5 is not a member of object Foo#Baz
+-- [E008] Not Found Error: tests/neg/i18686b.scala:20:20 ---------------------------------------------------------------
+20 |    println(foo.Baz.Bar6) // error
+   |            ^^^^^^^^^^^^
+   |            value Bar6 is not a member of object Foo#Baz

tests/neg/i18686b.scala

@@ -0,0 +1,22 @@
+class Foo:
+  private val Bar1: Int = 1
+  private[Foo] val Bar2: Int = 2
+  protected val Bar3: Int = 3
+  protected[Foo] val Bar4: Int = 5
+  class Baz:
+    private[Foo] val Bar5: Int = 5
+    protected[Foo] val Bar6: Int = 6
+end Foo
+
+def foo = new Foo
+
+object Main:
+  def main(args: Array[String]): Unit =
+    println(foo.Bar1) // error
+    println(foo.Bar2) // error
+    println(foo.Bar3) // error
+    println(foo.Bar4) // error
+    println(foo.Baz.Bar5) // error
+    println(foo.Baz.Bar6) // error
+  end main
+end Main

tests/neg/i18686c.check

@@ -0,0 +1,8 @@
+-- [E173] Reference Error: tests/neg/i18686c.scala:8:6 -----------------------------------------------------------------
+8 |  foo.foo // error
+  |  ^^^^^^^
+  |method foo cannot be accessed as a member of (foo² : Bar.Foo) from the top-level definitions in package <empty>.
+  |  protected[Bar] method foo can only be accessed from object Bar, or class Foo in object Bar or one of its subclasses.
+  |
+  |where:    foo  is a method in class Foo
+  |          foo² is a parameter in method test

tests/neg/i18686c.scala

@@ -0,0 +1,8 @@
+object Bar:
+  class Foo:
+    protected[Bar] def foo = 23
+  class Qux extends Foo:
+    val qux = foo
+
+def test(foo: Bar.Foo) =
+  foo.foo // error

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.