Improve error message for inaccessible types (#18406)

* If accessed from an object:
```diff
- from module class B$
+ from object B
```
* If accessed from a package:
```diff
- from module class <filename>$package$
+ from top-level definition in package foo
```

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

@@ -109,7 +109,7 @@ object NameOps {
         false
     }
 
-    /** is this the name of an object enclosing packagel-level definitions? */
+    /** is this the name of an object enclosing package-level definitions? */
     def isPackageObjectName: Boolean = name match {
       case name: TermName => name == nme.PACKAGE || name.endsWith(str.TOPLEVEL_SUFFIX)
       case name: TypeName =>
@@ -119,6 +119,16 @@ object NameOps {
         }
     }
 
+    /** is this the name of an object enclosing top-level definitions? */
+    def isTopLevelPackageObjectName: Boolean = name match {
+      case name: TermName => name.endsWith(str.TOPLEVEL_SUFFIX)
+      case name: TypeName =>
+        name.toTermName match {
+          case ModuleClassName(original) => original.isTopLevelPackageObjectName
+          case _ => false
+        }
+    }
+
     /** Convert this module name to corresponding module class name */
     def moduleClassName: TypeName = name.derived(ModuleClassName).toTypeName
 

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

@@ -672,6 +672,10 @@ object SymDenotations {
     def isPackageObject(using Context): Boolean =
       name.isPackageObjectName && owner.is(Package) && this.is(Module)
 
+    /** Is this symbol a package object containing top-level definitions? */
+    def isTopLevelDefinitionsObject(using Context): Boolean =
+      name.isTopLevelPackageObjectName && owner.is(Package) && this.is(Module)
+
     /** Is this symbol a toplevel definition in a package object? */
     def isWrappedToplevelDef(using Context): Boolean =
       !isConstructor && owner.isPackageObject
@@ -911,17 +915,17 @@ object SymDenotations {
             true
           else
             val encl = if ctx.owner.isConstructor then ctx.owner.enclosingClass.owner.enclosingClass else ctx.owner.enclosingClass
+            val location = if owner.is(Final) then owner.showLocated else owner.showLocated + " or one of its subclasses"
             fail(i"""
-                 | Access to protected $this not permitted because enclosing ${encl.showLocated}
-                 | is not a subclass of ${owner.showLocated} where target is defined""")
+                | Protected $this can only be accessed from $location.""")
         else if isType || pre.derivesFrom(cls) || isConstructor || owner.is(ModuleClass) then
           // allow accesses to types from arbitrary subclasses fixes #4737
           // don't perform this check for static members
           true
         else
+          val location = if cls.is(Final) then cls.showLocated else cls.showLocated + " or one of its subclasses"
           fail(i"""
-               | Access to protected ${symbol.show} not permitted because prefix type ${pre.widen.show}
-               | does not conform to ${cls.showLocated} where the access takes place""")
+               | Protected $this can only be accessed from $location.""")
       end isProtectedAccessOK
 
       if pre eq NoPrefix then true

compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala

@@ -1113,13 +1113,18 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
         fullNameString(sym)
       else if (sym.is(ModuleClass) && sym.isPackageObject && sym.name.stripModuleClassSuffix == tpnme.PACKAGE)
         nameString(sym.owner.name)
+      else if (sym.is(ModuleClass) && sym.isTopLevelDefinitionsObject)
+        nameString(sym.owner.name)
       else if (sym.is(ModuleClass))
         nameString(sym.name.stripModuleClassSuffix) + idString(sym)
       else if (hasMeaninglessName(sym))
         simpleNameString(sym.owner) + idString(sym)
       else
         nameString(sym)
-    (keywordText(kindString(sym)) ~~ {
+
+    if sym.is(ModuleClass) && sym.isTopLevelDefinitionsObject then
+      "the top-level definitions in package " + nameString(sym.owner.name)
+    else (keywordText(kindString(sym)) ~~ {
       if (sym.isAnonymousClass)
         toTextParents(sym.info.parents) ~~ "{...}"
       else

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

@@ -2984,7 +2984,7 @@ extends ReferenceMsg(CannotBeAccessedID):
         i"${if (sym.owner == pre.typeSymbol) sym.show else sym.showLocated} cannot"
       case _ =>
         i"none of the overloaded alternatives named $name can"
-    val where = if (ctx.owner.exists) s" from ${ctx.owner.enclosingClass}" else ""
+    val where = if (ctx.owner.exists) i" from ${ctx.owner.enclosingClass}" else ""
     val whyNot = new StringBuffer
     alts.foreach(_.isAccessibleFrom(pre, superAccess, whyNot))
     i"$whatCanNot be accessed as a member of $pre$where.$whyNot"

tests/neg-macros/annot-result-owner.check

@@ -2,7 +2,7 @@
 -- Error: tests/neg-macros/annot-result-owner/Test_2.scala:1:0 ---------------------------------------------------------
 1 |@insertVal // error
   |^^^^^^^^^^
-  |macro annotation @insertVal added value definitionWithWrongOwner$macro$1 with an inconsistent owner. Expected it to be owned by package object Test_2$package but was owned by method foo.
+  |macro annotation @insertVal added value definitionWithWrongOwner$macro$1 with an inconsistent owner. Expected it to be owned by the top-level definitions in package <empty> but was owned by method foo.
 -- Error: tests/neg-macros/annot-result-owner/Test_2.scala:5:2 ---------------------------------------------------------
 5 |  @insertVal // error
   |  ^^^^^^^^^^

tests/neg/i12573.check

@@ -1,9 +1,8 @@
 -- [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:
+   |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
+   |    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.

tests/neg/i7709.check

@@ -2,47 +2,39 @@
 5 |  class B extends X.Y // error
   |                  ^^^
   |                  class Y cannot be accessed as a member of X.type from class B.
-  |                   Access to protected class Y not permitted because enclosing object A
-  |                   is not a subclass of object X where target is defined
+  |                   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.
-  |                    Access to protected class Y not permitted because enclosing object A
-  |                    is not a subclass of object X where target is defined
+  |                    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.
-  |                           Access to protected class Y not permitted because enclosing object A
-  |                           is not a subclass of object X where target is defined
+  |                           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.
-   |                                 Access to protected class Y not permitted because enclosing object A
-   |                                 is not a subclass of object X where target is defined
+   |                                 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.
-   |                   Access to protected class Y not permitted because enclosing trait T
-   |                   is not a subclass of object X where target is defined
+   |                   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.
-   |                Access to protected class Y not permitted because enclosing class C
-   |                is not a subclass of class XX where target is defined
+   |                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.
-   |                  Access to protected class Y not permitted because enclosing class D
-   |                  is not a subclass of class XX where target is defined
+   |                  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.
-   |                   Access to protected class Y not permitted because enclosing package p
-   |                   is not a subclass of object X in package p where target is defined
+   |                   Protected class Y can only be accessed from object X in package p.

tests/neg/not-accessible.check

@@ -0,0 +1,20 @@
+-- [E173] Reference Error: tests/neg/not-accessible.scala:8:23 ---------------------------------------------------------
+8 |    def test(a: A) = a.x // error
+  |                     ^^^
+  |                     value x cannot be accessed as a member of (a : foo.A) from class B.
+-- [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.
+-- [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.
+-- [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.
+-- [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>.

tests/neg/not-accessible.scala

@@ -0,0 +1,15 @@
+package foo:
+
+  class A(private[A] val x: Int)
+
+  def test(a: A) = a.x // error
+
+  class B:
+    def test(a: A) = a.x // error
+  object B:
+    def test(a: A) = a.x // error
+
+  package bar:
+    def test(a: A) = a.x // error
+
+def test(a: foo.A) = a.x // error