Improve error message for inaccessible types

Author: nicolasstucki

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

@@ -2976,7 +2976,7 @@ 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/i12573.check

@@ -4,6 +4,6 @@
    |         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$package$.
+   |             method getDFType cannot be accessed as a member of DFType.type from package object 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
+   |              is not a subclass of object DFType where target is defined

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 package object not-accessible$package.
+-- [E173] Reference Error: tests/neg/not-accessible.scala:5:21 ---------------------------------------------------------
+5 |  def test(a: A) = a.x // error
+  |                   ^^^
+  |                 value x cannot be accessed as a member of (a : foo.A) from package object not-accessible$package.
+-- [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 package object not-accessible$package.

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