Detect symbols inherited into package objects in ambiguity warning

src/compiler/scala/tools/nsc/typechecker/Contexts.scala

@@ -1533,7 +1533,7 @@ trait Contexts { self: Analyzer =>
        *
        * Scala: Bindings of different kinds have a defined precedence order:
        *
-       *  1) Definitions and declarations in lexical scope that are not top-level have the highest precedence.
+       *  1) Definitions and declarations in lexical scope have the highest precedence.
        *  1b) Definitions and declarations that are either inherited, or made
        *      available by a package clause and also defined in the same compilation unit
        *      as the reference to them, have the next highest precedence.
@@ -1545,6 +1545,8 @@ trait Contexts { self: Analyzer =>
        *     as well as bindings supplied by the compiler but not explicitly written in source code,
        *     have the lowest precedence.
        */
+
+      /* Level 4 (see above) */
       def foreignDefined = defSym.exists && thisContext.isPackageOwnedInDifferentUnit(defSym)  // SI-2458
 
       // Find the first candidate import
@@ -1612,15 +1614,22 @@ trait Contexts { self: Analyzer =>
         val depth0  = symbolDepth
         val wasFoundInSuper = foundInSuper
         val foundCompetingSymbol: () => Boolean =
-          if (foreignDefined) () => !foreignDefined
-          else () => !defSym.owner.isPackageObjectOrClass && !foundInSuper && !foreignDefined
+          if (foreignDefined)
+            // if the first found symbol (defSym0) is level 4 (foreignDefined), a lower level (1 or 1b) defSym is competing
+            () => defSym.exists && !foreignDefined
+          else {
+            // if defSym0 is level 1 or 1b, another defSym is competing if defined in an outer scope in the same file
+            () => defSym.exists && !(pre.typeSymbol.isPackageClass && !defSym.owner.isPackageClass) && !foundInSuper && !foreignDefined
+            //                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+            //                       defined in package object (or inherited into package object)
+          }
         while ((cx ne NoContext) && cx.depth >= symbolDepth) cx = cx.outer
         if (wasFoundInSuper)
           while ((cx ne NoContext) && (cx.owner eq cx0.owner)) cx = cx.outer
         var done = false
         while (!done) {
           nextDefinition(defSym0, pre0)
-          done = (cx eq NoContext) || defSym.exists && foundCompetingSymbol()
+          done = (cx eq NoContext) || foundCompetingSymbol()
           if (!done && (cx ne NoContext)) cx = cx.outer
         }
         if (defSym.exists && (defSym ne defSym0)) {

test/files/neg/t12816.check

@@ -0,0 +1,20 @@
+t12816.scala:8: error: package object inheritance is deprecated (https://github.com/scala/scala-dev/issues/441);
+drop the `extends` clause or use a regular object instead
+package object p extends U {
+                 ^
+t12816.scala:29: warning: reference to c is ambiguous;
+it is both defined in the enclosing package p and inherited in the enclosing trait RR as method c (defined in trait T)
+In Scala 2, symbols inherited from a superclass shadow symbols defined in an outer scope.
+Such references are ambiguous in Scala 3. To continue using the inherited symbol, write `this.c`.
+Or use `-Wconf:msg=legacy-binding:s` to silence this warning.
+    def m3 = c // warn
+             ^
+t12816.scala:33: warning: reference to Z is ambiguous;
+it is both defined in the enclosing package p and inherited in the enclosing trait RR as trait Z (defined in trait T)
+In Scala 2, symbols inherited from a superclass shadow symbols defined in an outer scope.
+Such references are ambiguous in Scala 3. To continue using the inherited symbol, write `this.Z`.
+Or use `-Wconf:msg=legacy-binding:s` to silence this warning.
+    def n3: Z // warn
+            ^
+2 warnings
+1 error

test/files/neg/t12816.scala

@@ -0,0 +1,35 @@
+// scalac: -Xsource:3 -Werror
+
+trait U {
+  def a: Int = 0
+  trait X
+}
+
+package object p extends U {
+  def b: Int = 0
+  trait Y
+}
+
+package p {
+  object c
+  trait Z
+  trait T {
+    def a = 1
+    def b = 1
+    def c = 1
+
+    trait X
+    trait Y
+    trait Z
+  }
+
+  trait RR extends T {
+    def m1 = a // ok
+    def m2 = b // ok
+    def m3 = c // warn
+
+    def n1: X // ok
+    def n2: Y // ok
+    def n3: Z // warn
+  }
+}

test/files/neg/t12816b.check

@@ -0,0 +1,20 @@
+A.scala:5: error: package object inheritance is deprecated (https://github.com/scala/scala-dev/issues/441);
+drop the `extends` clause or use a regular object instead
+package object p extends U {
+                 ^
+B.scala:19: warning: reference to c is ambiguous;
+it is both defined in the enclosing package p and inherited in the enclosing trait RR as method c (defined in trait T)
+In Scala 2, symbols inherited from a superclass shadow symbols defined in an outer scope.
+Such references are ambiguous in Scala 3. To continue using the inherited symbol, write `this.c`.
+Or use `-Wconf:msg=legacy-binding:s` to silence this warning.
+    def m3 = c // warn
+             ^
+B.scala:23: warning: reference to Z is ambiguous;
+it is both defined in the enclosing package p and inherited in the enclosing trait RR as trait Z (defined in trait T)
+In Scala 2, symbols inherited from a superclass shadow symbols defined in an outer scope.
+Such references are ambiguous in Scala 3. To continue using the inherited symbol, write `this.Z`.
+Or use `-Wconf:msg=legacy-binding:s` to silence this warning.
+    def n3: Z // warn
+            ^
+2 warnings
+1 error

test/files/neg/t12816b/A.scala

@@ -0,0 +1,8 @@
+trait U {
+  def a: Int = 0
+  trait X
+}
+package object p extends U {
+  def b: Int = 0
+  trait Y
+}

test/files/neg/t12816b/B.scala

@@ -0,0 +1,25 @@
+// scalac: -Xsource:3 -Werror
+
+package p {
+  object c
+  trait Z
+  trait T {
+    def a = 1
+    def b = 1
+    def c = 1
+
+    trait X
+    trait Y
+    trait Z
+  }
+
+  trait RR extends T {
+    def m1 = a // ok
+    def m2 = b // ok
+    def m3 = c // warn
+
+    def n1: X // ok
+    def n2: Y // ok
+    def n3: Z // warn
+  }
+}