Include top-level symbols from same file in outer ambiguity error

When checking if an inherited definition is ambiguous with an
outer definition, include top-level outer definitions defined
in the same compilation unit.

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -366,11 +366,16 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
           // Does reference `tp` refer only to inherited symbols?
           def isInherited(denot: Denotation) =
             def isCurrent(mbr: SingleDenotation): Boolean =
-              !mbr.symbol.exists || mbr.symbol.owner == ctx.owner
+              !mbr.symbol.exists || mbr.symbol.owner == ctx.owner || ctx.owner.is(Package)
             denot match
               case denot: SingleDenotation => !isCurrent(denot)
               case denot => !denot.hasAltWith(isCurrent)
 
+          /* It is an error if an identifier x is available as an inherited member in an inner scope
+           * and the same name x is defined in an outer scope in the same source file, unless
+           * the inherited member (has an overloaded alternative that) coincides with
+           * (an overloaded alternative of) the definition x.
+           */
           def checkNoOuterDefs(denot: Denotation, last: Context, prevCtx: Context): Unit =
             def sameTermOrType(d1: SingleDenotation, d2: Denotation) =
               d2.containsSym(d1.symbol) || d2.hasUniqueSym && {
@@ -387,9 +392,15 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
             val owner = outer.owner
             if (owner eq last.owner) && (outer.scope eq last.scope) then
               checkNoOuterDefs(denot, outer, prevCtx)
-            else if !owner.is(Package) then
-              val scope = if owner.isClass then owner.info.decls else outer.scope
-              val competing = scope.denotsNamed(name).filterWithFlags(required, excluded)
+            else if !owner.isRoot then
+              val found =
+                if owner.is(Package) then
+                  owner.denot.asClass.membersNamed(name)
+                    .filterWithPredicate(d => !d.symbol.is(Package) && d.symbol.source == denot.symbol.source)
+                else
+                  val scope = if owner.isClass then owner.info.decls else outer.scope
+                  scope.denotsNamed(name)
+              val competing = found.filterWithFlags(required, excluded | Synthetic)
               if competing.exists then
                 val symsMatch = competing
                   .filterWithPredicate(sd => sameTermOrType(sd, denot))

tests/neg/ambiref.check

@@ -30,3 +30,19 @@
    |                and inherited subsequently in class E
    |
    | longer explanation available when compiling with `-explain`
+-- [E049] Reference Error: tests/neg/ambiref.scala:43:10 ---------------------------------------------------------------
+43 |  println(global)    // error
+   |          ^^^^^^
+   |          Reference to global is ambiguous.
+   |          It is both defined in package <empty>
+   |          and inherited subsequently in object D
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E049] Reference Error: tests/neg/ambiref.scala:49:16 ---------------------------------------------------------------
+49 |    def t = new T { } // error
+   |                ^
+   |                Reference to T is ambiguous.
+   |                It is both defined in package p
+   |                and inherited subsequently in class C
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg/ambiref.scala

@@ -40,4 +40,24 @@ val global = 0
 class C:
   val global = 1
 object D extends C:
-  println(global)    // OK, since global is defined in package
+  println(global)    // error
+
+package p:
+  class T
+  trait P { trait T }
+  class C extends P:
+    def t = new T { } // error
+
+package scala:
+  trait P { trait Option[+A] }
+  class C extends P:
+    def t = new Option[String] { } // OK, competing scala.Option is not defined in the same compilation unit
+
+object test5:
+  class Mu // generates a synthetic companion object with an apply method
+  trait A {
+    val Mu = 1
+  }
+  trait B extends A {
+    def t = Mu // don't warn about synthetic companion
+  }

tests/pos-special/fatal-warnings/i9260.scala

@@ -10,7 +10,7 @@ end AstImpl
 
 object untpd extends AstImpl[Null]:
 
-  def DefDef(ast: Ast): DefDef = ast match
+  def DefDef(ast: this.Ast): DefDef = ast match
     case ast: DefDef => ast
 
 end untpd

tests/run/protectedacc.scala

@@ -134,7 +134,7 @@ package p {
 
     abstract class X[T] extends PolyA[T] {
 
-      trait Inner extends B {
+      trait Inner extends this.B {
         def self: T;
         def self2: Node;
         def getB: Inner;