Package object members are not conflicting with inherited

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

@@ -418,7 +418,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
            * 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 checkNoOuterDefs(denot: Denotation, ctx: Context, origCtx: Context): Unit =
             def sameTermOrType(d1: SingleDenotation, d2: Denotation) =
               d2.containsSym(d1.symbol) || d2.hasUniqueSym && {
                 val sym1 = d1.symbol
@@ -430,19 +430,23 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                 else
                   (sym1.isAliasType || sym2.isAliasType) && d1.info =:= d2.info
               }
-            val outer = last.outer
-            val owner = outer.owner
-            if (owner eq last.owner) && (outer.scope eq last.scope) then
-              checkNoOuterDefs(denot, outer, prevCtx)
-            else if !owner.isRoot then
+            val outerCtx = ctx.outer
+            val outerOwner = outerCtx.owner
+            if (outerOwner eq ctx.owner) && (outerCtx.scope eq ctx.scope) then
+              checkNoOuterDefs(denot, outerCtx, origCtx)
+            else if !outerOwner.isRoot then
               val found =
-                if owner.is(Package) then
-                  owner.denot.asClass.membersNamed(name)
+                if outerOwner.is(Package) then
+                  def notInPackageObject(sym: Symbol) =
+                    sym.owner == outerOwner || // sym.owner.isPackageObject is false if sym is defined in a parent of the package object
+                      sym.owner.isPackageObject && sym.owner.name.endsWith(str.TOPLEVEL_SUFFIX) // top-level definitions
+                  outerOwner.denot.asClass.membersNamed(name)
                     .filterWithPredicate(d => !d.symbol.is(Package)
+                      && notInPackageObject(d.symbol)
                       && d.symbol.source.exists
                       && isDefinedInCurrentUnit(d))
                 else
-                  val scope = if owner.isClass then owner.info.decls else outer.scope
+                  val scope = if outerOwner.isClass then outerOwner.info.decls else outerCtx.scope
                   scope.denotsNamed(name)
               val competing = found.filterWithFlags(required, excluded | Synthetic)
               if competing.exists then
@@ -451,14 +455,14 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                   .exists
                 if !symsMatch && !suppressErrors then
                   report.errorOrMigrationWarning(
-                    AmbiguousReference(name, Definition, Inheritance, prevCtx)(using outer),
+                    AmbiguousReference(name, Definition, Inheritance, origCtx)(using outerCtx),
                     pos, from = `3.0`)
                   if migrateTo3 then
                     patch(Span(pos.span.start),
-                      if prevCtx.owner == refctx.owner.enclosingClass then "this."
-                      else s"${prevCtx.owner.name}.this.")
+                      if origCtx.owner == refctx.owner.enclosingClass then "this."
+                      else s"${origCtx.owner.name}.this.")
               else
-                checkNoOuterDefs(denot, outer, prevCtx)
+                checkNoOuterDefs(denot, outerCtx, origCtx)
 
           if isNewDefScope then
             val defDenot = ctx.denotNamed(name, required, excluded)

tests/neg/t12186.scala

@@ -0,0 +1,36 @@
+package object p extends p.U {
+  def b: Int = 0
+  trait Y
+}
+
+package p {
+  trait U {
+    def a: Int = 0
+    trait X
+  }
+
+  object c
+  def c1 = 0 // top-level def
+  trait Z
+  trait T {
+    def a = 1
+    def b = 1
+    def c = 1
+    def c1 = 1
+
+    trait X
+    trait Y
+    trait Z
+  }
+
+  trait RR extends T {
+    def m1 = a // ok
+    def m2 = b // ok
+    def m3 = c // error
+    def m4 = c1 // error
+
+    def n1: X // ok
+    def n2: Y // ok
+    def n3: Z // error
+  }
+}

tests/neg/t12186b/A.scala

@@ -0,0 +1,4 @@
+package object p extends p.U {
+  def b: Int = 0
+  trait Y
+}

tests/neg/t12186b/B.scala

@@ -0,0 +1,31 @@
+package p {
+  trait U {
+    def a: Int = 0
+    trait X
+  }
+
+  object c
+  def c1 = 0 // top-level def
+  trait Z
+  trait T {
+    def a = 1
+    def b = 1
+    def c = 1
+    def c1 = 1
+
+    trait X
+    trait Y
+    trait Z
+  }
+
+  trait RR extends T {
+    def m1 = a // ok
+    def m2 = b // ok
+    def m3 = c // error
+    def m4 = c1 // error
+
+    def n1: X // ok
+    def n2: Y // ok
+    def n3: Z // error
+  }
+}