Improve source check of conflicting definition

An inherited member cannot shadow a definition in scope
that was defined in the current compilation unit.
Aliases are given a pass of sorts.
Either the member or the definition may be overloaded,
which complicates both detection and mitigation.
Tests are for the status quo.

Author: som-snytt

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

@@ -439,8 +439,8 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                 if owner.is(Package) then
                   owner.denot.asClass.membersNamed(name)
                     .filterWithPredicate(d => !d.symbol.is(Package)
-                      && denot.symbol.source.exists
-                      && d.symbol.source == denot.symbol.source)
+                      && d.symbol.source.exists
+                      && isDefinedInCurrentUnit(d))
                 else
                   val scope = if owner.isClass then owner.info.decls else outer.scope
                   scope.denotsNamed(name)
@@ -481,7 +481,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                 result = checkNewOrShadowed(found, Definition) // no need to go further out, we found highest prec entry
                 found match
                   case found: NamedType
-                  if curOwner.isClass && found.denot.exists && isInherited(found.denot) && !ctx.compilationUnit.isJava =>
+                  if curOwner.isClass && isInherited(found.denot) && !ctx.compilationUnit.isJava =>
                     checkNoOuterDefs(found.denot, ctx, ctx)
                   case _ =>
               else

tests/neg/i17433.check

@@ -0,0 +1,8 @@
+-- [E049] Reference Error: tests/neg/i17433.scala:9:10 -----------------------------------------------------------------
+9 |  def g = f(42) // error
+  |          ^
+  |          Reference to f is ambiguous.
+  |          It is both defined in package <empty>
+  |          and inherited subsequently in class D
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg/i17433.scala

@@ -0,0 +1,12 @@
+
+class C:
+  def f(i: Int) = i + 1
+  def f(s: String) = s + "_1"
+
+def f = 42
+
+class D extends C:
+  def g = f(42) // error
+
+@main def test() = println:
+  D().g

tests/neg/i17433c.scala

@@ -0,0 +1,12 @@
+
+package p:
+  trait T:
+    def t(i: Int) = i + 1
+    def t(s: String) = s + "_1"
+
+  package object q extends T
+
+  package q:
+
+    class C extends T:
+      def c = t(42) // error

tests/neg/i17433d.scala

@@ -0,0 +1,13 @@
+
+package p:
+  trait T:
+    def t(i: Int) = i + 1
+
+  package object q extends T:
+    override def t(i: Int) = i + 2
+    def t(s: String) = s + "_2"
+
+  package q:
+
+    class C extends T:
+      def c = t(42) // error

tests/pos/17433/A_1.scala tests/pos/i17433a/A_1.scalatests/pos/17433/A_1.scala renamed to tests/pos/i17433a/A_1.scala

@@ -0,0 +1,7 @@
+
+package p:
+  trait T:
+    def t(i: Int) = i + 1
+    def t(s: String) = s + "_1"
+
+  package object q extends T

tests/pos/17433/B_2.scala tests/pos/i17433a/B_2.scalatests/pos/17433/B_2.scala renamed to tests/pos/i17433a/B_2.scala

@@ -0,0 +1,6 @@
+
+package p
+package q
+
+  class C extends T:
+    def c = t(42)

tests/pos/i17433b/p.scala

@@ -0,0 +1,12 @@
+
+package p:
+  trait T:
+    def t(i: Int) = i + 1
+    //def t(s: String) = s + "_1" // it won't try to reconcile the overloaded member
+
+  package object q extends T
+
+  package q:
+
+    class C extends T:
+      def c = t(42) // should error because this.t is not q.t, but is currently reconciled