Detect case where two alternatives are the same after widening ExprTypes (#18787)

In implicit or extension method search we might get two alternatives
that are different but that point to the same singleton type after
widening ExprTypes. In that case we can arbitrarily pick one of them
instead of declaring an ambiguity.

Fixes #18768

Author: odersky

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

@@ -1780,14 +1780,20 @@ trait Applications extends Compatibility {
       def winsType2 = isAsSpecific(alt2, tp2, alt1, tp1)
 
       overload.println(i"compare($alt1, $alt2)? $tp1 $tp2 $ownerScore $winsType1 $winsType2")
-      if (ownerScore == 1)
-        if (winsType1 || !winsType2) 1 else 0
-      else if (ownerScore == -1)
-        if (winsType2 || !winsType1) -1 else 0
-      else if (winsType1)
-        if (winsType2) 0 else 1
+      if winsType1 && winsType2
+          && alt1.widenExpr.isStable && (alt1.widenExpr frozen_=:= alt2.widenExpr)
+      then
+        // alternatives are the same after following ExprTypes, pick one of them
+        // (prefer the one that is not a method, but that's arbitrary).
+        if alt1.widenExpr =:= alt2 then -1 else 1
+      else if ownerScore == 1 then
+        if winsType1 || !winsType2 then 1 else 0
+      else if ownerScore == -1 then
+        if winsType2 || !winsType1 then -1 else 0
+      else if winsType1 then
+        if winsType2 then 0 else 1
       else
-        if (winsType2) -1 else 0
+        if winsType2 then -1 else 0
     }
 
     if alt1.symbol.is(ConstructorProxy) && !alt2.symbol.is(ConstructorProxy) then -1

tests/pos/i18768.scala

@@ -0,0 +1,44 @@
+package minimized:
+  object Module:
+    object Exportee:
+
+      opaque type Id = Long
+
+      def apply(): Id = ???
+
+      extension (e: Id)
+        def updated: Id = ???
+
+
+  object Client:
+    export Module.*
+    val x = Exportee().updated
+
+package original:
+  object Module:
+    trait EntityDef:
+      type Id
+      type Record
+      type Entity = (Id, Record)
+
+      extension (e: Entity)
+        def updated: Entity = e
+
+    case class Exportee()
+    object Exportee extends EntityDef:
+      opaque type Id = Long
+      type Record = Exportee
+
+      def apply(id: Long): Entity = (id, Exportee())
+
+  object Client:
+    export Module.*
+    val x = Exportee(1L).updated
+
+
+  object ClientWorkingWithManualExport:
+    export Module.{Exportee as _, *}
+    type Exportee = Module.Exportee
+    val Exportee = Module.Exportee
+
+    val x = Exportee(1L).updated