Avoid necessaryEither because it could constrain too much

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -823,7 +823,15 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         }
         val tp1norm = simplifyAndTypeWithFallback(tp11, tp12, tp1)
         if (tp1 ne tp1norm) recur(tp1norm, tp2)
-        else either(recur(tp11, tp2), recur(tp12, tp2))
+        else
+          if useNecessaryEither && !frozenConstraint then
+            if tp11 frozen_<:< tp12 then recur(tp11, tp2)
+            else if tp12 frozen_<:< tp11 then recur(tp12, tp2)
+            else false
+          else
+            val saved = constraint
+            either(recur(tp11, tp2), recur(tp12, tp2))
+              .showing(i"constraining andtype $tp11 & $tp12 <: $tp2 in $saved ----> $constraint", subtyping)
       case tp1: MatchType =>
         def compareMatch = tp2 match {
           case tp2: MatchType =>

tests/pos/i12306.scala

@@ -0,0 +1,22 @@
+class Record(elems: Map[String, Any]) extends Selectable:
+  val fields = elems.toMap
+  def selectDynamic(name: String): Any = fields(name)
+
+extension [A <: Record] (a:A) {
+  def join[B <: Record] (b:B): A & B = {
+    Record(a.fields ++ b.fields).asInstanceOf[A & B]
+  }
+}
+
+type Person = Record { val name: String; val age: Int }
+type Child = Record { val parent: String }
+type PersonAndChild = Record { val name: String; val age: Int; val parent: String }
+
+@main def hello = {
+  val person = Record(Map("name" -> "Emma", "age" -> 42)).asInstanceOf[Person]
+  val child = Record(Map("parent" -> "Alice")).asInstanceOf[Child]
+  //val personAndChild = person.join(child)
+
+  //val v1: PersonAndChild = personAndChild
+  val v2: PersonAndChild = person.join(child)
+}