Simplify merging in lub/glb, avoid unnecessary constraints

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

@@ -1983,8 +1983,7 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] w
   /** Merge `t1` into `tp2` if t1 is a subtype of some &-summand of tp2.
    */
   private def mergeIfSub(tp1: Type, tp2: Type): Type =
-    if (isSubTypeWhenFrozen(tp1, tp2))
-      if (isSubTypeWhenFrozen(tp2, tp1)) tp2 else tp1 // keep existing type if possible
+    if (isSubTypeWhenFrozen(tp1, tp2)) tp1
     else tp2 match {
       case tp2 @ AndType(tp21, tp22) =>
         val lower1 = mergeIfSub(tp1, tp21)
@@ -2004,8 +2003,7 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] w
    *  @param canConstrain  If true, new constraints might be added to make the merge possible.
    */
   private def mergeIfSuper(tp1: Type, tp2: Type, canConstrain: Boolean): Type =
-    if (isSubType(tp2, tp1, whenFrozen = !canConstrain))
-      if (isSubType(tp1, tp2, whenFrozen = !canConstrain)) tp2 else tp1 // keep existing type if possible
+    if (isSubType(tp2, tp1, whenFrozen = !canConstrain)) tp1
     else tp2 match {
       case tp2 @ OrType(tp21, tp22) =>
         val higher1 = mergeIfSuper(tp1, tp21, canConstrain)

tests/neg/i4564.scala

@@ -42,5 +42,4 @@ class BaseCNSP[T] {
 }
 
 object ClashNoSigPoly extends BaseCNSP[Int]
-// TODO: improve error message
-case class ClashNoSigPoly private(x: Int)  // error: found: ClashNoSigPoly required: Nothing
+case class ClashNoSigPoly private(x: Int)

tests/pos/merge-constraint.scala

@@ -0,0 +1,11 @@
+class Hi
+class Lo extends Hi
+
+object Test {
+  def foo[T, U <: T](t: T, f: T => U): U = ???
+
+  def test(hi: Hi, lo: Lo): Unit = {
+    val ret = foo(hi, x => lo) // This used to infer U := Hi
+    val y: Lo = ret
+  }
+}