tryCompiletimeConstantFold in disjointnessBoundary (#20168)

Fixes #20166

Alternatively placing the tryCompiletimeConstantFold in
`AppliedType#superType` also seems to fix the issue and not break
anything. But I'm not very sure about the spec here either way.

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

@@ -2905,6 +2905,8 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         tp
       case tp: ConstantType =>
         tp
+      case tp: AppliedType if tp.tryCompiletimeConstantFold.exists =>
+        tp.tryCompiletimeConstantFold
       case tp: HKTypeLambda =>
         tp
       case tp: ParamRef =>

tests/pos/i20166.scala

@@ -0,0 +1,24 @@
+import scala.compiletime.ops.int.*
+
+// NOTE ops.int.S is documented as equivalent to MyS
+
+type MyS[X] = X match
+  case 0 => 1
+  case 1 => 2
+  case 2 => 3
+
+type M[I <: Int] = 4 match
+  case 1 - 1 => "0"
+  case MyS[I] => "2"
+  case S[I] => "2" // Not provablyDisjoint before changes
+  case 2 + I => "3"
+  case I + 3 => "4"
+
+val _: M[1] = "4"
+
+
+type M2[I <: Int, P] = I match
+  case P => "b"
+  case _ => "c"
+
+val _: M2[5, 2 + 3] = "b"