Dealias arg type to detect opaque types

Fixes #14653

Author: nicolasstucki

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

@@ -677,7 +677,7 @@ class Inliner(call: tpd.Tree, rhsToInline: tpd.Tree)(using Context) {
    *  map them to their opaque proxies.
    */
   def mapOpaquesInValueArg(arg: Tree)(using Context): Tree =
-    val argType = arg.tpe.widen
+    val argType = arg.tpe.widenDealias
     addOpaqueProxies(argType, arg.span, forThisProxy = false)
     if opaqueProxies.nonEmpty then
       val mappedType = mapOpaques.typeMap(argType)

tests/neg/i14653.scala renamed to tests/pos/i14653.scala

@@ -11,8 +11,8 @@ object Amount:
   val aa: Amount = Amount(1)
   val ab: Amount = Amount(2)
   val ac: Amount = Amount(2)
-  val as1: Amount = aa + ab  // error
-  val as2: Amount = aa + ab + ac // error
+  val as1: Amount = aa + ab
+  val as2: Amount = aa + ab + ac
 
   println(s"aa + ab = ${as1}")
   println(s"aa + ab = ${as2}")

tests/pos/i14653b.scala

@@ -0,0 +1,12 @@
+type Amount = Amount.Type
+object Amount:
+  opaque type Type = Int
+  inline def twice(x: Type): Type = x + x
+
+def a: Amount = ???
+def b: Amount.Type = ???
+def test(c: Amount, d: Amount.Type): Unit =
+   Amount.twice(a)
+   Amount.twice(b)
+   Amount.twice(c)
+   Amount.twice(d)