Generate opaque proxies also for base classes

Fixes #14656

Author: odersky

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

@@ -597,7 +597,7 @@ class Inliner(call: tpd.Tree, rhsToInline: tpd.Tree)(using Context) {
   def addOpaqueProxies(tp: Type, span: Span, forThisProxy: Boolean)(using Context): Unit =
     tp.foreachPart {
       case ref: TermRef =>
-        for cls <- ref.widen.classSymbols do
+        for cls <- ref.widen.baseClasses do
           if cls.containsOpaques
              && (forThisProxy || inlinedMethod.isContainedIn(cls))
              && mapRef(ref).isEmpty

tests/pos/i14656.scala

@@ -0,0 +1,12 @@
+
+trait BigDecimalNewType:
+  opaque type Type = BigDecimal
+  def apply(value: BigDecimal): Type = value
+  extension (self: Type)
+    def value: BigDecimal = self
+    inline def +(y: Type): Type = apply(self.value + y.value)
+
+object Amount extends BigDecimalNewType {
+  val x = Amount(0)
+  val y = (x + x) + x
+}