Warn universal extensions on opaque types

Author: rochala

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

@@ -1168,8 +1168,9 @@ object RefChecks {
         def strippedResultType = Applications.stripImplicit(tp.stripPoly, wildcardOnly = true).resultType
         def firstExplicitParamTypes = Applications.stripImplicit(tp.stripPoly, wildcardOnly = true).firstParamTypes
         def hasImplicitParams = tp.stripPoly match { case mt: MethodType => mt.isImplicitMethod case _ => false }
-      val target = sym.info.firstExplicitParamTypes.head // required for extension method, the putative receiver
+      val target = sym.info.firstExplicitParamTypes.head.typeSymbol.info // required for extension method, the putative receiver
       val methTp = sym.info.strippedResultType // skip leading implicits and the "receiver" parameter
+
       def hidden =
         target.nonPrivateMember(sym.name)
         .filterWithPredicate:
@@ -1190,7 +1191,7 @@ object RefChecks {
             }
           }
         .exists
-      if !target.typeSymbol.isOpaqueAlias && hidden
+      if hidden
       then report.warning(ExtensionNullifiedByMember(sym, target.typeSymbol), sym.srcPos)
   end checkExtensionMethods
 

tests/warn/i22232.scala

@@ -0,0 +1,19 @@
+class C
+object C:
+  extension (c: C) def equals(that: Any): Boolean = false // warn
+
+object X:
+  class C
+  opaque type D <: C = C
+  object D:
+    extension (d: D) def equals(that: Any): Boolean = false // warn
+
+object Upperbound:
+  opaque type IArray[+T] <: String = String
+  extension (arr: IArray[Byte]) def length: Int = 0 // warn
+  extension [T <: Int](arr: T) def length: Int = 0 // nowarn
+
+object NonUpperbound:
+  opaque type IArray[+T] = String
+  extension (arr: IArray[Byte]) def length: Int = 0 // nowarn
+