Dealias for unused param check

Author: som-snytt

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

@@ -685,7 +685,7 @@ object SymDenotations {
       isAbstractOrAliasType && !isAbstractOrParamType
 
     /** Is this symbol an abstract or alias type? */
-    final def isAbstractOrAliasType: Boolean = isType & !isClass
+    final def isAbstractOrAliasType: Boolean = isType && !isClass
 
     /** Is this symbol an abstract type or type parameter? */
     final def isAbstractOrParamType(using Context): Boolean = this.isOneOf(DeferredOrTypeParam)

compiler/src/dotty/tools/dotc/transform/CheckUnused.scala

@@ -575,14 +575,14 @@ object CheckUnused:
         || m.hasAnnotation(dd.UnusedAnnot)          // param of unused method
         || sym.name.is(ContextFunctionParamName)    // a ubiquitous parameter
         || sym.isCanEqual
-        || sym.info.typeSymbol.match                // more ubiquity
+        || sym.info.dealias.typeSymbol.match        // more ubiquity
            case dd.DummyImplicitClass | dd.SubTypeClass | dd.SameTypeClass => true
            case tps =>
              tps.isMarkerTrait // no members to use; was only if sym.name.is(ContextBoundParamName)
              ||                // but consider NotGiven
              tps.hasAnnotation(dd.LanguageFeatureMetaAnnot)
         || sym.info.isSingleton // DSL friendly
-        || sym.info.isInstanceOf[RefinedType] // can't be expressed as a context bound
+        || sym.info.dealias.isInstanceOf[RefinedType] // can't be expressed as a context bound
       if ctx.settings.WunusedHas.implicits
         && !infos.skip(m)
         && !m.isEffectivelyOverride

tests/warn/i23250.scala

@@ -0,0 +1,17 @@
+//> using options -Wunused:all -Werror
+
+trait MonadError[F[_], E]
+type MonadThrow[F[_]] = MonadError[F, Throwable]
+
+trait MetaStreams[F[_]]:
+  def use[A]: F[A] = ???
+trait WriteResult
+
+trait MetaStreamsSyntax:
+  extension [F[_]](ms: MetaStreams[F])(using MonadThrow[F])
+    def setMaxAge(): F[WriteResult] =
+      summon[MonadThrow[F]]
+      ms.use[WriteResult]
+
+    def setTruncateBefore(): F[WriteResult] =
+      ms.use[WriteResult]