Fix #2901: Make asClass cast only when sure it is a class

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

@@ -101,15 +101,15 @@ class SuperAccessors(thisTransformer: DenotTransformer) {
       val Select(sup @ Super(_, mix), name) = sel
       val sym   = sel.symbol
       assert(sup.symbol.exists, s"missing symbol in $sel: ${sup.tpe}")
-      val clazz = sup.symbol.asClass
+      val clazz = sup.symbol
 
       if (sym.isTerm && !sym.is(Method, butNot = Accessor) && !ctx.owner.is(ParamForwarder))
         // ParamForwaders as installed ParamForwarding.scala do use super calls to vals
         ctx.error(s"super may be not be used on ${sym.underlyingSymbol}", sel.pos)
       else if (isDisallowed(sym))
         ctx.error(s"super not allowed here: use this.${sel.name} instead", sel.pos)
       else if (sym is Deferred) {
-        val member = sym.overridingSymbol(clazz)
+        val member = sym.overridingSymbol(clazz.asClass)
         if (!mix.name.isEmpty ||
             !member.exists ||
             !((member is AbsOverride) && member.isIncompleteIn(clazz)))

tests/neg/i2901.scala

@@ -0,0 +1,9 @@
+trait Foo {
+  def foo = 4
+}
+object Bar extends Foo {
+  inline def bar = super[Foo].foo // error
+}
+object Main {
+  Bar.bar
+}