Avoid crash in erasure when reference cannot be emitted (#18056)

Fixes #17391

Author: odersky

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

@@ -764,7 +764,9 @@ object Erasure {
         val symIsPrimitive = sym.owner.isPrimitiveValueClass
 
         def originalQual: Type =
-          erasure(tree.qualifier.typeOpt.widen.finalResultType)
+          erasure(
+            inContext(preErasureCtx):
+              tree.qualifier.typeOpt.widen.finalResultType)
 
         if (qualIsPrimitive && !symIsPrimitive || qual.tpe.widenDealias.isErasedValueType)
           recur(box(qual))
@@ -869,7 +871,7 @@ object Erasure {
 
           app(fun1)
         case t =>
-          if ownArgs.isEmpty then fun1
+          if ownArgs.isEmpty || t.isError then fun1
           else throw new MatchError(i"tree $tree has unexpected type of function $fun/$fun1: $t, was $origFunType, args = $ownArgs")
     end typedApply
 

tests/pos/i17391/Bar.java

@@ -0,0 +1,7 @@
+package mypkg;
+
+class Base<E> {
+    public void retainAll(String x) {}
+}
+
+public class Bar<E> extends Base<E> {}

tests/pos/i17391/test.scala

@@ -0,0 +1,11 @@
+def test(): Unit =
+{
+  val x: Foo[mypkg.Bar[String]] = ???
+  val y: mypkg.Bar[String] = ???
+
+  y.retainAll("fd") // works
+  x.f.retainAll("fd"); // error
+
+}
+
+class Foo[T](val f: T)