Fix #5090: Fix condition in erasedResult type

Author: odersky

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

@@ -507,7 +507,7 @@ class TypeErasure(isJava: Boolean, semiEraseVCs: Boolean, isConstructor: Boolean
       // constructor method should not be semi-erased.
       else if (isConstructor && isDerivedValueClass(sym)) eraseNormalClassRef(tp)
       else this(tp)
-    case AppliedType(tycon, _) if !erasureDependsOnArgs(tycon) =>
+    case AppliedType(tycon, _) if tycon.typeSymbol.isClass && !erasureDependsOnArgs(tycon) =>
       eraseResult(tycon)
     case _ =>
       this(tp)

tests/pos/i5090.scala

@@ -0,0 +1,4 @@
+class TypeAlias {
+  type T[X] = X
+  def a(i: T[Int]): T[Int] = i
+}