Fix #5006: Enforce asInstanceOf to be a Select node

Author: nicolasstucki

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

@@ -31,6 +31,8 @@ object FirstTransform {
  *   - drops branches of ifs using the rules
  *          if (true) A else B    ==> A
  *          if (false) A else B   ==> B
+ *   - Notmalize asInsanceOf to always be a selection
+ *          asInsanceOf[X] ==> this.asInsanceOf[X]
  */
 class FirstTransform extends MiniPhase with InfoTransformer { thisPhase =>
   import ast.tpd._
@@ -54,6 +56,8 @@ class FirstTransform extends MiniPhase with InfoTransformer { thisPhase =>
           qual.tpe.derivesFrom(tree.symbol.owner) ||
             tree.symbol.is(JavaStatic) && qual.tpe.derivesFrom(tree.symbol.enclosingClass),
           i"non member selection of ${tree.symbol.showLocated} from ${qual.tpe} in $tree")
+      case _: Ident =>
+        assert(tree.symbol != defn.Any_asInstanceOf)
       case _: TypeTree =>
       case _: Import | _: NamedArg | _: TypTree =>
         assert(false, i"illegal tree: $tree")
@@ -143,8 +147,15 @@ class FirstTransform extends MiniPhase with InfoTransformer { thisPhase =>
     case tree => if (tree.isType) toTypeTree(tree) else tree
   }
 
-  override def transformIdent(tree: Ident)(implicit ctx: Context): Tree =
-    if (tree.isType) toTypeTree(tree) else constToLiteral(tree)
+  override def transformIdent(tree: Ident)(implicit ctx: Context): Tree = {
+    if (tree.symbol == defn.Any_asInstanceOf) {
+      // Add missing `this` prefix
+      val TermRef(ths: ThisType, _) = tree.tpe
+      ref(ths.termSymbol).select(tree.name).withSpan(tree.span)
+    }
+    else if (tree.isType) toTypeTree(tree)
+    else constToLiteral(tree)
+  }
 
   override def transformSelect(tree: Select)(implicit ctx: Context): Tree =
     if (tree.isType) toTypeTree(tree) else constToLiteral(tree)

tests/pos/i5006.scala

@@ -0,0 +1,11 @@
+object i0 {
+  def f: Int = asInstanceOf[Int].toInt
+}
+
+class i2 {
+  def f: Int = asInstanceOf[Int].toInt
+}
+
+trait i3 {
+  def f: Int = asInstanceOf[Int].toInt
+}