Fix #4205: Select ClassTag directly for Unit, Any and AnyVal

Author: nicolasstucki

compiler/src/dotty/tools/dotc/typer/Implicits.scala

@@ -554,11 +554,15 @@ trait Implicits { self: Typer =>
             val etag = inferImplicitArg(defn.ClassTagType.appliedTo(elemTp), pos)
             if (etag.tpe.isError) EmptyTree else etag.select(nme.wrap)
           case tp if hasStableErasure(tp) && !defn.isBottomClass(tp.typeSymbol) =>
-            ref(defn.ClassTagModule)
-              .select(nme.apply)
-              .appliedToType(tp)
-              .appliedTo(clsOf(erasure(tp)))
-              .withPos(pos)
+            val sym = tp.typeSymbol
+            if (sym == defn.UnitClass || sym == defn.AnyClass || sym == defn.AnyValClass)
+              ref(defn.ClassTagModule).select(sym.name.toTermName).withPos(pos)
+            else
+              ref(defn.ClassTagModule)
+                .select(nme.apply)
+                .appliedToType(tp)
+                .appliedTo(clsOf(erasure(tp)))
+                .withPos(pos)
           case tp =>
             EmptyTree
         }

tests/run/i4205.check

@@ -0,0 +1,14 @@
+true
+true
+true
+true
+true
+true
+true
+true
+true
+true
+true
+true
+true
+true

tests/run/i4205.scala

@@ -0,0 +1,22 @@
+import scala.reflect.ClassTag
+import scala.runtime.BoxedUnit
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    println(implicitly[ClassTag[Unit]] == ClassTag.Unit)
+    println(implicitly[ClassTag[Boolean]] == ClassTag.Boolean)
+    println(implicitly[ClassTag[Byte]] == ClassTag.Byte)
+    println(implicitly[ClassTag[Char]] == ClassTag.Char)
+    println(implicitly[ClassTag[Short]] == ClassTag.Short)
+    println(implicitly[ClassTag[Int]] == ClassTag.Int)
+    println(implicitly[ClassTag[Long]] == ClassTag.Long)
+    println(implicitly[ClassTag[Float]] == ClassTag.Float)
+    println(implicitly[ClassTag[Double]] == ClassTag.Double)
+    println(implicitly[ClassTag[Object]] == ClassTag.Object)
+    println(implicitly[ClassTag[Any]] == ClassTag.Any)
+    println(implicitly[ClassTag[AnyRef]] == ClassTag.AnyRef)
+    println(implicitly[ClassTag[AnyVal]] == ClassTag.AnyVal)
+
+    println(implicitly[ClassTag[BoxedUnit]] != ClassTag.Unit)
+  }
+}