Don't keep Unit type ascriptions in erasure

They lead to inefficient code:

    (x: Unit)

became

    (x; BoxedUnit: BoxedUnit)

Now it's just `x`.

Note: Such type ascriptions come up when inlining a unit-typed non-transparent
method. For instance, `assert(p, msg)`.

Author: odersky

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

@@ -588,16 +588,17 @@ object Erasure {
       tree.withType(erasure(tree.tpe))
 
     /** This override is only needed to semi-erase type ascriptions */
-    override def typedTyped(tree: untpd.Typed, pt: Type)(using Context): Tree = {
+    override def typedTyped(tree: untpd.Typed, pt: Type)(using Context): Tree =
       val Typed(expr, tpt) = tree
-      val tpt1 = tpt match {
-        case Block(_, tpt) => tpt // erase type aliases (statements) from type block
-        case tpt => tpt
-      }
-      val tpt2 = typedType(tpt1)
-      val expr1 = typed(expr, tpt2.tpe)
-      assignType(untpd.cpy.Typed(tree)(expr1, tpt2), tpt2)
-    }
+      if tpt.typeOpt.typeSymbol == defn.UnitClass then
+        typed(expr, defn.UnitType)
+      else
+        val tpt1 = tpt match
+          case Block(_, tpt) => tpt // erase type aliases (statements) from type block
+          case tpt => tpt
+        val tpt2 = typedType(tpt1)
+        val expr1 = typed(expr, tpt2.tpe)
+        assignType(untpd.cpy.Typed(tree)(expr1, tpt2), tpt2)
 
     override def typedLiteral(tree: untpd.Literal)(using Context): Tree =
       if (tree.typeOpt.isRef(defn.UnitClass))

library/src/dotty/DottyPredef.scala

@@ -3,15 +3,11 @@ package dotty
 object DottyPredef {
   import compiletime.summonFrom
 
-  inline final def assert(inline assertion: Boolean, inline message: => Any): Unit = {
-    if (!assertion)
-      scala.runtime.Scala3RunTime.assertFailed(message)
-  }
+  inline def assert(inline assertion: Boolean, inline message: => Any): Unit =
+    if !assertion then scala.runtime.Scala3RunTime.assertFailed(message)
 
-  transparent inline final def assert(inline assertion: Boolean): Unit = {
-    if (!assertion)
-      scala.runtime.Scala3RunTime.assertFailed()
-  }
+  inline def assert(inline assertion: Boolean): Unit =
+    if !assertion then scala.runtime.Scala3RunTime.assertFailed()
 
   /**
    * Retrieve the single value of a type with a unique inhabitant.