Erase arguments of phantom type

This commit removes arguments and parameter of phantom type from all applications.
Arguments are evaluated before the application in case they have side effects.

```
def foo(i: Int, p: SomePhantom) = ???

foo(42, { println("hello"); getSomePhantom })
```

becomes

```
def foo(i: Int) = ???

val x$0 = 42
val x$1 = { println("hello"); getSomePhantom }
foo(x$0)
```

Note that now `def foo(i: Int)` and def `foo(i: Int, p: SomePhantom)` erase to the same signature.

Tests for this commit where added in `Extra tests for Phantoms 1`, they where back ported as the semantics and runtime is not affected.

Author: nicolasstucki

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

@@ -0,0 +1,105 @@
+package dotty.tools.dotc.transform
+
+import dotty.tools.dotc.ast.tpd
+import dotty.tools.dotc.core.Contexts._
+import dotty.tools.dotc.core.Flags._
+import dotty.tools.dotc.core.NameKinds._
+import dotty.tools.dotc.core.Types._
+import dotty.tools.dotc.transform.TreeTransforms.{MiniPhaseTransform, TransformerInfo}
+
+/** This phase extracts the arguments of phantom type before the application to avoid losing any
+ *  effects in the argument tree. This trivializes the removal of parameter in the Erasure phase.
+ *
+ *  `f(x1,...)(y1,...)...(...)` with at least one phantom argument
+ *
+ *    -->
+ *
+ *  `val ev$f = f` // if `f` is some expression that needs evaluation
+ *  `val ev$x1 = x1`
+  *  ...
+ *  `val ev$y1 = y1`
+  *  ...
+ *  `ev$f(ev$x1,...)(ev$y1,...)...(...)`
+ *
+ */
+class PhantomArgumentEval extends MiniPhaseTransform {
+  import tpd._
+
+  override def phaseName: String = "phantomArgumentEval"
+
+  /** Check what the phase achieves, to be called at any point after it is finished. */
+  override def checkPostCondition(tree: Tree)(implicit ctx: Context): Unit = tree match {
+    case tree: Apply =>
+      tree.args.foreach { arg =>
+        assert(
+          !arg.tpe.isPhantom ||
+          (arg.isInstanceOf[Ident] && !arg.symbol.is(Method) && !arg.symbol.is(Lazy))
+        )
+      }
+    case _ =>
+  }
+
+  /* Tree transform */
+
+  override def transformApply(tree: Apply)(implicit ctx: Context, info: TransformerInfo): Tree = tree.tpe match {
+    case _: MethodType => tree // Do the transformation higher in the tree if needed
+    case _ =>
+      def mkNewBlock(newApply: Tree, synthVals: List[ValDef]) = Block(synthVals, newApply)
+      if (!hasPhantomArgs(tree)) tree
+      else transformApplication(tree, mkNewBlock)
+  }
+
+  override def transformAssign(tree: Assign)(implicit ctx: Context, info: TransformerInfo): Tree = {
+    if (!tree.rhs.tpe.isPhantom) super.transformAssign(tree)
+    else {
+      // Apply the same transformation to setters before their creation.
+      val (synthVal, synthValRef) = mkSynthVal(tree.rhs)
+      Block(List(synthVal), Assign(tree.lhs, synthValRef))
+    }
+  }
+
+  /* private methods */
+
+  /** Returns true if at least on of the arguments is a phantom.
+   *  Inner applies are also checked in case of multiple parameter list.
+   */
+  private def hasPhantomArgs(tree: Apply)(implicit ctx: Context): Boolean = {
+    tree.args.exists {
+      case arg: Ident if !arg.symbol.is(Method) && !arg.symbol.is(Lazy) => false
+      case arg => arg.tpe.isPhantom
+    } || {
+      tree.fun match {
+        case fun: Apply => hasPhantomArgs(fun)
+        case _ => false
+      }
+    }
+  }
+
+  /** Collects all args (and possibly the function) as synthetic vals and replaces them in the tree by the reference to
+   *  the lifted its val.
+   */
+  private def transformApplication(tree: Tree, mkTree: (Tree, List[ValDef]) => Tree)(implicit ctx: Context): Tree = tree match {
+    case tree: Apply =>
+      def mkNewApply(newFun: Tree, accSynthVals: List[ValDef]) = {
+        val (synthVals, synthValRefs) = tree.args.map(mkSynthVal).unzip
+        mkTree(cpy.Apply(tree)(newFun, synthValRefs), accSynthVals ::: synthVals)
+      }
+      transformApplication(tree.fun, mkNewApply)
+    case tree: TypeApply =>
+      def mkNewTypeApply(newFun: Tree, accSynthVals: List[ValDef]) =
+        mkTree(cpy.TypeApply(tree)(newFun, tree.args), accSynthVals)
+      transformApplication(tree.fun, mkNewTypeApply)
+    case tree: Select if !tree.qualifier.isInstanceOf[New] =>
+      val (accSynthVal, synthValRef) = mkSynthVal(tree.qualifier)
+      mkTree(Select(synthValRef, tree.name), List(accSynthVal))
+    case _ => mkTree(tree, Nil)
+  }
+
+  /** Makes a synthetic val with the tree and creates a reference to it.
+   *    `tree` --> (`val ev$x = tree`, `ev$x`)
+   */
+  private def mkSynthVal(tree: Tree)(implicit ctx: Context): (ValDef, Tree) = {
+    val synthVal = SyntheticValDef(TempResultName.fresh(), tree)
+    (synthVal, ref(synthVal.symbol))
+  }
+}