Merge pull request scala#2094 from scalamacros/ticket/6591

 SI-6591 Reify and path-dependent types

Author: JamesIry

src/compiler/scala/reflect/reify/codegen/GenSymbols.scala

@@ -1,6 +1,8 @@
 package scala.reflect.reify
 package codegen
 
+import scala.reflect.internal.Flags._
+
 trait GenSymbols {
   self: Reifier =>
 
@@ -100,6 +102,33 @@ trait GenSymbols {
     reifyIntoSymtab(binding.symbol) { sym =>
       if (reifyDebug) println("Free term" + (if (sym.isCapturedVariable) " (captured)" else "") + ": " + sym + "(" + sym.accurateKindString + ")")
       val name = newTermName(nme.REIFY_FREE_PREFIX + sym.name + (if (sym.isType) nme.REIFY_FREE_THIS_SUFFIX else ""))
+      // We need to note whether the free value being reified is stable or not to guide subsequent reflective compilation.
+      // Here's why reflection compilation needs our help.
+      //
+      // When dealing with a tree, which contain free values, toolboxes extract those and wrap the entire tree in a Function
+      // having parameters defined for every free values in the tree. For example, evaluating
+      //
+      //   Ident(setTypeSignature(newFreeTerm("x", 2), <Int>))
+      //
+      // Will generate something like
+      //
+      //   object wrapper {
+      //     def wrapper(x: () => Int) = {
+      //       x()
+      //     }
+      //   }
+      //
+      // Note that free values get transformed into, effectively, by-name parameters. This is done to make sure
+      // that evaluation order is kept intact. And indeed, we cannot just evaluate all free values at once in order
+      // to obtain arguments for wrapper.wrapper, because if some of the free values end up being unused during evaluation,
+      // we might end up doing unnecessary calculations.
+      //
+      // So far, so good - we didn't need any flags at all. However, if the code being reified contains path-dependent types,
+      // we're in trouble, because valid code like `free.T` ends up being transformed into `free.apply().T`, which won't compile.
+      //
+      // To overcome this glitch, we note whether a given free term is stable or not (because vars can also end up being free terms).
+      // Then, if a free term is stable, we tell the compiler to treat `free.apply()` specially and assume that it's stable.
+      if (!sym.isMutable) sym setFlag STABLE
       if (sym.isCapturedVariable) {
         assert(binding.isInstanceOf[Ident], showRaw(binding))
         val capturedBinding = referenceCapturedVariable(sym)

src/compiler/scala/reflect/reify/codegen/GenTrees.scala

@@ -153,21 +153,23 @@ trait GenTrees {
         else mirrorCall(nme.Ident, reify(name))
 
       case Select(qual, name) =>
-        if (sym == NoSymbol || sym.name == name)
-          reifyProduct(tree)
-        else
-          reifyProduct(Select(qual, sym.name))
+        if (qual.symbol != null && qual.symbol.isPackage) {
+          mirrorBuildCall(nme.Ident, reify(sym))
+        } else {
+          val effectiveName = if (sym != null && sym != NoSymbol) sym.name else name
+          reifyProduct(Select(qual, effectiveName))
+        }
 
       case _ =>
         throw new Error("internal error: %s (%s, %s) is not supported".format(tree, tree.productPrefix, tree.getClass))
     }
   }
 
-  private def reifyBoundType(tree: Tree): Tree = {
+  private def reifyBoundType(tree: RefTree): Tree = {
     val sym = tree.symbol
     val tpe = tree.tpe
 
-    def reifyBoundType(tree: Tree): Tree = {
+    def reifyBoundType(tree: RefTree): Tree = {
       assert(tpe != null, "unexpected: bound type that doesn't have a tpe: " + showRaw(tree))
 
       // if a symbol or a type of the scrutinee are local to reifee
@@ -196,13 +198,19 @@ trait GenTrees {
               mirrorBuildCall(nme.TypeTree, spliced)
           }
         }
-        else if (sym.isLocatable) {
-          if (reifyDebug) println("tpe is locatable: reify as Ident(%s)".format(sym))
-          mirrorBuildCall(nme.Ident, reify(sym))
-        }
-        else {
-          if (reifyDebug) println("tpe is not locatable: reify as TypeTree(%s)".format(tpe))
-          mirrorBuildCall(nme.TypeTree, reify(tpe))
+        else tree match {
+          case Select(qual, name) if !qual.symbol.isPackage =>
+            if (reifyDebug) println(s"reifying Select($qual, $name)")
+            mirrorCall(nme.Select, reify(qual), reify(name))
+          case SelectFromTypeTree(qual, name) =>
+            if (reifyDebug) println(s"reifying SelectFromTypeTree($qual, $name)")
+            mirrorCall(nme.SelectFromTypeTree, reify(qual), reify(name))
+          case _ if sym.isLocatable =>
+            if (reifyDebug) println(s"tpe is locatable: reify as Ident($sym)")
+            mirrorBuildCall(nme.Ident, reify(sym))
+          case _ =>
+            if (reifyDebug) println(s"tpe is not locatable: reify as TypeTree($tpe)")
+            mirrorBuildCall(nme.TypeTree, reify(tpe))
         }
       }
     }

src/compiler/scala/reflect/reify/utils/Extractors.scala

@@ -263,12 +263,12 @@ trait Extractors {
   }
 
   object BoundType {
-    def unapply(tree: Tree): Option[Tree] = tree match {
-      case Select(_, name) if name.isTypeName =>
+    def unapply(tree: Tree): Option[RefTree] = tree match {
+      case tree @ Select(_, name) if name.isTypeName =>
         Some(tree)
-      case SelectFromTypeTree(_, name) if name.isTypeName =>
+      case tree @ SelectFromTypeTree(_, _) =>
         Some(tree)
-      case Ident(name) if name.isTypeName =>
+      case tree @ Ident(name) if name.isTypeName =>
         Some(tree)
       case _ =>
         None

src/compiler/scala/tools/reflect/ToolBoxFactory.scala

@@ -8,7 +8,7 @@ import scala.tools.nsc.typechecker.Modes
 import scala.tools.nsc.io.VirtualDirectory
 import scala.tools.nsc.interpreter.AbstractFileClassLoader
 import scala.tools.nsc.util.FreshNameCreator
-import scala.reflect.internal.Flags
+import scala.reflect.internal.Flags._
 import scala.reflect.internal.util.{BatchSourceFile, NoSourceFile, NoFile}
 import java.lang.{Class => jClass}
 import scala.compat.Platform.EOL
@@ -211,8 +211,9 @@ abstract class ToolBoxFactory[U <: JavaUniverse](val u: U) { factorySelf =>
 
           val meth = obj.moduleClass.newMethod(newTermName(wrapperMethodName))
           def makeParam(schema: (FreeTermSymbol, TermName)) = {
+            // see a detailed explanation of the STABLE trick in `GenSymbols.reifyFreeTerm`
             val (fv, name) = schema
-            meth.newValueParameter(name) setInfo appliedType(definitions.FunctionClass(0).tpe, List(fv.tpe.resultType))
+            meth.newValueParameter(name, newFlags = if (fv.hasStableFlag) STABLE else 0) setInfo appliedType(definitions.FunctionClass(0).tpe, List(fv.tpe.resultType))
           }
           meth setInfo MethodType(freeTerms.map(makeParam).toList, AnyClass.tpe)
           minfo.decls enter meth
@@ -418,4 +419,3 @@ abstract class ToolBoxFactory[U <: JavaUniverse](val u: U) { factorySelf =>
     def eval(tree: u.Tree): Any = compile(tree)()
   }
 }
-

src/reflect/scala/reflect/internal/Flags.scala

@@ -274,7 +274,7 @@ class Flags extends ModifierFlags {
    *  from Modifiers.  Others which may be applied at creation time are:
    *  SYNTHETIC.
    */
-  final val ValueParameterFlags = BYNAMEPARAM | IMPLICIT | DEFAULTPARAM
+  final val ValueParameterFlags = BYNAMEPARAM | IMPLICIT | DEFAULTPARAM | STABLE
   final val BeanPropertyFlags   = DEFERRED | OVERRIDE | STATIC
   final val VarianceFlags       = COVARIANT | CONTRAVARIANT
 

src/reflect/scala/reflect/internal/StdNames.scala

@@ -607,6 +607,7 @@ trait StdNames {
     val RootPackage: NameType          = "RootPackage"
     val RootClass: NameType            = "RootClass"
     val Select: NameType               = "Select"
+    val SelectFromTypeTree: NameType   = "SelectFromTypeTree"
     val StringContext: NameType        = "StringContext"
     val This: NameType                 = "This"
     val ThisType: NameType             = "ThisType"

src/reflect/scala/reflect/internal/TreeInfo.scala

@@ -89,12 +89,15 @@ abstract class TreeInfo {
       tree.symbol.isStable && isExprSafeToInline(qual)
     case TypeApply(fn, _) =>
       isExprSafeToInline(fn)
+    case Apply(Select(free @ Ident(_), nme.apply), _) if free.symbol.name endsWith nme.REIFY_FREE_VALUE_SUFFIX =>
+      // see a detailed explanation of this trick in `GenSymbols.reifyFreeTerm`
+      free.symbol.hasStableFlag && isExprSafeToInline(free)
     case Apply(fn, List()) =>
-      /* Note: After uncurry, field accesses are represented as Apply(getter, Nil),
-       * so an Apply can also be pure.
-       * However, before typing, applications of nullary functional values are also
-       * Apply(function, Nil) trees. To prevent them from being treated as pure,
-       * we check that the callee is a method. */
+      // Note: After uncurry, field accesses are represented as Apply(getter, Nil),
+      // so an Apply can also be pure.
+      // However, before typing, applications of nullary functional values are also
+      // Apply(function, Nil) trees. To prevent them from being treated as pure,
+      // we check that the callee is a method.
       fn.symbol.isMethod && !fn.symbol.isLazy && isExprSafeToInline(fn)
     case Typed(expr, _) =>
       isExprSafeToInline(expr)

test/files/run/freetypes_false_alarm1.check

@@ -1 +1 @@
-List[Int]
+scala.List[Int]

test/files/run/idempotency-case-classes.check

@@ -50,6 +50,6 @@ C(2,3)
       Some.apply[(Int, Int)](Tuple2.apply[Int, Int](x$0.x, x$0.y));
     <synthetic> private def readResolve(): Object = C
   };
-  scala.this.Predef.println(C.apply(2, 3))
+  Predef.println(C.apply(2, 3))
 }
 error!

test/files/run/idempotency-lazy-vals.check

@@ -18,6 +18,6 @@
   };
   val c: C = new C();
   import c._;
-  c.x.*(scala.this.Predef.implicitly[Int](c.y))
+  c.x.*(Predef.implicitly[Int](c.y))
 }
 error!