Merge pull request #2821 from xeno-by/topic/fundep-materialization

SI-7470 implements fundep materialization

7 files changed, 108 insertions, 2 deletions

diff --git a/test/files/run/t5923a/Macros_1.scala b/test/files/run/t5923a/Macros_1.scala
index 6d21362c4d..97076eb102 100644
--- a/test/files/run/t5923a/Macros_1.scala
+++ b/test/files/run/t5923a/Macros_1.scala
@@ -7,8 +7,46 @@ object C {
 }
 
 object Macros {
-  def impl[T: c.WeakTypeTag](c: Context) = {
+  def impl[T](c: Context)(ttag: c.WeakTypeTag[T]) = {
     import c.universe._
-    reify(C[T](c.literal(weakTypeOf[T].toString).splice))
+    val ttag0 = ttag;
+    {
+      // When we're expanding implicitly[C[Nothing]], the type inferencer will see
+      // that foo[T] returns C[T] and that we request an implicit of type C[Nothing].
+      //
+      // Then the type inferencer will try to match C[T] against C[Nothing] and infer everything it can infer
+      // from that match, but not more (e.g. if we were returning Iso[T, U] and the type we were looking at was Iso[Foo, L],
+      // we wouldn't want U to be auto-inferred to Nothing, as it usually happens with normal methods,
+      // but would rather want it to remain unknown, so that our macro could take a stab at inferring it:
+      // see the comments in this commit for more information).
+      //
+      // Equipped with common sense, in our case of C[T] and C[Nothing] we would expect T to be inferred as Nothing, and then we
+      // would expect T in the corresponding macro invocation to be Nothing. Unfortunately it is not that simple.
+      //
+      // Internally the type inferencer uses Nothing as a dummy value, which stands for "don't know how to
+      // infer this type parameter". In the Iso example, matching Iso[T, U] against Iso[Foo, L] would result in
+      // T being inferred as Foo and U being inferred as Nothing (!!). Then the type inferencer will think:
+      // "Aha! U ended up being Nothing. This means that I failed to infer it,
+      // therefore the result of my work is: T -> Foo, U -> still unknown".
+      //
+      // That's all very good and works very well until Nothing is a genuine result of type inference,
+      // as in our original example of inferring T in C[T] from C[Nothing]. In that case, the inferencer becomes confused
+      // and here in the macro implementation we get weakTypeOf[T] equal to some dummy type carrying a type parameter
+      // instead of Nothing.
+      //
+      // This eccentric behavior of the type inferencer is a long-standing problem in scalac,
+      // so the best one can do for now until it's fixed is to work around, manually converting
+      // suspicious T's into Nothings. Of course, this means that we would have to approximate,
+      // because there's no way to know whether having T here stands for a failed attempt to infer Nothing
+      // or for a failed attempt to infer anything, but at least we're in full control of making the best
+      // of this sad situation.
+      implicit def ttag: WeakTypeTag[T] = {
+        val tpe = ttag0.tpe
+        val sym = tpe.typeSymbol.asType
+        if (sym.isParameter && !sym.isSkolem) TypeTag.Nothing.asInstanceOf[TypeTag[T]]
+        else ttag0
+      }
+      reify(C[T](c.literal(weakTypeOf[T].toString).splice))
+    }
   }
 }
\ No newline at end of file
diff --git a/test/files/run/t5923c.check b/test/files/run/t5923c.check
new file mode 100644
index 0000000000..bed7429108
--- /dev/null
+++ b/test/files/run/t5923c.check
@@ -0,0 +1 @@
+(23,foo,true)
diff --git a/test/files/run/t5923c/Macros_1.scala b/test/files/run/t5923c/Macros_1.scala
new file mode 100644
index 0000000000..0b7a3399e2
--- /dev/null
+++ b/test/files/run/t5923c/Macros_1.scala
@@ -0,0 +1,39 @@
+import language.experimental.macros
+import scala.reflect.macros.Context
+
+trait Iso[T, U] {
+  def to(t : T) : U
+  // def from(u : U) : T
+}
+
+object Iso {
+  implicit def materializeIso[T, U]: Iso[T, U] = macro impl[T, U]
+  def impl[T: c.WeakTypeTag, U: c.WeakTypeTag](c: Context): c.Expr[Iso[T, U]] = {
+    import c.universe._
+    import definitions._
+    import Flag._
+
+    val sym = c.weakTypeOf[T].typeSymbol
+    if (!sym.isClass || !sym.asClass.isCaseClass) c.abort(c.enclosingPosition, s"$sym is not a case class")
+    val fields = sym.typeSignature.declarations.toList.collect{ case x: TermSymbol if x.isVal && x.isCaseAccessor => x }
+
+    def mkTpt() = {
+      val core = Ident(TupleClass(fields.length) orElse UnitClass)
+      if (fields.length == 0) core
+      else AppliedTypeTree(core, fields map (f => TypeTree(f.typeSignature)))
+    }
+
+    def mkFrom() = {
+      if (fields.length == 0) Literal(Constant(Unit))
+      else Apply(Ident(newTermName("Tuple" + fields.length)), fields map (f => Select(Ident(newTermName("f")), newTermName(f.name.toString.trim))))
+    }
+
+    val evidenceClass = ClassDef(Modifiers(FINAL), newTypeName("$anon"), List(), Template(
+      List(AppliedTypeTree(Ident(newTypeName("Iso")), List(Ident(sym), mkTpt()))),
+      emptyValDef,
+      List(
+        DefDef(Modifiers(), nme.CONSTRUCTOR, List(), List(List()), TypeTree(), Block(List(Apply(Select(Super(This(tpnme.EMPTY), tpnme.EMPTY), nme.CONSTRUCTOR), List())), Literal(Constant(())))),
+        DefDef(Modifiers(), newTermName("to"), List(), List(List(ValDef(Modifiers(PARAM), newTermName("f"), Ident(sym), EmptyTree))), TypeTree(), mkFrom()))))
+    c.Expr[Iso[T, U]](Block(List(evidenceClass), Apply(Select(New(Ident(newTypeName("$anon"))), nme.CONSTRUCTOR), List())))
+  }
+}
diff --git a/test/files/run/t5923c/Test_2.scala b/test/files/run/t5923c/Test_2.scala
new file mode 100644
index 0000000000..a00f4ed7db
--- /dev/null
+++ b/test/files/run/t5923c/Test_2.scala
@@ -0,0 +1,12 @@
+// see the comments for macroExpandApply.onDelayed for an explanation of what's tested here
+object Test extends App {
+  case class Foo(i: Int, s: String, b: Boolean)
+  def foo[C, L](c: C)(implicit iso: Iso[C, L]): L = iso.to(c)
+
+  {
+    val equiv = foo(Foo(23, "foo", true))
+    def typed[T](t: => T) {}
+    typed[(Int, String, Boolean)](equiv)
+    println(equiv)
+  }
+}
\ No newline at end of file
diff --git a/test/files/run/t5923d.check b/test/files/run/t5923d.check
new file mode 100644
index 0000000000..e69de29bb2
--- /dev/null
+++ b/test/files/run/t5923d.check
diff --git a/test/files/run/t5923d/Macros_1.scala b/test/files/run/t5923d/Macros_1.scala
new file mode 100644
index 0000000000..f32d1af704
--- /dev/null
+++ b/test/files/run/t5923d/Macros_1.scala
@@ -0,0 +1,9 @@
+import scala.language.experimental.macros
+import scala.reflect.macros.Context
+
+trait MappedRow
+trait RowMapper[T <: MappedRow]
+object RowMapper {
+  implicit def mapper[T <: MappedRow]: RowMapper[T] = macro impl[T]
+  def impl[T <: MappedRow : c.WeakTypeTag](c: Context) = c.universe.reify(new RowMapper[T]{})
+}
\ No newline at end of file
diff --git a/test/files/run/t5923d/Test_2.scala b/test/files/run/t5923d/Test_2.scala
new file mode 100644
index 0000000000..6be10227c2
--- /dev/null
+++ b/test/files/run/t5923d/Test_2.scala
@@ -0,0 +1,7 @@
+class RowA extends MappedRow
+class RowB extends MappedRow
+
+object Test extends App {
+  implicitly[RowMapper[RowA]]
+  implicitly[RowMapper[RowB]]
+}
\ No newline at end of file