SI-7470 implements fundep materialization

This fix provides implicit macros with an ability to affect type inference
in a more or less sane manner. That's crucial for materialization of
multi-parametric type class instances (e.g. Iso's from shapeless).
Details of the technique can be found in comments.

2 files changed, 51 insertions, 0 deletions

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)
+  }
+}
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