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Diffstat (limited to 'tests/disabled/macro/run/t5923a/Macros_1.scala')
| -rw-r--r-- | tests/disabled/macro/run/t5923a/Macros_1.scala | 53 |
1 files changed, 53 insertions, 0 deletions
diff --git a/tests/disabled/macro/run/t5923a/Macros_1.scala b/tests/disabled/macro/run/t5923a/Macros_1.scala new file mode 100644 index 000000000..9050fd4b1 --- /dev/null +++ b/tests/disabled/macro/run/t5923a/Macros_1.scala @@ -0,0 +1,53 @@ +import scala.reflect.macros.whitebox.Context +import language.experimental.macros + +case class C[T](t: String) +object C { + implicit def foo[T]: C[T] = macro Macros.impl[T] +} + +object Macros { + def impl[T](c: Context)(ttag: c.WeakTypeTag[T]) = { + import c.universe._ + import internal._ + 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 && !isSkolem(sym)) TypeTag.Nothing.asInstanceOf[TypeTag[T]] + else ttag0 + } + reify(C[T](c.Expr[String](Literal(Constant(weakTypeOf[T].toString))).splice)) + } + } +}
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