SI-6186 TypeTags no longer supported in macros

The original idea was to support both both TypeTags and ConcreteTypeTags as
context bounds on macro implementations.

Back then TypeTags were the implied default flavor of type tags. Basically
because "TypeTag" is shorter than "ConcreteTypeTag" everyone jumped onto
them and used them everywhere.

That led to problems, because at that time TypeTags could reify unresolved type
parameters ("unresolved" = not having TypeTag annotations for them). This
led to a series of creepy errors, when one forgets to add a context bound
in the middle of a chain of methods that all pass a type tag around, and then
suddenly all the tags turn into pumpkins (because that unlucky method just
reifies TypeRef(NoPrefix, <type parameter symbol>, Nil and passes it down
the chain).

Hence we decided to rename ConcreteTypeTag => TypeTag & TypeTag => AbsTypeTag,
which makes a lot of sense from a reflection point of view.

Unfortunately this broke macros (in a sense), because now everyone writes
TypeTag context bounds on macro implementations, which breaks in trivial
situations like: "def foo[T](x: T) = identity_macro(x)" (the type of x
is not concrete, so macro expansion will emit an error when trying to
materialize the corresponding TypeTag).

Now we restore the broken balance by banning TypeTag from macro impls.
This forces anyone to use AbsTypeTags, and if someone wants to check the input
for presence of abstract types, it's possible to do that manually.

1 files changed, 4 insertions, 8 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Macros.scala b/src/compiler/scala/tools/nsc/typechecker/Macros.scala
index 1381450970..7c5d458fee 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Macros.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Macros.scala
@@ -23,7 +23,7 @@ import java.lang.reflect.{Array => jArray, Method => jMethod}
  *
  *  Then fooBar needs to point to a static method of the following form:
  *
- *    def fooBar[T: c.TypeTag]
+ *    def fooBar[T: c.AbsTypeTag]
  *           (c: scala.reflect.macros.Context)
  *           (xs: c.Expr[List[T]])
  *           : c.Expr[T] = {
@@ -156,7 +156,7 @@ trait Macros extends scala.tools.reflect.FastTrack with Traces {
       case TypeRef(SingleType(NoPrefix, contextParam), sym, List(tparam)) =>
         var wannabe = sym
         while (wannabe.isAliasType) wannabe = wannabe.info.typeSymbol
-        if (wannabe != definitions.AbsTypeTagClass && wannabe != definitions.TypeTagClass)
+        if (wannabe != definitions.AbsTypeTagClass)
           List(param)
         else
           transform(param, tparam.typeSymbol) map (_ :: Nil) getOrElse Nil
@@ -202,7 +202,6 @@ trait Macros extends scala.tools.reflect.FastTrack with Traces {
       def abbreviateCoreAliases: String = { // hack!
         var result = s
         result = result.replace("c.universe.AbsTypeTag", "c.AbsTypeTag")
-        result = result.replace("c.universe.TypeTag", "c.TypeTag")
         result = result.replace("c.universe.Expr", "c.Expr")
         result
       }
@@ -440,7 +439,7 @@ trait Macros extends scala.tools.reflect.FastTrack with Traces {
       // we don't have to do this, but it appears to be more clear than allowing them
       val implicitParams = actparamss.flatten filter (_.isImplicit)
       if (implicitParams.length > 0) {
-        reportError(implicitParams.head.pos, "macro implementations cannot have implicit parameters other than TypeTag evidences")
+        reportError(implicitParams.head.pos, "macro implementations cannot have implicit parameters other than AbsTypeTag evidences")
         macroTraceVerbose("macro def failed to satisfy trivial preconditions: ")(macroDef)
       }
 
@@ -854,9 +853,6 @@ trait Macros extends scala.tools.reflect.FastTrack with Traces {
           param.tpe.typeSymbol match {
             case definitions.AbsTypeTagClass =>
               // do nothing
-            case definitions.TypeTagClass =>
-              if (!tpe.isConcrete) context.abort(context.enclosingPosition, "cannot create TypeTag from a type %s having unresolved type parameters".format(tpe))
-              // otherwise do nothing
             case _ =>
               throw new Error("unsupported tpe: " + tpe)
           }
@@ -1019,7 +1015,7 @@ trait Macros extends scala.tools.reflect.FastTrack with Traces {
       )
       val forgotten = (
         if (sym.isTerm) "splice when splicing this variable into a reifee"
-        else "c.TypeTag annotation for this type parameter"
+        else "c.AbsTypeTag annotation for this type parameter"
       )
       typer.context.error(expandee.pos,
         template.replaceAllLiterally("@kind@", sym.name.nameKind).format(