SI-7899 Allow by-name inference under -Yinfer-by-name

As usual, the hole has been exploited in the wild. While you
can't abstract over by-name-ness without running into the
ClassCastException or an un-applied Function0, there are cases
like the enclosed test where you can tiptoe around those
cases.

I've proposed a small change to Scalaz to avoid tripping over
this problem:

   https://github.com/scalaz/scalaz/pull/562/files

But this commit I've also added a transitional flag, -Yinfer-by-name,
that they could use to back-publish older versions without code
changes. It is also an insurance policy for other projects that
might be exploiting the same hole.

1 files changed, 5 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Infer.scala b/src/compiler/scala/tools/nsc/typechecker/Infer.scala
index ccf4a5e46f..dd0923a696 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Infer.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Infer.scala
@@ -913,7 +913,11 @@ trait Infer extends Checkable {
       val targs  = exprTypeArgs(tvars, tparams, treeTp, pt, useWeaklyCompatible)
       def infer_s = map3(tparams, tvars, targs)((tparam, tvar, targ) => s"$tparam=$tvar/$targ") mkString ","
       printTyping(tree, s"infer expr instance from pt=$pt, $infer_s")
-      def targsStrict = if (targs eq null) null else targs mapConserve dropByName
+
+      // SI-7899 infering by-name types is unsound. The correct behaviour is conditional because the hole is
+      //         exploited in Scalaz (Free.scala), as seen in: run/t7899-regression.
+      def dropByNameIfStrict(tp: Type): Type = if (settings.inferByName) tp else dropByName(tp)
+      def targsStrict = if (targs eq null) null else targs mapConserve dropByNameIfStrict
 
       if (keepNothings || (targs eq null)) { //@M: adjustTypeArgs fails if targs==null, neg/t0226
         substExpr(tree, tparams, targsStrict, pt)