Better explanation for SI-5969.

The comment should now better reflect the joys of tryTwice
as discussed during the code review.

1 files changed, 6 insertions, 2 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Infer.scala b/src/compiler/scala/tools/nsc/typechecker/Infer.scala
index e1aabb5469..d1974a058c 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Infer.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Infer.scala
@@ -1612,8 +1612,12 @@ trait Infer {
         val saved = context.state
         var fallback = false
         context.setBufferErrors()
-        // Need to test first attempt with an empty buffer.
-        // Otherwise, any previous errors will get lost.
+        // We cache the current buffer because it is impossible to 
+        // distinguish errors that occurred before entering tryTwice
+        // and our first attempt in 'withImplicitsDisabled'. If the
+        // first attempt fails we try with implicits on *and* clean
+        // buffer but that would also flush any pre-tryTwice valid
+        // errors, hence some manual buffer tweaking is necessary.
         val errorsToRestore = context.flushAndReturnBuffer()
         try {
           context.withImplicitsDisabled(infer(false))