Improve presentation compilation of annotations

A trio of problems were hampering autocompletion of annotations.

First, given that that annotation is written before the annotated
member, it is very common to end parse incomplete code that has a
floating annotation without an anotatee.

The parser was discarding the annotations (ie, the modifiers) and
emitting an `EmptyTree`.

Second, the presetation compiler was only looking for annotations
in the Modifiers of a member def, but after typechecking annotations
are moved into the symbol.

Third, if an annotation failed to typecheck, it was being discarded
in place of `ErroneousAnnotation`.

This commit:
  - modifies the parser to uses a dummy class- or type-def tree,
    instead of EmptyTree, which can carry the annotations.
  - updates the locator to look in the symbol annotations of the
    modifiers contains no annotations.
  - uses a separate instance of `ErroneousAnnotation` for each
    erroneous annotation, and stores the original tree in its
    `original` tree.

1 files changed, 8 insertions, 2 deletions

diff --git a/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala b/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
index 70abdf6bc0..4494a8ac8d 100644
--- a/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
+++ b/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
@@ -2680,7 +2680,10 @@ self =>
           case t if t == SUPERTYPE || t == SUBTYPE || t == COMMA || t == RBRACE || isStatSep(t) =>
             TypeDef(mods | Flags.DEFERRED, name, tparams, typeBounds())
           case _ =>
-            syntaxErrorOrIncompleteAnd("`=', `>:', or `<:' expected", skipIt = true)(EmptyTree)
+            syntaxErrorOrIncompleteAnd("`=', `>:', or `<:' expected", skipIt = true)(
+              // assume a dummy type def so as to have somewhere to stash the annotations
+              TypeDef(mods, tpnme.ERROR, Nil, EmptyTree)
+            )
         }
       }
     }
@@ -2713,7 +2716,10 @@ self =>
         case CASEOBJECT =>
           objectDef(pos, (mods | Flags.CASE) withPosition (Flags.CASE, tokenRange(in.prev /*scanner skips on 'case' to 'object', thus take prev*/)))
         case _ =>
-          syntaxErrorOrIncompleteAnd("expected start of definition", skipIt = true)(EmptyTree)
+          syntaxErrorOrIncompleteAnd("expected start of definition", skipIt = true)(
+            // assume a class definition so as to have somewhere to stash the annotations
+            atPos(pos)(gen.mkClassDef(mods, tpnme.ERROR, Nil, Template(Nil, noSelfType, Nil)))
+          )
       }
     }