Deal gracefully with missing return types of abstract menthods.

1) Better error message: "missing return type" instead of `=' expected.
2) Allow them under language:Scala2

Fixes #871

1 files changed, 15 insertions, 3 deletions

diff --git a/src/dotty/tools/dotc/parsing/Parsers.scala b/src/dotty/tools/dotc/parsing/Parsers.scala
index 4b22eac95..12cef900a 100644
--- a/src/dotty/tools/dotc/parsing/Parsers.scala
+++ b/src/dotty/tools/dotc/parsing/Parsers.scala
@@ -1741,10 +1741,22 @@ object Parsers {
         val vparamss = paramClauses(name)
         var tpt = fromWithinReturnType(typedOpt())
         val rhs =
-          if (tpt.isEmpty || in.token == EQUALS) {
-            if (atScala2Brace) tpt = scalaUnit else accept(EQUALS)
+          if (in.token == EQUALS) {
+            in.nextToken()
+            expr
+          }
+          else if (!tpt.isEmpty)
+            EmptyTree
+          else if (scala2mode) {
+            tpt = scalaUnit
+            if (in.token == LBRACE) expr()
+            else EmptyTree
+          }
+          else {
+            if (!isExprIntro) syntaxError("missing return type", in.lastOffset)
+            accept(EQUALS)
             expr()
-          } else EmptyTree
+          }
         DefDef(name, tparams, vparamss, tpt, rhs).withMods(mods1)
       }
     }