Deal gracefully with missing return types of abstract menthods.

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)
       }
     }

test/dotc/tests.scala

@@ -35,6 +35,7 @@ class tests extends CompilerTest {
 
   val allowDeepSubtypes = defaultOptions diff List("-Yno-deep-subtypes")
   val allowDoubleBindings = defaultOptions diff List("-Yno-double-bindings")
+  val scala2mode = List("-language:Scala2")
 
   val testsDir      = "./tests/"
   val posDir        = testsDir + "pos/"
@@ -90,14 +91,16 @@ class tests extends CompilerTest {
   @Test def pos_valueclasses = compileFiles(posDir + "valueclasses/", twice)
   @Test def pos_nullarify = compileFile(posDir, "nullarify", args = "-Ycheck:nullarify" :: Nil)
   @Test def pos_subtyping = compileFile(posDir, "subtyping", twice)
-  @Test def pos_t2613 = compileFile(posSpecialDir, "t2613")(allowDeepSubtypes)
   @Test def pos_packageObj = compileFile(posDir, "i0239", twice)
   @Test def pos_anonClassSubtyping = compileFile(posDir, "anonClassSubtyping", twice)
   @Test def pos_extmethods = compileFile(posDir, "extmethods", twice)
   @Test def pos_companions = compileFile(posDir, "companions", twice)
 
   @Test def pos_all = compileFiles(posDir) // twice omitted to make tests run faster
 
+  @Test def pos_t2613 = compileFile(posSpecialDir, "t2613")(allowDeepSubtypes)
+  @Test def pos_i871 = compileFile(posSpecialDir, "i871", scala2mode)
+
   @Test def new_all = compileFiles(newDir, twice)
 
   @Test def neg_abstractOverride() = compileFile(negDir, "abstract-override", xerrors = 2)
@@ -132,6 +135,7 @@ class tests extends CompilerTest {
   @Test def neg_t2994 = compileFile(negDir, "t2994", xerrors = 2)
   @Test def neg_subtyping = compileFile(negDir, "subtyping", xerrors = 5)
   @Test def neg_variances = compileFile(negDir, "variances", xerrors = 2)
+  @Test def neg_i871_missingReturnType = compileFile(negDir, "i871", xerrors = 2)
   @Test def neg_badAuxConstr = compileFile(negDir, "badAuxConstr", xerrors = 2)
   @Test def neg_typetest = compileFile(negDir, "typetest", xerrors = 1)
   @Test def neg_t1569_failedAvoid = compileFile(negDir, "t1569-failedAvoid", xerrors = 1)

tests/neg/i871.scala

@@ -0,0 +1,5 @@
+trait Message {
+  def first(x: Int)
+  def second
+    1
+}

tests/pos-special/i871.flags

@@ -0,0 +1 @@
+-language:Scala2

tests/pos-special/i871.scala

@@ -0,0 +1,4 @@
+trait Message {
+  def first(x: Int)
+  def second
+}