Disallow eliding when Nothing is expected

Author: som-snytt

src/compiler/scala/tools/nsc/backend/icode/GenICode.scala

@@ -885,6 +885,7 @@ abstract class GenICode extends SubComponent  {
               ctx.bb.emit(CONSTANT(global.gen.mkConstantZero(expectedType.toType)), tree.pos)
               generatedType = expectedType
             }
+            else if (expectedType.isNothingType) unit.error(tree.pos, "Cannot elide where Nothing is required.")
             else {
               ctx.bb.emit(CONSTANT(Constant(null)), tree.pos)
               generatedType = NullReference

test/files/neg/elide-to-nothing.check

@@ -0,0 +1,4 @@
+elide-to-nothing.scala:14: error: Cannot elide where Nothing is required.
+  val b: Nothing = unimplemented()
+                                ^
+one error found

test/files/neg/elide-to-nothing.flags

@@ -0,0 +1 @@
+-Xelide-below 500

test/files/neg/elide-to-nothing.scala

@@ -0,0 +1,31 @@
+
+/** Test which should fail compilation */
+class ElysianFailed {
+
+  import ElysianField._
+
+  // fine
+  val a: Int = myInt
+
+  // fine
+  unimplemented()
+
+  // not fine
+  val b: Nothing = unimplemented()
+  
+}
+
+object ElysianField {
+
+  import annotation.elidable
+
+  @elidable(100) def unimplemented(): Nothing = throw new UnsupportedOperationException
+
+  @elidable(100) def myInt: Int = 17
+
+}
+
+
+
+
+