Fixes #4351.

Added an "Abstract" method info to the specialized phase, which denotes
that no implementation should be generated.

Previously:

trait A[@specialized(Boolean) T] {
  def foo: T
}

used to generate:

trait A$mcZ$sp extends A[Boolean] {
  def foo$mcZ$sp = this.foo
}

which caused cyclic calls because of linearization rules when several
traits are mixed together. Now, the following is generated:

trait A$mcZ$sp extends A[Boolean] {
  def foo$mcZ$sp: Boolean
}

Review by dragos.

2 files changed, 21 insertions, 0 deletions

diff --git a/test/files/pos/t4351.check b/test/files/pos/t4351.check
new file mode 100644
index 0000000000..cb5d407e13
--- /dev/null
+++ b/test/files/pos/t4351.check
@@ -0,0 +1 @@
+runtime exception
diff --git a/test/files/pos/t4351.scala b/test/files/pos/t4351.scala
new file mode 100644
index 0000000000..2d57588793
--- /dev/null
+++ b/test/files/pos/t4351.scala
@@ -0,0 +1,20 @@
+object Test {
+  def main(args: Array[String]): Unit = {
+    try new BooleanPropImpl() value
+    catch {
+      case e: RuntimeException => println("runtime exception")
+    }
+  }
+}
+
+trait Prop[@specialized(Boolean) +T] {
+  def value: T
+}
+
+class PropImpl[+T] extends Prop[T] {
+  def value: T = scala.sys.error("")
+}
+
+trait BooleanProp extends Prop[Boolean]
+
+class BooleanPropImpl() extends PropImpl[Boolean] with BooleanProp