SD-128 fix override checks for default methods

The check for inheriting two conflicting members was wrong for default
methods, leading to a missing error message.

We were also not issuing "needs `override' modifier" when overriding a
default method.

Removes two methods:

  - `isDeferredOrJavaDefault` had a single use that is removed in this commit.

  - `isDeferredNotJavaDefault` is redundant with `isDeferred`, because
no default method has the `DEFERRED` flag:
    - For symbols originating in the classfile parser this was the case
      from day one: default methods don't receive the `DEFERRED` flag.
      Only abstract interface methods do, as they have the `JAVA_ACC_ABSTRACT`
      flag in bytecode, which the classfile parser translates to `DEFERRED`.

    - For symbols created by the Java source parser, we don't add the
      `DEFERRED` to default methods anymore since 373db1e.

Fixes scala/scala-dev#128

2 files changed, 22 insertions, 0 deletions

diff --git a/test/files/neg/sd128/A.java b/test/files/neg/sd128/A.java
new file mode 100644
index 0000000000..6774deba2e
--- /dev/null
+++ b/test/files/neg/sd128/A.java
@@ -0,0 +1,3 @@
+interface A {
+  default int f() { return -10; }
+}
diff --git a/test/files/neg/sd128/Test.scala b/test/files/neg/sd128/Test.scala
new file mode 100644
index 0000000000..66ca3d0940
--- /dev/null
+++ b/test/files/neg/sd128/Test.scala
@@ -0,0 +1,19 @@
+trait T {
+  def f = 99
+}
+class C1 extends A with T // error
+class C2 extends T with A // error
+
+trait U extends A {
+  override def f = 999
+}
+class D1 extends A with U // OK
+class D2 extends U with A // OK
+
+class E1 extends A {
+  def f() = 9999 // need override modifier
+}
+
+class E2 extends A {
+  override def f() = 9999 // OK
+}