SI-9834 Improve error on failed op=

If rewriting `x += y` fails to typecheck, emit error messages
for both the original tree and the assignment.

If rewrite is not attempted because `x` is a val, then say so.

The error message at `tree.pos` is updated with the additional advice.

SI-8763 Crash in update conversion

When there are already errors, don't attempt mechanical rewrites.

1 files changed, 11 insertions, 0 deletions

diff --git a/test/files/neg/t8763.scala b/test/files/neg/t8763.scala
new file mode 100644
index 0000000000..08ce1b471a
--- /dev/null
+++ b/test/files/neg/t8763.scala
@@ -0,0 +1,11 @@
+
+import collection.mutable
+
+object Foo {
+  def bar() {
+    val names_times = mutable.Map[String, mutable.Set[Long]]()
+    val line = ""
+    val Array(fields) = line.split("\t")
+    names_times(fields(0)) += fields(1).toLong
+  }
+}