Add a pending test for a modified member of a trait

The test shows that name hashing optimization is broken for members
defined in traits. The problem is that modification of a member of a trait
triggers change of a hash of trait's name.

The behavior covered by this test regressed in
40ebc82531
The bug is tracked in #2436.

sbt/src/sbt-test/compiler-project/trait-member-modified/build.sbt

@@ -0,0 +1,27 @@
+/* Performs checks related to compilations:
+ *  a) checks in which compilation given set of files was recompiled
+ *  b) checks overall number of compilations performed
+ */
+TaskKey[Unit]("check-compilations") := {
+  val analysis = (compile in Compile).value
+  val srcDir = (scalaSource in Compile).value
+  def relative(f: java.io.File): java.io.File =  f.relativeTo(srcDir) getOrElse f
+  val allCompilations = analysis.compilations.allCompilations
+  val recompiledFiles: Seq[Set[java.io.File]] = allCompilations map { c =>
+    val recompiledFiles = analysis.apis.internal.collect {
+      case (file, api) if api.compilation.startTime == c.startTime => relative(file)
+    }
+    recompiledFiles.toSet
+  }
+  def recompiledFilesInIteration(iteration: Int, fileNames: Set[String]) = {
+    val files = fileNames.map(new java.io.File(_))
+    assert(recompiledFiles(iteration) == files, "%s != %s".format(recompiledFiles(iteration), files))
+  }
+  assert(allCompilations.size == 2)
+  // B.scala is just compiled at the beginning
+  recompiledFilesInIteration(0, Set("B.scala"))
+  // A.scala is changed and recompiled
+  recompiledFilesInIteration(1, Set("A.scala"))
+}
+
+logLevel := Level.Debug

sbt/src/sbt-test/compiler-project/trait-member-modified/changes/A1.scala

@@ -0,0 +1,3 @@
+trait A {
+  def foo: Int = 12
+}

sbt/src/sbt-test/compiler-project/trait-member-modified/pending

@@ -0,0 +1,9 @@
+# Test if adding a member to a trait affects classes that refer to that trait
+# by a member reference
+> compile
+# add `foo` method to `A`
+$ copy-file changes/A1.scala src/main/scala/A.scala
+# only A.scala should be recompiled
+> compile
+# check if there are only two compile iterations performed
+> check-compilations

sbt/src/sbt-test/compiler-project/trait-member-modified/src/main/scala/A.scala

@@ -0,0 +1 @@
+trait A

sbt/src/sbt-test/compiler-project/trait-member-modified/src/main/scala/B.scala

@@ -0,0 +1 @@
+class B(a: A)