test for #678

sbt/src/sbt-test/source-dependencies/ext/A.scala

@@ -0,0 +1,5 @@
+import sun.net.spi.nameservice.dns.DNSNameService
+
+object A {
+	val x = new DNSNameService
+}

sbt/src/sbt-test/source-dependencies/ext/build.sbt

@@ -0,0 +1,9 @@
+import complete.DefaultParsers._
+
+val checkIterations = inputKey[Unit]("Verifies the accumlated number of iterations of incremental compilation.")
+
+checkIterations := {
+	val expected: Int = (Space ~> NatBasic).parsed
+	val actual: Int = (compile in Compile).value.compilations.allCompilations.size
+	assert(expected == actual, s"Expected $expected compilations, got $actual")
+}

sbt/src/sbt-test/source-dependencies/ext/test

@@ -0,0 +1,7 @@
+# initial compilation
+> checkIterations 1
+
+# no further compilation should be necessary, since nothing changed
+# previously, a dependency on a jar in <java.home>lib/ext/ would
+# always force recompilation
+> checkIterations 1