SI-5717 error when bytecode cannot be written

If there's an existing file foo when emitting a class file for foo.Bar,
then emit an error at foo.Bar, similar to what javac does.

The error message looks something like:

foo.scala:4: error: error writing object Foo: ./mypkg/Foo.class: ./mypkg is not a directory

1 files changed, 21 insertions, 0 deletions

diff --git a/test/files/run/t5717.scala b/test/files/run/t5717.scala
new file mode 100755
index 0000000000..a0997f5a49
--- /dev/null
+++ b/test/files/run/t5717.scala
@@ -0,0 +1,21 @@
+import scala.tools.partest._
+import java.io.File
+
+object Test extends StoreReporterDirectTest {
+  def code = ???
+
+  def compileCode(code: String) = {
+    val classpath = List(sys.props("partest.lib"), testOutput.path) mkString sys.props("path.separator")
+    compileString(newCompiler("-cp", classpath, "-d", testOutput.path))(code)
+  }
+  // TODO
+  // Don't assume output is on physical disk
+  // Let the compiler tell us output dir
+  // val sc = newCompiler("-cp", classpath, "-d", testOutput.path)
+  // val out = sc.settings.outputDirs.getSingleOutput.get
+  def show(): Unit = {
+    // Don't crash when we find a file 'a' where package 'a' should go.
+    scala.reflect.io.File(testOutput.path + "/a").writeAll("a")
+    compileCode("package a { class B }")
+  }
+}