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, 16 insertions, 9 deletions

diff --git a/src/compiler/scala/tools/nsc/backend/jvm/BytecodeWriters.scala b/src/compiler/scala/tools/nsc/backend/jvm/BytecodeWriters.scala
index c1cd3204e0..66aed14d1c 100644
--- a/src/compiler/scala/tools/nsc/backend/jvm/BytecodeWriters.scala
+++ b/src/compiler/scala/tools/nsc/backend/jvm/BytecodeWriters.scala
@@ -6,12 +6,15 @@
 package scala.tools.nsc
 package backend.jvm
 
-import java.io.{ DataOutputStream, FileOutputStream, OutputStream, File => JFile }
+import java.io.{ DataOutputStream, FileOutputStream, IOException, OutputStream, File => JFile }
 import scala.tools.nsc.io._
 import scala.tools.nsc.util.ScalaClassLoader
 import java.util.jar.Attributes.Name
 import scala.language.postfixOps
 
+/** Can't output a file due to the state of the file system. */
+class FileConflictException(msg: String, val file: AbstractFile) extends IOException(msg)
+
 /** For the last mile: turning generated bytecode in memory into
  *  something you can use.  Has implementations for writing to class
  *  files, jars, and disassembled/javap output.
@@ -20,16 +23,20 @@ trait BytecodeWriters {
   val global: Global
   import global._
 
-  private def outputDirectory(sym: Symbol): AbstractFile = (
-    settings.outputDirs.outputDirFor(enteringFlatten(sym.sourceFile))
-  )
-  private def getFile(base: AbstractFile, /*cls.getName()*/ clsName: String, suffix: String): AbstractFile = {
+  private def outputDirectory(sym: Symbol): AbstractFile =
+    settings.outputDirs outputDirFor enteringFlatten(sym.sourceFile)
+
+  /**
+   * @param clsName cls.getName
+   */
+  private def getFile(base: AbstractFile, clsName: String, suffix: String): AbstractFile = {
+    def ensureDirectory(dir: AbstractFile): AbstractFile =
+      if (dir.isDirectory) dir
+      else throw new FileConflictException(s"${base.path}/$clsName$suffix: ${dir.path} is not a directory", dir)
     var dir = base
     val pathParts = clsName.split("[./]").toList
-    for (part <- pathParts.init) {
-      dir = dir.subdirectoryNamed(part)
-    }
-    dir.fileNamed(pathParts.last + suffix)
+    for (part <- pathParts.init) dir = ensureDirectory(dir) subdirectoryNamed part
+    ensureDirectory(dir) fileNamed pathParts.last + suffix
   }
   private def getFile(sym: Symbol, clsName: String, suffix: String): AbstractFile =
     getFile(outputDirectory(sym), clsName, suffix)