diff --git a/compiler/src/dotty/tools/io/AbstractFile.scala b/compiler/src/dotty/tools/io/AbstractFile.scala
index 67bd964ecc3d..532dc315da7c 100644
--- a/compiler/src/dotty/tools/io/AbstractFile.scala
+++ b/compiler/src/dotty/tools/io/AbstractFile.scala
@@ -10,7 +10,7 @@ import java.io.{
   ByteArrayOutputStream
 }
 import java.net.URL
-import java.nio.file.{Files, Paths}
+import java.nio.file.{FileAlreadyExistsException, Files, Paths}
 
 /**
  * An abstraction over files for use in the reflection/compiler libraries.
@@ -235,17 +235,21 @@ abstract class AbstractFile extends Iterable[AbstractFile] {
     file
   }
 
-  private def fileOrSubdirectoryNamed(name: String, isDir: Boolean): AbstractFile = {
-    val lookup = lookupName(name, isDir)
-    if (lookup != null) lookup
-    else {
-      Files.createDirectories(jpath)
-      val path = jpath.resolve(name)
-      if (isDir) Files.createDirectory(path)
-      else Files.createFile(path)
-      new PlainFile(new File(path))
+  private def fileOrSubdirectoryNamed(name: String, isDir: Boolean): AbstractFile =
+    lookupName(name, isDir) match {
+      case null =>
+        // the optional exception may be thrown for symlinks, notably /tmp on MacOS.
+        // isDirectory tests for existing directory. The default behavior is hypothetical isDirectory(jpath, FOLLOW_LINKS).
+        try Files.createDirectories(jpath)
+        catch { case _: FileAlreadyExistsException if Files.isDirectory(jpath) => }
+
+        // a race condition in creating the entry after the failed lookup may throw
+        val path = jpath.resolve(name)
+        if (isDir) Files.createDirectory(path)
+        else Files.createFile(path)
+        new PlainFile(new File(path))
+      case lookup => lookup
     }
-  }
 
   /**
    * Get the file in this directory with the given name,
diff --git a/compiler/test/dotty/tools/io/AbstractFileTest.scala b/compiler/test/dotty/tools/io/AbstractFileTest.scala
new file mode 100644
index 000000000000..221df8dfc227
--- /dev/null
+++ b/compiler/test/dotty/tools/io/AbstractFileTest.scala
@@ -0,0 +1,55 @@
+package dotty.tools.io
+
+import org.junit.Test
+
+import dotty.tools.io.AbstractFile
+import java.nio.file.Files._
+
+class AbstractFileTest {
+  //
+  // Cope with symbolic links. Exercised by -d output.
+  //
+  // BytecodeWriters#getFile ensures d.isDirectory for elements in path,
+  // but d.fileNamed and d.subdirectoryNamed also Files.createDirectories
+  // for prefixes, which may optionally fail on an existing symbolic link.
+  //
+  private def exerciseSymbolicLinks(temp: Directory): Unit = {
+    val base = {
+      val target = createTempDirectory(temp.jpath, "real")
+      val link   = temp.jpath.resolve("link")
+      createSymbolicLink(link, target)     // may bail early if unsupported
+      AbstractFile.getDirectory(link)
+    }
+
+    val file = base.fileNamed("foo")
+    assert(file.exists && !file.isDirectory)
+
+    val dir = base.subdirectoryNamed("bar")
+    assert(dir.isDirectory)
+    val leaf = dir.fileNamed("basement")
+    assert(leaf.exists && !leaf.isDirectory)
+
+    val nested = AbstractFile.getDirectory(createSymbolicLink(dir.jpath.resolve("link"), dir.subdirectoryNamed("nested").jpath))
+    val doubly = nested.fileNamed("doubly")
+    assert(nested.exists && nested.isDirectory)  // /tmp/link/bar/link
+    assert(doubly.exists && !doubly.isDirectory) // /tmp/link/bar/link/doubly
+    assert(dir.subdirectoryNamed("link").exists)
+  }
+  @Test def t6450(): Unit =
+    try Directory.inTempDirectory(exerciseSymbolicLinks)
+    catch { case _: UnsupportedOperationException => () }
+}
+
+/* Was:
+[error] Test dotty.tools.io.AbstractFileTest.t6450 failed: java.nio.file.FileAlreadyExistsException: /var/folders/2_/xb149z895wb5f1y632xp2bw40000gq/T/temp9110019868196066936/link, took 0.124 sec
+[error]     at sun.nio.fs.UnixException.translateToIOException(UnixException.java:88)
+[error]     at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:102)
+[error]     at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:107)
+[error]     at sun.nio.fs.UnixFileSystemProvider.createDirectory(UnixFileSystemProvider.java:384)
+[error]     at java.nio.file.Files.createDirectory(Files.java:674)
+[error]     at java.nio.file.Files.createAndCheckIsDirectory(Files.java:781)
+[error]     at java.nio.file.Files.createDirectories(Files.java:727)
+[error]     at dotty.tools.io.AbstractFile.fileOrSubdirectoryNamed(AbstractFile.scala:242)
+[error]     at dotty.tools.io.AbstractFile.fileNamed(AbstractFile.scala:262)
+[error]     at dotty.tools.io.AbstractFileTest.exerciseSymbolicLinks(AbstractFileTest.scala:25)
+ */