Compile an empty source in InteractiveDriver ctor

This is necessary because we may be using this InteractiveDriver before
asking it to compile anything (which would have initialized it). This
happens, for instance, if we use this Driver to resolve a symbol coming
from a different Driver.

Drivers are not meant to be used before being initialized and can crash.

Author: Duhemm

compiler/src/dotty/tools/dotc/interactive/InteractiveDriver.scala

@@ -86,6 +86,8 @@ class InteractiveDriver(val settings: List[String]) extends Driver {
     names
   }
 
+  initialize()
+
   /**
    * The trees for all the source files in this project.
    *
@@ -298,6 +300,18 @@ class InteractiveDriver(val settings: List[String]) extends Driver {
     new SourceFile(virtualFile, Codec.UTF8)
   }
 
+  /**
+   * Initialize this driver and compiler by "compiling" a fake, empty source file.
+   *
+   * This is necessary because an `InteractiveDriver` can be put to work without having
+   * compiled anything (for instance, resolving a symbol coming from a different compiler in
+   * this compiler). In those cases, an un-initialized compiler will crash.
+   */
+  private[this] def initialize(): Unit = {
+    val fakeSource = new File("fake.scala")
+    run(fakeSource.toURI, "")
+  }
+
 }
 
 object InteractiveDriver {