5 files changed, 22 insertions, 7 deletions
diff --git a/src/compiler/scala/reflect/internal/Required.scala b/src/compiler/scala/reflect/internal/Required.scala
index 874eb2f9ae..1bf1a2e97e 100644
--- a/src/compiler/scala/reflect/internal/Required.scala
+++ b/src/compiler/scala/reflect/internal/Required.scala
@@ -5,7 +5,10 @@ import settings.MutableSettings
 
 trait Required { self: SymbolTable =>
 
-  type AbstractFileType >: Null <: { def path: String }
+  type AbstractFileType >: Null <: {
+    def path: String
+    def canonicalPath: String
+  }
 
   def picklerPhase: Phase
 
diff --git a/src/compiler/scala/reflect/internal/Symbols.scala b/src/compiler/scala/reflect/internal/Symbols.scala
index 4ded91c16c..cd60b85ff5 100644
--- a/src/compiler/scala/reflect/internal/Symbols.scala
+++ b/src/compiler/scala/reflect/internal/Symbols.scala
@@ -1446,8 +1446,12 @@ trait Symbols extends api.Symbols { self: SymbolTable =>
         (this.sourceFile == that.sourceFile) || {
           // recognize companion object in separate file and fail, else compilation
           // appears to succeed but highly opaque errors come later: see bug #1286
-          if (this.sourceFile.path != that.sourceFile.path)
-            throw InvalidCompanions(this, that)
+          if (this.sourceFile.path != that.sourceFile.path) {
+            // The cheaper check can be wrong: do the expensive normalization
+            // before failing.
+            if (this.sourceFile.canonicalPath != that.sourceFile.canonicalPath)
+              throw InvalidCompanions(this, that)
+          }
 
           false
         }
@@ -2404,8 +2408,10 @@ trait Symbols extends api.Symbols { self: SymbolTable =>
     // printStackTrace() // debug
   }
 
-  case class InvalidCompanions(sym1: Symbol, sym2: Symbol)
-  extends Throwable("Companions '" + sym1 + "' and '" + sym2 + "' must be defined in same file") {
+  case class InvalidCompanions(sym1: Symbol, sym2: Symbol) extends Throwable(
+    "Companions '" + sym1 + "' and '" + sym2 + "' must be defined in same file:\n" +
+    "  Found in " + sym1.sourceFile.canonicalPath + " and " + sym2.sourceFile.canonicalPath
+  ) {
       override def toString = getMessage
   }
 
diff --git a/src/compiler/scala/reflect/runtime/AbstractFile.scala b/src/compiler/scala/reflect/runtime/AbstractFile.scala
index 04f18bcd3b..bf3b47298b 100644
--- a/src/compiler/scala/reflect/runtime/AbstractFile.scala
+++ b/src/compiler/scala/reflect/runtime/AbstractFile.scala
@@ -2,4 +2,5 @@ package scala.reflect.runtime
 
 class AbstractFile(val jfile: java.io.File) {
  def path: String = jfile.getPath()
+ def canonicalPath: String = jfile.getCanonicalPath()
 }
 \ No newline at end of file
diff --git a/src/compiler/scala/tools/nsc/io/AbstractFile.scala b/src/compiler/scala/tools/nsc/io/AbstractFile.scala
index d60ecd8734..27dd8625a5 100644
--- a/src/compiler/scala/tools/nsc/io/AbstractFile.scala
+++ b/src/compiler/scala/tools/nsc/io/AbstractFile.scala
@@ -89,6 +89,9 @@ abstract class AbstractFile extends AnyRef with Iterable[AbstractFile] {
   /** Returns the path of this abstract file. */
   def path: String
 
+  /** Returns the path of this abstract file in a canonical form. */
+  def canonicalPath: String = if (file == null) path else file.getCanonicalPath
+
   /** Checks extension case insensitively. */
   def hasExtension(other: String) = extension == other.toLowerCase
   private lazy val extension: String = Path.extension(name)
diff --git a/test/files/neg/t1286.check b/test/files/neg/t1286.check
index 734964e9cf..2699f0789d 100644
--- a/test/files/neg/t1286.check
+++ b/test/files/neg/t1286.check
@@ -1,7 +1,9 @@
-a.scala:1: error: Companions 'object Foo' and 'trait Foo' must be defined in same file
+a.scala:1: error: Companions 'object Foo' and 'trait Foo' must be defined in same file:
+  Found in files/neg/t1286/b.scala and files/neg/t1286/a.scala
 trait Foo {
       ^
-b.scala:1: error: Companions 'trait Foo' and 'object Foo' must be defined in same file
+b.scala:1: error: Companions 'trait Foo' and 'object Foo' must be defined in same file:
+  Found in files/neg/t1286/a.scala and files/neg/t1286/b.scala
 object Foo extends Foo {
        ^
 two errors found