Better error message for case class/object matc...

Better error message for case class/object match confusion. Closes
SI-4879, no review.

4 files changed, 53 insertions, 2 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Infer.scala b/src/compiler/scala/tools/nsc/typechecker/Infer.scala
index b70f20ea89..354eb52913 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Infer.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Infer.scala
@@ -1540,8 +1540,30 @@ trait Infer {
         val pt1 = pt.instantiateTypeParams(ptparams, ptvars)
         if (pat.tpe <:< pt1)
           ptvars foreach instantiateTypeVar
-        else
-          error(pat.pos, "pattern type is incompatible with expected type"+foundReqMsg(pat.tpe, pt))
+        else {
+          val sym   = pat.tpe.typeSymbol
+          val clazz = sym.companionClass
+          val addendum = (
+            if (sym.isModuleClass && clazz.isCaseClass && (clazz isSubClass pt1.typeSymbol)) {
+              // TODO: move these somewhere reusable.
+              val typeString = clazz.typeParams match {
+                case Nil  => "" + clazz.name
+                case xs   => xs map (_ => "_") mkString (clazz.name + "[", ",", "]")
+              }
+              val caseString = (
+                clazz.caseFieldAccessors
+                map (_ => "_")    // could use the actual param names here
+                mkString (clazz.name + "(", ",", ")")
+              )
+              (
+                "\nNote: if you intended to match against the class, try `case _: " +
+                typeString + "` or `case " + caseString + "`"
+              )
+            }
+            else ""
+          )
+          error(pat.pos, "pattern type is incompatible with expected type"+foundReqMsg(pat.tpe, pt) + addendum)
+        }
       }
 
     object toOrigin extends TypeMap {
diff --git a/test/files/neg/bug414.check b/test/files/neg/bug414.check
index ec23e26337..91bb39caf2 100644
--- a/test/files/neg/bug414.check
+++ b/test/files/neg/bug414.check
@@ -1,6 +1,7 @@
 bug414.scala:5: error: pattern type is incompatible with expected type;
  found   : object Empty
  required: IntMap[a]
+Note: if you intended to match against the class, try `case _: Empty[_]` or `case Empty()`
                 case Empty =>
                      ^
 bug414.scala:7: error: type mismatch;
diff --git a/test/files/neg/bug4879.check b/test/files/neg/bug4879.check
new file mode 100644
index 0000000000..6b9e452109
--- /dev/null
+++ b/test/files/neg/bug4879.check
@@ -0,0 +1,13 @@
+bug4879.scala:6: error: pattern type is incompatible with expected type;
+ found   : object C
+ required: C
+Note: if you intended to match against the class, try `case _: C` or `case C(_)`
+    case C  => true
+         ^
+bug4879.scala:10: error: pattern type is incompatible with expected type;
+ found   : object D
+ required: D[T,U,V]
+Note: if you intended to match against the class, try `case _: D[_,_,_]` or `case D(_,_,_)`
+    case D  => true
+         ^
+two errors found
diff --git a/test/files/neg/bug4879.scala b/test/files/neg/bug4879.scala
new file mode 100644
index 0000000000..7d6561e9e0
--- /dev/null
+++ b/test/files/neg/bug4879.scala
@@ -0,0 +1,15 @@
+case class C(d: Double) { }
+case class D[T, U, V](bingo: Int, donkey: String, private val vegas: Set[A])(jehovah: Int) { }
+
+class A {
+  def f = (new C(5)) match {
+    case C  => true
+    case _  => false
+  }
+  def g[T, U, V](x: D[T, U, V]) = x match {
+    case D  => true
+    case _  => false
+  }
+}
+
+