Github user srowen commented on a diff in the pull request:

    https://github.com/apache/spark/pull/15177#discussion_r79790212
  
    --- Diff: 
mllib/src/main/scala/org/apache/spark/mllib/linalg/CholeskyDecomposition.scala 
---
    @@ -52,8 +51,20 @@ private[spark] object CholeskyDecomposition {
       def inverse(UAi: Array[Double], k: Int): Array[Double] = {
         val info = new intW(0)
         lapack.dpptri("U", k, UAi, info)
    -    val code = info.`val`
    -    assert(code == 0, s"lapack.dpptri returned $code.")
    +    checkReturnValue(info, "dpptri")
         UAi
       }
    +
    +  private def checkReturnValue(info: intW, method: String): Unit = {
    +    info.`val` match {
    +      case code if code < 0 =>
    +        throw new IllegalStateException(s"LAPACK.$method returned $code; 
arg ${-code} is illegal")
    --- End diff --
    
    Yes, none of this info is really useful to the end user in this case 
because it's a bug. It may be useful in an error report. I figured it's worth 
saying _something_ and why not say a little more about the cause to help anyone 
who cares save the step of looking up what "-2" means.


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