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

    https://github.com/apache/spark/pull/14834#discussion_r78243482
  
    --- Diff: 
mllib/src/main/scala/org/apache/spark/ml/classification/LogisticRegression.scala
 ---
    @@ -333,22 +387,18 @@ class LogisticRegression @Since("1.2.0") (
     
           val isConstantLabel = histogram.count(_ != 0) == 1
     
    -      if (numClasses > 2) {
    -        val msg = s"LogisticRegression with ElasticNet in ML package only 
supports " +
    -          s"binary classification. Found $numClasses in the input dataset. 
Consider using " +
    -          s"MultinomialLogisticRegression instead."
    -        logError(msg)
    -        throw new SparkException(msg)
    -      } else if ($(fitIntercept) && numClasses == 2 && isConstantLabel) {
    -        logWarning(s"All labels are one and fitIntercept=true, so the 
coefficients will be " +
    -          s"zeros and the intercept will be positive infinity; as a 
result, " +
    -          s"training is not needed.")
    -        (Vectors.sparse(numFeatures, Seq()), Double.PositiveInfinity, 
Array.empty[Double])
    -      } else if ($(fitIntercept) && numClasses == 1) {
    -        logWarning(s"All labels are zero and fitIntercept=true, so the 
coefficients will be " +
    -          s"zeros and the intercept will be negative infinity; as a 
result, " +
    -          s"training is not needed.")
    -        (Vectors.sparse(numFeatures, Seq()), Double.NegativeInfinity, 
Array.empty[Double])
    +      if ($(fitIntercept) && isConstantLabel) {
    +        logWarning(s"All labels are the same value and fitIntercept=true, 
so the coefficients " +
    +          s"will be zeros. Training is not needed.")
    +        val constantLabelIndex = Vectors.dense(histogram).argmax
    +        val coefficientMatrix = Matrices.sparse(numCoefficientSets, 
numFeatures,
    +          Array.fill(numFeatures + 1)(0), Array.empty[Int], 
Array.empty[Double])
    --- End diff --
    
    I added this logic and also added a test to check it. We can add to that 
test when we complete the compressed logic.


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