gdalwarp zeroes all alpha pixels from grey+alpha image

[cdestigter]

When warping a grey+alpha image, gdal appears to zero out all the alpha pixels, resulting in a completely invisible image.

test case:

gdalinfo -stats ~/c/in.tif | grep Minimum
rm -f ~/c/out.tif ; gdalwarp ~/c/in.tif ~/c/out.tif
gdalinfo -stats ~/c/out.tif | grep Minimum

  Minimum=0.000, Maximum=176.000, Mean=86.546, StdDev=59.195
  Minimum=0.000, Maximum=255.000, Mean=176.375, StdDev=117.760
Creating output file that is 16P x 15L.
Processing input file /Users/cdestigter/c/in.tif.
0...10...20...30...40...50...60...70...80...90...100 - done.
  Minimum=0.000, Maximum=176.000, Mean=86.546, StdDev=59.195
  Minimum=0.000, Maximum=0.000, Mean=0.000, StdDev=0.000

Adding -setci doesn't help. If I add -dstalpha I get a good alpha band in band 3, but then I have to remove band 2 somehow.

Tested on both stock 1.11.0 and a recent trunk fork (specifically ​https://github.com/koordinates/gdal/commit/164434f0759ed2b5fac2f0533abe5e099e56b4e4 )

Will attach in.tif.