Silent truncation when converting I to RGBA

[anntzer]

What did you do?

I converted an I-mode PNG with values >255 to RGBA.

What did you expect to happen?

Conversion without truncation (converting to the lower available bit depth in RGBA mode), or at least a warning about truncation.

What actually happened?

Values were silently truncated to 255

What are your OS, Python and Pillow versions?

• OS: arch linux
• Python: 3.7
• Pillow: 6.1

The attached example file comes from http://www.schaik.com/pngsuite/pngsuite_bas_png.html.

import PIL, numpy as np
np.asarray(PIL.Image.open("basn0g16.png").convert("RGBA"))

=> nearly all values (except the top right corner, which has value zero) were truncated to 255.

I appreciate that resampling may be too complex to be automatically performed, but at least truncation should not be silent.

[radarhere]

I'm going to say this is part of #3159.