User isn't informed that information from proj4 string has been ignored in making CRS

[wjbenfold]

Example of problem

from pyproj.crs import CRS

crss = [
    CRS.from_user_input("+proj=merc +datum=WGS84 +ellps=WGS84 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +units=m +no_defs +type=crs"),
    CRS.from_user_input("+proj=merc +a=6378137.0 +rf=298.257223563 +datum=WGS84 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +units=m +no_defs +type=crs"),
]

for crs in crss:
    print(crs)
    print(crs.datum)
    print()

+proj=merc +datum=WGS84 +ellps=WGS84 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +units=m +no_defs +type=crs
World Geodetic System 1984

+proj=merc +a=6378137.0 +rf=298.257223563 +datum=WGS84 +lon_0=0.0 +x_0=0.0 +y_0=0.0 +units=m +no_defs +type=crs
unknown

Problem description

Currently, information (the datum) is dropped in the second example. I'm informed (pyproj4/pyproj#1061) that this is because the string parsing in proj has identified that the ellipse information conflicts with the datum and thus chosen to drop the datum.

The lack of warning that this has been done (along with pyproj's choice to show the string used to create the crs even though it's inaccurate) makes it harder to decode what's gone wrong. I think that proj should warn or error on provision of strings that make it drop data like this.

Environment Information

• PROJ version (proj)
  Rel. 8.2.0, November 1st, 2021
• Operation System Information
  System:
  python: 3.8.12 | packaged by conda-forge | (default, Oct 12 2021, 21:59:51) [GCC 9.4.0]
  executable: .../.conda/envs/iris-dev/bin/python
  machine: Linux-3.10.0-1160.62.1.el7.x86_64-x86_64-with-glibc2.10

Installation method

• conda

[kbevers]

How do you propose we issue such a warning? Currently we don't really have a mechanism for that

[rouault]

Proposed resolution in #3171

[wjbenfold]

I don't know enough to tell everything that's going on in #3171 but the stated intent seems like a good resolution