Custom CRS does not always switch parameters correctly from wkt to proj string

[DelazJ]

1. Open Settings --> Custom Projection... dialog
2. With "proj string" format enabled, paste the following parameters: +proj=vandg +lon_0=0 +x_0=0 +y_0=0 +R_A +a=6371000 +b=6371000 +units=m +no_defs. You can test; it's returned as valid definition.
3. Switch to wkt format: you get a conversion of the parameters. Great!

PROJCRS["unknown",
    BASEGEOGCRS["unknown",
        DATUM["unknown",
            ELLIPSOID["unknown",6371000,0,
                LENGTHUNIT["metre",1,
                    ID["EPSG",9001]]]],
        PRIMEM["Greenwich",0,
            ANGLEUNIT["degree",0.0174532925199433],
            ID["EPSG",8901]]],
    CONVERSION["unknown",
        METHOD["Van Der Grinten"],
        PARAMETER["Longitude of natural origin",0,
            ANGLEUNIT["degree",0.0174532925199433],
            ID["EPSG",8802]],
        PARAMETER["False easting",0,
            LENGTHUNIT["metre",1],
            ID["EPSG",8806]],
        PARAMETER["False northing",0,
            LENGTHUNIT["metre",1],
            ID["EPSG",8807]]],
    CS[Cartesian,2],
        AXIS["(E)",east,
            ORDER[1],
            LENGTHUNIT["metre",1,
                ID["EPSG",9001]]],
        AXIS["(N)",north,
            ORDER[2],
            LENGTHUNIT["metre",1,
                ID["EPSG",9001]]]]

1. Switch back to proj string format: you get the same text as for wkt (except some indentation changes) and not the expression I enter at the beginning
   

NOTA: I tried with copy-pasting the epsg4326 proj parameters and the switch between the formats works as expected

QGIS version	3.15.0-Master	QGIS code revision	5c241c8
Compiled against Qt	5.12.8	Running against Qt	5.12.8
Compiled against GDAL/OGR	3.0.4	Running against GDAL/OGR	3.0.4
Compiled against GEOS	3.8.0-CAPI-1.13.1	Running against GEOS	3.8.0-CAPI-1.13.1
Compiled against SQLite	3.31.1	Running against SQLite	3.31.1
PostgreSQL Client Version	12.4 (Ubuntu 12.4-0ubuntu0.20.04.1)	SpatiaLite Version	4.3.0a
QWT Version	6.1.4	QScintilla2 Version	2.11.2
Compiled against PROJ	6.3.1	Running against PROJ	Rel. 6.3.1, February 10th, 2020
OS Version	Ubuntu 20.04.1 LTS	This copy of QGIS writes debugging output.
Active python plugins	db_manager; MetaSearch; processing

[nyalldawson]

Can the projinfo tool convert this wkt to proj string? Not all wkt is representable as a proj string...

[rouault]

I can't reproduce this with QGIS master and PROJ master (with PROJ 6.3 branch, converting the WKT to PROJ.4 string works, so I don't think this is a PROJ issue)

[github-actions]

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[github-actions]

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