Implicitly convert strings to geometries

Postgis and H2GIS implicitly transform strings into a geometries so that one can write the following query:

SELECT ST_AsText('POINT(-71.064544 42.28787)');

Instead of:

SELECT ST_AsText(ST_GeomFromText('POINT(-71.064544 42.28787)'));

 supports this kind of conversion. Should I introduce a new SqlTypeTranform (TO_GEOMETRY)? Thanks a lot for your help, I'm still discovering the internals of calcite. 

The idea would be to introduce a SqlTypeTransform, change the SqlTypeAssignmentRule (see CALCITE-3550), add tests in TypeCoercionTest, and update the doc.