Currently MariaDB has a very relaxed function parameter data type control. It's only prohibited to pass a ROW-type value to a function expecting a scalar parameter, or the other way around. The responsible code resides in:

virtual Item_func::check_allowed_arg_cols(uint argno);

MariaDB has had mostly general purpose data types so far (except GEOMETRY), so the code assumes that all data types implement all value format methods:

• val_int()
• val_real()
• val_decimal()
• val_str()
• get_date()

Function parameter data type control needs some improvements, because:

• Pluggable data types might not want to implement all value methods. For example, the UUID data type may not want to implement get_date(), as UUID-to-temporal conversion has no any sense.
• Data type plugins might want stricter control on their function parameter data types. For example, the function GEOMEFROMTEXT(txt) returning a GEOMETRY-type value, might want to return an error at fix_fields time on attempts to pass a number (instead of a string) as a parameter txt, because the result will be NULL anyway for all possible numbers.
• After introduction of the ROW-type variables (MDEV-10914), in some cases it's now not possible to distinguish between a scalar parameter and a ROW value with one element.

Under terms of this task we'll improve function parameter control methods:

• We'll replace Item_func::check_allowed_arg_cols(uint argno)) to a more generic method:

  virtual bool check_arguments() const;

  This method will be called from Item_func::fix_fields(), after the loop doing fix_fields() for the arguments.

• As a prove of the concept, we'll fix functions in item_geofunc.h to disallow various senseless function parameter data type combinations. For example, passing a number as a GEOMETRY parameter, or passing a GEOMETRY value as a numeric parameter, will be disallowed.

Note, passing GEOMETRY values to functions implemented outside of item_geofunc.h will still be possible for now, e.g.:

SELECT TIMEDIFF(POINT(1,1));

This will be fixed under terms of a separate task.