Standard."-" provided to "with function "-" (VAL1 : in VALUE_TYPE) return VALUE_TYPE is <>;" refused:

[Mehdi Saada <00120260a@gmail.com>]

The parameter is
   with function "-" (VAL1 : in VALUE_TYPE) return VALUE_TYPE is <>;
from the (simplified) package
generic
   type VALUE_TYPE is private;
   with function "-" (VAL1 : in VALUE_TYPE) return VALUE_TYPE is <>;
package NUMERIC_SIGNATURE is end Numeric_Signature;

The instantiation is:
package P_Num is new NUMERIC_SIGNATURE (VALUE_TYPE => Float);
Package_needing_a_numeric_signature is new SPREADSHEETS.Formula_Cells (P_NUM);

why do I have:
instantiation error
 operator "-" not defined for type "VALUE_TYPE" defined at ...
gprbuild: *** compilation phase failed
And putting "-" => Standard."-" "association not allowed for overloaded formal"

... Isn't able to choose the "-" with the right parameters number ?!