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Date: Fri, 6 Apr 2018 10:20:21 -0700 (PDT)
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Subject: Standard."-" provided to "with function "-" (VAL1 : in VALUE_TYPE)
 return VALUE_TYPE is <>;" refused:
From: Mehdi Saada <00120260a@gmail.com>
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Xref: reader02.eternal-september.org comp.lang.ada:51359
Date: 2018-04-06T10:20:21-07:00
List-Id: <comp.lang.ada>

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 ?!