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Newsgroups: comp.lang.ada
Date: Mon, 7 Oct 2019 08:03:10 -0700 (PDT)
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Subject: Re: GNAT: no visible subprogram matches the specification for "Put"
From: Stephen Leake <stephen_leake@stephe-leake.org>
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Xref: reader01.eternal-september.org comp.lang.ada:57255
Date: 2019-10-07T08:03:10-07:00
List-Id: <comp.lang.ada>

On Sunday, October 6, 2019 at 4:45:22 AM UTC-7, marc...@earthlink.net wrote=
:
> I think that both are: both the one that can be passed a string and the o=
ne that=20
> can be passed an integer.  I thing that the "test signature" procedure at=
 least=20
> shows that the string one and the put_line one should be visible because =
it
> runs as expected and the string one is the one that GNAT is complaining a=
bout.
> I am thinking that GNAT's compilation - and my subsequent running - of th=
e
> Signature test program would imply that?  Do you agree?

To be precise, this works as expected:

$ gnatchop marc.ada # produces individual files from above code
$ gnatmake signature_package_instance_test
$ ./signature_package_instance_test

Direct Instance Put_Line("string")
Direct Instance Put     ("string")

However, this gives a compiler error:

$ gnatmake signature_package_reexport_instance
gcc -c signature_package_reexport_instance.ads
signature_package_reexport_instance.ads:6:01: instantiation error at signat=
ure_package_reexport_generic.ads:15
signature_package_reexport_instance.ads:6:01: no visible subprogram matches=
 the specification for "Put"
gnatmake: "signature_package_reexport_instance.ads" compilation error

signature_package_reexport_generic.ads:15 is:

  procedure Put (X: in String) renames Instance. Put; -- instantiation erro=
r

"Instance.Put" is the name of a generic parameter of "Signature_Package_Gen=
eric".

The visibility of generic formal parameters outside the generic body is pro=
blematic in GNAT. LRM 8.2 (8) says:
  =20
      * The visible part of a generic unit includes the
     generic_formal_part. ...

So your code should work. However, I've often had problems referring to the=
m from outside the generic, and the problems seem to change with each versi=
on of GNAT. Note that this is not an intended use of the generic formal par=
ameters, but it is sometimes useful (if it works).

So I agree this is (probably) a compiler bug.

The fact that signature_package_instance compiles is not relevant; it is no=
t using the name of a generic formal parameter from outside the generic; it=
 is only referring to them in the body of the generic, which is where they =
are intended to be used.

One workaround is to put a rename declaration in the generic package:

package Signature_Package_Generic is

   procedure Put_Param (X    : in String) renames Put;
end;

then use it in Signature_Package_Reexport_Generic:

  procedure Put      (X    : in String)       renames Instance. Put_Param;

This compiles.