[Ada] Fix Ada 2005/95 difference

[Arnaud Charlet <charlet at adacore dot com>]

Tested on i686-linux, committed on trunk

Ada 2005 introduces several new operations in predefined packages.
These operations should not be visible when compiling in Ada 95 mode.
The following must compile quietly:
gcc -c -gnat95 -gnatws main.adb
--
package P is
     function Get_Line return String;
end P;
--
with P; use P;
with Ada.Text_IO; use Ada.Text_IO;
procedure Main is
     X: constant String := Get_Line;
begin
     null;
end Main;

In Ada 2005, an abstract operation does not participate in overloading
resolution, and is removed from the list of interpretations during the
first pass of resolution. However, if the operands are universal, the
universal interpretation must be preserved until the second pass. If
the node is still ambiguous and the context is not universal, it may be
due to the now incorrect presence of a numeric interpretation, which has
to be removed to resolve the ambiguity.
The following must compile quietly:
gcc -c -gnat05 amb.adb
--
procedure Amb is
  type whole is new integer;
  function "/" (Left, Right: Whole) return Whole is abstract;  -- 1
  function "+" (      Right: Whole) return Whole is abstract;  -- 2
--
  type Rational is record
    Numerator  : Whole;
    Denominator: Whole;
  end record;
--
  function "+" (      Right: Whole   ) return Rational is
  begin
      return (Right, 1);
  end;
  function "/" (Left, Right: Whole   ) return Rational is
  begin
     return (Left, Right);
  end;
  function "+" (      Right: Rational) return Rational is
  begin
     return Right;
  end;
--
  R: Rational := +1/2;  --  ambiguous in Ada 95, OK in Ada 2005
begin
   R := (1/3) + (3/4);
end Amb;

2005-12-05  Ed Schonberg  <schonberg@adacore.com>

	* sem_ch4.adb (Remove_Abstract_Operations): Do not apply preference
	rule prematurely when operands are universal, remaining ambiguities
	will be removed during resolution.
	Code cleanup.

	* sem_type.adb (Disambiguate): In Ada95 mode, discard interpretations
	that are Ada 2005 functions.
	(Has_Abstract_Interpretation): Subsidiary to
	Remove_Conversions, to remove ambiguities caused by abstract operations
	on numeric types when operands are universal.

Attachment: difs.22
Description: Text document