[Ada] Avoid false positive on subtype conversion warning

[Arnaud Charlet <charlet at adacore dot com>]

In some circumstances it is necessary to use a conversion which at
first glance has the same source type and destination type, and
therefore looks redundant. But in fact the expression's type has
non-static bounds because of technical rules as to what is or is
not static. So the conversion is not after all redundant. This
patch avoids the false positive warning in this case.

The following example shows the case in question:

Compiling: badconversionwarn.adb

     1. procedure badconversionwarn is
     2.    type INTRA_CHECK_T is RANGE 1..11;
     3.
     4.    subtype TT is INTRA_CHECK_T range 1 .. 1;
     5.    type INTRA_CHECK_TO_DO_T is array (TT) of BOOLEAN;
     6.
     7.     type R is record
     8.        X : INTRA_CHECK_TO_DO_T;
     9.     end record;
    10.
    11.     A : constant R := R'(X => (1 => False));
    12. begin
    13.      for I in A.X'range loop
    14.        case TT (I) is
    15.            when 1 => null;
    16.        end case;
    17.
    18.        case I is
               1    3
        >>> missing case values: -128 .. 0
        >>> missing case values: 2 .. 127
        >>> bounds of "I" are not static, alternatives must
            cover base type

    19.           when 1 => null;
    20.        end case;
    21.
    22.        case TT'(I) is
    23.            when 1 => null;
    24.        end case;
    25.      end loop;
    26. end;

This is compiled with -gnatwa -gnatld7 -gnatj60. Before the patch,
the apparently redundant conversion on line 14 was flagged with a
warning. But as line 18 shows, this conversion si not redundant.

Tested on x86_64-pc-linux-gnu, committed on trunk

2009-10-30  Robert Dewar  <dewar@adacore.com>

	* sem_res.adb (Resolve_Type_Conversion): Avoid false positive when
	converting non-static subtype to "identical" static subtype.

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