[Ada] Overriding_Indicators not legal on protected subprogram bodies

[Arnaud Charlet <charlet at adacore dot com>]

The compiler incorrectly allows overriding_indicators to be applied
to protected subprogram bodies (and flags a style error with -gnatyO
when they're missing), but those are disallowed by the Ada RM (see
RM-8.3.1(3-6) and AC95-00213 for confirmation of intent). This is
fixed, but the error can be changed to a warning with -gnatd.E to
ease transition for programs that were using such overriding_indicators.

The test below must report the following style warning and error
when compiled with:

$ gcc -c -gnatyO -gnatj60 prot_subp_indicator_bug.adb

prot_subp_indicator_bug.adb:17:07: (style) missing
                                   "overriding" indicator
                                   in declaration of "P"
prot_subp_indicator_bug.adb:32:07: overriding indicator not
                                   allowed for protected
                                   subprogram body

and the following warnings when compiled with:

$ gcc -c -gnatyO -gnatd.E -gnatj60 prot_subp_indicator_bug.adb

prot_subp_indicator_bug.adb:17:07: (style) missing
                                   "overriding" indicator
                                   in declaration of "P"
prot_subp_indicator_bug.adb:32:07: warning: overriding
                                   indicator not allowed
                                   for protected subprogram
                                   body

procedure Prot_Subp_Indicator_Bug is

   package Synch_Pkg is

      type Synch_Interface is synchronized interface;

      procedure P (X : out Synch_Interface) is abstract;

      procedure Q (X : in out Synch_Interface) is abstract;

   end Synch_Pkg;

   use Synch_Pkg;

   protected type Prot_Type is new Synch_Interface with

      procedure P;    -- Warning "missing overriding indicator" OK with -gnatyO

      overriding      -- OK
      procedure Q;

   end Prot_Type;

   protected body Prot_Type is

      procedure P is -- Shouldn't get warning about adding overriding indicator
      begin
         null;
      end P;

      overriding     -- Illegal (but only give a warning when using -gnatd.E)
      procedure Q is
      begin
         null;
      end Q;

   end Prot_Type;

begin
   null;
end Prot_Subp_Indicator_Bug;

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

2014-05-21  Gary Dismukes  <dismukes@adacore.com>

	* debug.adb: Add case of illegal overriding_indicator for a
	protected subprogram body to description of -gnatd.E switch.
	* sem_ch6.adb (Verify_Overriding_Indicator): Issue error message
	for cases of giving overriding_indicators on protected subprogram
	bodies, but change this to a warning if -gnatd.E is enabled. No
	longer give a style warning about missing indicators on protected
	subprogram bodies.

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