[Ada] Warning needed on anonymous access type allocators

[Ada] Warning needed on anonymous access type allocators

This patch enhances the compiler to add an optional warning for
allocators of anonymous access types due to the somewhat confusing
runtime accessibility checks that they generate. For more details see RM
3.10.2 sections 14/3, 14.1/3, and 14.2/3.

These warnings can now be enabled with -gnatw_a, disabled with -gnatw_A
and are part of the "all warnings" flag -gnatwa.

------------
-- Source --
------------

--  main.adb

procedure Main is

   type Int_Ptr is access all Integer;

   Ptr_Obj          :          Int_Ptr;
   Ptr_Not_Null_Obj : not null Int_Ptr := new Integer;
   Ptr_Anon_Acc_Obj : not null access Integer :=
     new Integer;                     --  WARNING

   procedure Update_Ptr (Item : access Integer) is
   begin
      Ptr_Obj := Int_Ptr (Item);      --  RUNTIME ERROR
   end;

   procedure Update_Ptr_With_Anonymous_Allocator is
      Item : access Integer := new Integer;
   begin
      Update_Ptr (Item);
   end;

   type Rec_With_Coextension_A (Disc : access Integer)
     is null record;

   type Rec_With_Coextension_B (Disc : access Integer) is record
      Comp : Integer;
   end record;

   Obj : Rec_With_Coextension_A :=
     (Disc => new Integer'(32));      -- WARNING

   procedure Test_Param (Param : access Integer) is
   begin
      null;
   end;

   function Test_Simple_Return return access Integer is
   begin
      return new Integer;             --  WARNING
   end;

   function Test_Coextension_Return_A return Rec_With_Coextension_A is
   begin
      return (Disc => new Integer);   --  WARNING
   end;

   function Test_Coextension_Return_B return Rec_With_Coextension_B is
   begin
      return (new Integer, 32);       --  WARNING
   end;

begin
   Test_Param (new Integer);          --  WARNING
   Test_Param (Param => new Integer); --  WARNING
   Update_Ptr_With_Anonymous_Allocator;
end;

-----------------
-- Compilation --
-----------------

$ gnatmake -q -gnatw_a main.adb
$ rm *.ali
$ gnatmake -q -gnatwa -gnatw_A main.adb
$ rm *.ali
$ gnatmake -q -gnatwa main.adb
$ main
main.adb:8:06: warning: use of an anonymous access type allocator
main.adb:16:32: warning: use of an anonymous access type allocator
main.adb:29:15: warning: use of an anonymous access type allocator
main.adb:38:14: warning: use of an anonymous access type allocator
main.adb:43:23: warning: coextension will not be deallocated when
its associated owner is deallocated
main.adb:43:23: warning: use of an anonymous access type allocator
main.adb:48:15: warning: coextension will not be deallocated when
its associated owner is deallocated
main.adb:48:15: warning: use of an anonymous access type allocator
main.adb:52:16: warning: use of an anonymous access type allocator
main.adb:53:25: warning: use of an anonymous access type allocator
main.adb:5:04: warning: variable "Ptr_Obj" is assigned but never read
main.adb:6:04: warning: variable "Ptr_Not_Null_Obj" is not referenced
main.adb:7:04: warning: variable "Ptr_Anon_Acc_Obj" is not referenced
main.adb:16:07: warning: "Item" is not modified, could be declared constant
main.adb:28:04: warning: variable "Obj" is not referenced
main.adb:36:13: warning: function "Test_Simple_Return" is not referenced
main.adb:41:13: warning: function "Test_Coextension_Return_A" is not referenced
main.adb:43:23: warning: coextension will not be deallocated when its
associated owner is deallocated
main.adb:46:13: warning: function "Test_Coextension_Return_B" is not referenced
main.adb:48:15: warning: coextension will not be deallocated when its
associated owner is deallocated
main.adb:5:04: warning: variable "Ptr_Obj" is assigned but never read
main.adb:6:04: warning: variable "Ptr_Not_Null_Obj" is not referenced
main.adb:7:04: warning: variable "Ptr_Anon_Acc_Obj" is not referenced
main.adb:8:06: warning: use of an anonymous access type allocator
main.adb:16:07: warning: "Item" is not modified, could be declared constant
main.adb:16:32: warning: use of an anonymous access type allocator
main.adb:28:04: warning: variable "Obj" is not referenced
main.adb:29:15: warning: use of an anonymous access type allocator
main.adb:36:13: warning: function "Test_Simple_Return" is not referenced
main.adb:38:14: warning: use of an anonymous access type allocator
main.adb:41:13: warning: function "Test_Coextension_Return_A" is not referenced
main.adb:43:23: warning: coextension will not be deallocated when its
associated owner is deallocated
main.adb:43:23: warning: use of an anonymous access type allocator
main.adb:46:13: warning: function "Test_Coextension_Return_B" is not referenced
main.adb:48:15: warning: coextension will not be deallocated when its
associated owner is deallocated
main.adb:48:15: warning: use of an anonymous access type allocator
main.adb:52:16: warning: use of an anonymous access type allocator
main.adb:53:25: warning: use of an anonymous access type allocator

raised PROGRAM_ERROR : main.adb:12 accessibility check failed

2019-07-09  Justin Squirek  <squirek@adacore.com>

gcc/ada/

	* exp_ch4.adb (Expand_N_Allocator): Add conditional to detect
	the presence of anoymous access type allocators and issue a
	warning if the appropriate warning flag is enabled.
	* warnsw.ads: Add new warning flag for anonymous allocators
	* warnsw.adb (All_Warnings, Restore_Warnings, Save_Warnings,
	Set_Underscore_Warning_Switch): Register new flags.
	(WA_Warnings): Register new flag as an "all warnings" switch
	* usage.adb,
	doc/gnat_ugn/building_executable_programs_with_gnat.rst:
	Document new warning switches -gnatw_a and -gnatw_A.
	* gnat_ugn.texi: Regenerate.

From-SVN: r273290