[Ada] Spurious warning on uninitialized entity during code generation

This patch suppresses a spurious warning coming from the GCC backend, on
an aggregate that cannot be built in place and for which a temporary
variable must be created. If the type of the aggregate is a packed
boolean array, the generated code may appear to use an uninitialized
value for a component of the array, when in fact the code simply sets a
single bit of that array.

2018-10-09  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* exp_aggr.adb (Expand_Array_Aggregate): If it is not possible
	to build in place an aggregate with component associations, set
	the Warnings_Off flag on the generated temporary, to prevent
	spurious warnings from the backend when compiling with the
	-Wuninitialized gcc flag.

gcc/testsuite/

	* gnat.dg/warn18.adb: New testcase.

From-SVN: r264960

gcc/ada/ChangeLog

@@ -1,3 +1,11 @@
+2018-10-09  Ed Schonberg  <schonberg@adacore.com>
+
+	* exp_aggr.adb (Expand_Array_Aggregate): If it is not possible
+	to build in place an aggregate with component associations, set
+	the Warnings_Off flag on the generated temporary, to prevent
+	spurious warnings from the backend when compiling with the
+	-Wuninitialized gcc flag.
+
 2018-09-30  Alexandre Oliva <oliva@adacore.com>
 
 	* gcc-interface/lang-specs.h (default_compilers): When given

gcc/ada/exp_aggr.adb

@@ -6354,6 +6354,7 @@ package body Exp_Aggr is
              Defining_Identifier => Tmp,
              Object_Definition   => New_Occurrence_Of (Typ, Loc));
          Set_No_Initialization (Tmp_Decl, True);
+         Set_Warnings_Off (Tmp);
 
          --  If we are within a loop, the temporary will be pushed on the
          --  stack at each iteration. If the aggregate is the expression

gcc/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2018-10-09  Ed Schonberg  <schonberg@adacore.com>
+
+	* gnat.dg/warn18.adb: New testcase.
+
 2018-10-09  Martin Liska  <mliska@suse.cz>
 
 	* c-c++-common/asan/pr64820.c: Add line number to scanned

gcc/testsuite/gnat.dg/warn18.adb

@@ -0,0 +1,13 @@
+--  { dg-do compile }
+--  { dg-options "-Wuninitialized" }
+
+with Ada.Text_IO; use Ada.Text_IO;
+
+procedure Warn18 is
+   type Set is array (Natural range <>) of Boolean;
+   pragma Pack (Set);
+
+   O : constant Set (0 .. 255) := (28 => True, others => False);
+begin
+   Put_Line (O (1)'Img);
+end Warn18;