[COMMITTED] ada: Avoid renaming_decl in case of constrained array

[COMMITTED] ada: Avoid renaming_decl in case of constrained array

[Marc Poulhiès]

From: Bob Duff <duff@adacore.com>

This patch avoids rewriting "X: S := F(...);" as "X: S renames F(...);".
That rewrite is incorrect if S is a constrained array subtype,
because it changes the semantics. In the original, the
bounds of X are that of S. But constraints are ignored in
renamings, so the bounds of X would come from F'Result.
This can cause spurious Constraint_Errors in some obscure
cases. It causes unnecessary checks to be inserted, and even
when such checks pass (more common case), they might be less
efficient.

gcc/ada/

	* exp_ch3.adb (Expand_N_Object_Declaration): Avoid transforming to
	a renaming in case of constrained array that comes from source.

Tested on x86_64-pc-linux-gnu, committed on master.

---
 gcc/ada/exp_ch3.adb | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/gcc/ada/exp_ch3.adb b/gcc/ada/exp_ch3.adb
index daf27fb25e9..db27a5f68b6 100644
--- a/gcc/ada/exp_ch3.adb
+++ b/gcc/ada/exp_ch3.adb
@@ -7275,6 +7275,13 @@ package body Exp_Ch3 is
       Rewrite_As_Renaming : Boolean := False;
       --  Whether to turn the declaration into a renaming at the end
 
+      Nominal_Subtype_Is_Constrained_Array : constant Boolean :=
+        Comes_From_Source (Obj_Def)
+        and then Is_Array_Type (Typ) and then Is_Constrained (Typ);
+      --  Used to avoid rewriting as a renaming for constrained arrays,
+      --  which is only a problem for source arrays; others have the
+      --  correct bounds (see below).
+
    --  Start of processing for Expand_N_Object_Declaration
 
    begin
@@ -8030,7 +8037,14 @@ package body Exp_Ch3 is
 
                    or else (Nkind (Expr_Q) = N_Slice
                              and then OK_To_Rename_Ref (Prefix (Expr_Q))
-                             and then not Special_Ret_Obj));
+                             and then not Special_Ret_Obj))
+
+                --  If we have "X : S := ...;", and S is a constrained array
+                --  subtype, then we cannot rename, because renamings ignore
+                --  the constraints of S, so that would change the semantics
+                --  (sliding would not occur on the initial value).
+
+                and then not Nominal_Subtype_Is_Constrained_Array;
 
             --  If the type needs finalization and is not inherently limited,
             --  then the target is adjusted after the copy and attached to the
-- 
2.40.0