[gcc r12-6233] [Ada] Prevent Get_Current_Value_Condition from returning the same node

[Pierre-Marie de Rodat]

https://gcc.gnu.org/g:a6505936a35e7a5c66e36f7c4e2032c5c7f64838

commit r12-6233-ga6505936a35e7a5c66e36f7c4e2032c5c7f64838
Author: Bob Duff <duff@adacore.com>
Date:   Tue Nov 30 16:00:40 2021 -0500

    [Ada] Prevent Get_Current_Value_Condition from returning the same node
    
    gcc/ada/
    
            * exp_util.ads (Get_Current_Value_Condition): Belt: Add a
            postcondition that Val /= Var.
            * sem_util.adb (Known_Null): Suspenders: Raise Program_Error if
            Get_Current_Value_Condition returned the same value. This will
            be enabled even without assertions, because infinite recursion
            is a nuisance -- better to crash if this bug ever occurs.

Diff:
---
 gcc/ada/exp_util.ads | 2 +-
 gcc/ada/sem_util.adb | 7 +++++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/gcc/ada/exp_util.ads b/gcc/ada/exp_util.ads
index 2b61132107c..773fa01e871 100644
--- a/gcc/ada/exp_util.ads
+++ b/gcc/ada/exp_util.ads
@@ -703,7 +703,7 @@ package Exp_Util is
    procedure Get_Current_Value_Condition
      (Var : Node_Id;
       Op  : out Node_Kind;
-      Val : out Node_Id);
+      Val : out Node_Id) with Post => Val /= Var;
    --  This routine processes the Current_Value field of the variable Var. If
    --  the Current_Value field is null or if it represents a known value, then
    --  on return Cond is set to N_Empty, and Val is set to Empty.
diff --git a/gcc/ada/sem_util.adb b/gcc/ada/sem_util.adb
index 49a58e3c615..7064dfac8d8 100644
--- a/gcc/ada/sem_util.adb
+++ b/gcc/ada/sem_util.adb
@@ -22062,6 +22062,13 @@ package body Sem_Util is
 
          Get_Current_Value_Condition (N, Op, Val);
 
+         --  If Get_Current_Value_Condition were to return Val = N, then the
+         --  recursion below could be infinite.
+
+         if Val = N then
+            raise Program_Error;
+         end if;
+
          if Known_Null (Val) then
             if Op = N_Op_Eq then
                return True;