[Ada] Prevent Get_Current_Value_Condition from returning the same node

[Pierre-Marie de Rodat via Gcc-patches]

Get_Current_Value_Condition should never return Val = Var, because that
could cause infinite recursion in Known_Null.

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

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 --git a/gcc/ada/exp_util.ads b/gcc/ada/exp_util.ads
--- 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
--- 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;