[Ada] Infinite loop on illegal declaration

[Pierre-Marie de Rodat <derodat@adacore.com>]

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This patch updates predicate Null_Status to prevent an infinite
recursion when the argument is an illegal object declaration of an
access type.

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

2019-07-11  Hristian Kirtchev  <kirtchev@adacore.com>

gcc/ada/

	* sem_util.adb (Null_Status): Assume that an erroneous construct
	has an undefined null status.

gcc/testsuite/

	* gnat.dg/self_ref1.adb: New testcase.

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--- gcc/ada/sem_util.adb
+++ gcc/ada/sem_util.adb
@@ -22367,9 +22367,15 @@ package body Sem_Util is
    --  Start of processing for Null_Status
 
    begin
+      --  Prevent cascaded errors or infinite loops when trying to determine
+      --  the null status of an erroneous construct.
+
+      if Error_Posted (N) then
+         return Unknown;
+
       --  An allocator always creates a non-null value
 
-      if Nkind (N) = N_Allocator then
+      elsif Nkind (N) = N_Allocator then
          return Is_Non_Null;
 
       --  Taking the 'Access of something yields a non-null value

--- /dev/null
new file mode 100644
+++ gcc/testsuite/gnat.dg/self_ref1.adb
@@ -0,0 +1,11 @@
+--  { dg-do compile }
+
+procedure Self_Ref1 is
+   type Integer_Ptr is access all Integer;
+   Ptr : constant Integer_Ptr := Integer_Ptr (Ptr); --  { dg-error "object \"Ptr\" cannot be used before end of its declaration" }
+
+begin
+   if Ptr /= null then
+      null;
+   end if;
+end Self_Ref1;