[Ada] Reinitialize Private_Dependents when it is vanishing

[Ada] Reinitialize Private_Dependents when it is vanishing

[Pierre-Marie de Rodat]

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We call Set_Ekind on a E_Incomplete_Subtype entity, and the
Private_Dependents field vanishes. This patch resets it to zero, as
required for vanishing fields.

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

gcc/ada/

	* sem_ch3.adb (Process_Incomplete_Dependents): Reset
	Private_Dependents field to zero before calling Set_Ekind.  Also
	move Set_Etype to after Set_Ekind, because it's always best to
	set the Ekind as early as possible.
	* atree.adb: Improve debugging facilities for vanishing fields.

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diff --git a/gcc/ada/atree.adb b/gcc/ada/atree.adb
--- a/gcc/ada/atree.adb
+++ b/gcc/ada/atree.adb
@@ -29,6 +29,8 @@
 
 --  Checks and assertions in this package are too slow, and are mostly needed
 --  when working on this package itself, or on gen_il, so we disable them.
+--  To debug low-level bugs in this area, comment out the following pragmas,
+--  and run with -gnatd_v.
 
 pragma Suppress (All_Checks);
 pragma Assertion_Policy (Ignore);
@@ -868,7 +870,9 @@ package body Atree is
       Old_Kind : constant Entity_Kind := Ekind (Old_N);
 
       --  If this fails, it means you need to call Reinit_Field_To_Zero before
-      --  calling Set_Ekind.
+      --  calling Set_Ekind. But we have many cases where vanishing fields are
+      --  expected to reappear after converting to/from E_Void. Other cases are
+      --  more problematic; set a breakpoint on "(non-E_Void case)" below.
 
    begin
       for J in Entity_Field_Table (Old_Kind)'Range loop
@@ -882,12 +886,14 @@ package body Atree is
                   Write_Str (New_Kind'Img);
                   Write_Str (" Nonzero field ");
                   Write_Str (F'Img);
-                  Write_Str (" is vanishing");
+                  Write_Str (" is vanishing ");
                   Write_Eol;
 
-                  pragma Assert (New_Kind = E_Void or else Old_Kind = E_Void);
-
-                  raise Program_Error;
+                  if New_Kind = E_Void or else Old_Kind = E_Void then
+                     Write_Line ("    (E_Void case)");
+                  else
+                     Write_Line ("    (non-E_Void case)");
+                  end if;
                end if;
             end if;
          end;


diff --git a/gcc/ada/sem_ch3.adb b/gcc/ada/sem_ch3.adb
--- a/gcc/ada/sem_ch3.adb
+++ b/gcc/ada/sem_ch3.adb
@@ -21299,8 +21299,11 @@ package body Sem_Ch3 is
          then
             Set_Subtype_Indication
               (Parent (Priv_Dep), New_Occurrence_Of (Full_T, Sloc (Priv_Dep)));
-            Set_Etype (Priv_Dep, Full_T);
+            Reinit_Field_To_Zero
+              (Priv_Dep, Private_Dependents,
+               Old_Ekind => E_Incomplete_Subtype);
             Set_Ekind (Priv_Dep, Subtype_Kind (Ekind (Full_T)));
+            Set_Etype (Priv_Dep, Full_T);
             Set_Analyzed (Parent (Priv_Dep), False);
 
             --  Reanalyze the declaration, suppressing the call to Enter_Name