[Ada] Declaration_Node for Itypes returns Empty or declaration

[Pierre-Marie de Rodat via Gcc-patches]

This patch changes Declaration_Node for Itypes so that it either returns
Empty or returns a proper declaration node. If the tree structure is
that of a normal type or subtype declaration, so the parent of the Itype
is that declaration, then we return the declaration. Otherwise, we
return Empty rather than returning some more-or-less arbitrary node.

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

gcc/ada/

        * einfo.ads (Declaration_Node): Document that Declaration_Node
        for Itypes.
        * einfo-utils.adb (Declaration_Node): Make it return Empty for
        Itypes, or a proper type or subtype declaration.
        * gen_il-gen.adb: Minor comment improvement.
diff --git a/gcc/ada/einfo-utils.adb b/gcc/ada/einfo-utils.adb
--- a/gcc/ada/einfo-utils.adb
+++ b/gcc/ada/einfo-utils.adb
@@ -655,16 +655,21 @@ package body Einfo.Utils is
          P := Parent (Id);
       end if;
 
+      while Nkind (P) in N_Selected_Component | N_Expanded_Name
+        or else (Nkind (P) = N_Defining_Program_Unit_Name
+                   and then Is_Child_Unit (Id))
       loop
-         if Nkind (P) in N_Selected_Component | N_Expanded_Name
-           or else (Nkind (P) = N_Defining_Program_Unit_Name
-                     and then Is_Child_Unit (Id))
-         then
-            P := Parent (P);
-         else
-            return P;
-         end if;
+         P := Parent (P);
       end loop;
+
+      if Is_Itype (Id)
+        and then Nkind (P) not in
+          N_Full_Type_Declaration | N_Subtype_Declaration
+      then
+         P := Empty;
+      end if;
+
+      return P;
    end Declaration_Node;
 
    ---------------------


diff --git a/gcc/ada/einfo.ads b/gcc/ada/einfo.ads
--- a/gcc/ada/einfo.ads
+++ b/gcc/ada/einfo.ads
@@ -829,7 +829,9 @@ package Einfo is
 --       a private type, then we obtain the declaration node denoted by the
 --       full type, i.e. the full type declaration node. Also note that for
 --       subprograms, this returns the {function,procedure}_specification, not
---       the subprogram_declaration.
+--       the subprogram_declaration. If the parent of an Itype is a type or
+--       subtype declaration, we return the declaration node as for any other
+--       type. For other Itypes, we return Empty.
 
 --    Default_Aspect_Component_Value [base type only]
 --       Defined in array types. Holds the static value specified in a


diff --git a/gcc/ada/gen_il-gen.adb b/gcc/ada/gen_il-gen.adb
--- a/gcc/ada/gen_il-gen.adb
+++ b/gcc/ada/gen_il-gen.adb
@@ -710,6 +710,8 @@ package body Gen_IL.Gen is
                   Type_Table (T).Last  := T;
                   Add_Concrete_Descendant_To_Ancestors
                     (Type_Table (T).Parent, T);
+                  --  Parent cannot be No_Type here, because T is a concrete
+                  --  type, and therefore not a root type.
 
                when Abstract_Type =>
                   declare