Fix handling of null nodes in memoized properties

TN: T203-028

langkit/templates/properties/def_ada.mako

@@ -143,7 +143,7 @@ begin
    ## If this property uses env, we want to make sure lexical env caches are up
    ## to date.
    % if property.uses_envs:
-      if Node /= null then
+      if Self /= null then
          Reset_Caches (Self.Unit);
 
          ## And if it is also public, we need to ensure that lexical
@@ -155,6 +155,10 @@ begin
    % endif
 
    % if memoized:
+      if Self = null then
+         raise Property_Error with "property called on null node";
+      end if;
+
       ## If memoization is enabled for this property, look for an already
       ## computed result for this property. See the declaration of
       ## Analysis_Context_Type.In_Populate_Lexical_Env for the rationale about
@@ -295,16 +299,18 @@ exception
       % endif
 
       % if memoized:
-         % if not property.memoize_in_populate:
-         if not Self.Unit.Context.In_Populate_Lexical_Env then
-         % endif
+         if Self /= null then
+            % if not property.memoize_in_populate:
+            if not Self.Unit.Context.In_Populate_Lexical_Env then
+            % endif
 
-            Add_Memoized_Value
-              (Self.Unit, Mmz_Handle, (Kind => Mmz_Property_Error));
+               Add_Memoized_Value
+                 (Self.Unit, Mmz_Handle, (Kind => Mmz_Property_Error));
 
-         % if not property.memoize_in_populate:
+            % if not property.memoize_in_populate:
+            end if;
+            % endif
          end if;
-         % endif
       % endif
 
       % if has_logging:

testsuite/tests/properties/mmz_on_null/expected_concrete_syntax.lkt

@@ -0,0 +1,12 @@
+grammar None_grammar {
+    main_rule <- Example(@Example)
+
+}
+
+class FooNode {
+
+    @export memoized fun prop (): Boolean = true
+}
+
+class Example : FooNode {
+}

testsuite/tests/properties/mmz_on_null/main.adb

@@ -0,0 +1,33 @@
+with Ada.Exceptions; use Ada.Exceptions;
+with Ada.Text_IO;    use Ada.Text_IO;
+
+with Langkit_Support.Diagnostics; use Langkit_Support.Diagnostics;
+
+with Libfoolang.Analysis; use Libfoolang.Analysis;
+with Libfoolang.Common;   use Libfoolang.Common;
+
+procedure Main is
+   Ctx  : constant Analysis_Context := Create_Context;
+   Unit : constant Analysis_Unit :=
+      Get_From_Buffer (Ctx, "foo.txt", Buffer => "example");
+begin
+   if Has_Diagnostics (Unit) then
+      for D of Diagnostics (Unit) loop
+         Put_Line (To_Pretty_String (D));
+      end loop;
+      raise Program_Error;
+   end if;
+
+   declare
+      Dummy : Boolean;
+   begin
+      Dummy := Unit.Root.P_Prop;
+      Put_Line ("No exception...");
+   exception
+      when Exc : Property_Error =>
+         Put_Line ("Got an exception!");
+         Put_Line (Exception_Name (Exc) & ": " & Exception_Message (Exc));
+   end;
+
+   Put_Line ("main.adb: Done");
+end Main;

testsuite/tests/properties/mmz_on_null/test.out

@@ -0,0 +1,3 @@
+No exception...
+main.adb: Done
+Done

testsuite/tests/properties/mmz_on_null/test.py

@@ -0,0 +1,29 @@
+"""
+Test that calling a memoized property on a null node triggers the expected
+error.
+"""
+
+from __future__ import absolute_import, division, print_function
+
+from langkit.dsl import ASTNode
+from langkit.expressions import langkit_property
+from langkit.parsers import Grammar
+
+from utils import build_and_run
+
+
+class FooNode(ASTNode):
+
+    @langkit_property(public=True, memoized=True)
+    def prop():
+        return True
+
+
+class Example(FooNode):
+    pass
+
+
+g = Grammar('main_rule')
+g.add_rules(main_rule=Example('example'))
+build_and_run(g, ada_main='main.adb')
+print('Done')

testsuite/tests/properties/mmz_on_null/test.yaml

@@ -0,0 +1 @@
+driver: python