patch 9.0.1796: Vim9 problems with null_objects

Problem:  Vim9 problems with null_objects
Solution: Vim9 improve null_object usage

Fix "xvar == null", where xvar might have been assigned null_object.

Fix compilation failure: "var o2: C = null_object".

closes: #12890

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Ernie Rael <errael@raelity.com>

src/proto/vim9instr.pro

@@ -28,6 +28,7 @@ int generate_PUSHCHANNEL(cctx_T *cctx);
 int generate_PUSHJOB(cctx_T *cctx);
 int generate_PUSHBLOB(cctx_T *cctx, blob_T *blob);
 int generate_PUSHFUNC(cctx_T *cctx, char_u *name, type_T *type, int may_prefix);
+int generate_PUSHOBJ(cctx_T *cctx);
 int generate_AUTOLOAD(cctx_T *cctx, char_u *name, type_T *type);
 int generate_GETITEM(cctx_T *cctx, int index, int with_op);
 int generate_SLICE(cctx_T *cctx, int count);

src/testdir/test_vim9_class.vim

@@ -337,7 +337,7 @@ def Test_object_not_set()
       var bg: Background           # UNINITIALIZED
       echo Colorscheme.new(bg).GetBackground()
   END
-  v9.CheckScriptFailure(lines, 'E1012: Type mismatch; expected object<Background> but got object<Unknown>')
+  v9.CheckScriptFailure(lines, 'E1360:')
 
   # TODO: this should not give an error but be handled at runtime
   lines =<< trim END
@@ -359,6 +359,46 @@ def Test_object_not_set()
   v9.CheckScriptFailure(lines, 'E1363:')
 enddef
 
+def Test_null_object_assign_compare()
+  var lines =<< trim END
+    vim9script
+
+    var nullo = null_object
+    def F(): any
+      return nullo
+    enddef
+    assert_equal('object<Unknown>', typename(F()))
+
+    var o0 = F()
+    assert_true(o0 == null_object)
+    assert_true(o0 == null)
+
+    var o1: any = nullo
+    assert_true(o1 == null_object)
+    assert_true(o1 == null)
+
+    def G()
+      var x = null_object
+    enddef
+
+    class C
+    endclass
+    var o2: C
+    assert_true(o2 == null_object)
+    assert_true(o2 == null)
+
+    o2 = null_object
+    assert_true(o2 == null)
+
+    o2 = C.new()
+    assert_true(o2 != null)
+
+    o2 = null_object
+    assert_true(o2 == null)
+  END
+  v9.CheckScriptSuccess(lines)
+enddef
+
 def Test_class_member_initializer()
   var lines =<< trim END
       vim9script

src/typval.c

@@ -1578,12 +1578,15 @@ typval_compare_null(typval_T *tv1, typval_T *tv2)
 #ifdef FEAT_JOB_CHANNEL
 	    case VAR_CHANNEL: return tv->vval.v_channel == NULL;
 #endif
+	    // TODO: null_class handling
+	    // case VAR_CLASS: return tv->vval.v_class == NULL;
 	    case VAR_DICT: return tv->vval.v_dict == NULL;
 	    case VAR_FUNC: return tv->vval.v_string == NULL;
 #ifdef FEAT_JOB_CHANNEL
 	    case VAR_JOB: return tv->vval.v_job == NULL;
 #endif
 	    case VAR_LIST: return tv->vval.v_list == NULL;
+	    case VAR_OBJECT: return tv->vval.v_object == NULL;
 	    case VAR_PARTIAL: return tv->vval.v_partial == NULL;
 	    case VAR_STRING: return tv->vval.v_string == NULL;
 

src/version.c

@@ -699,6 +699,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1796,
 /**/
     1795,
 /**/

src/vim9compile.c

@@ -2299,6 +2299,9 @@ push_default_value(
 	case VAR_CHANNEL:
 	    r = generate_PUSHCHANNEL(cctx);
 	    break;
+	case VAR_OBJECT:
+	    r = generate_PUSHOBJ(cctx);
+	    break;
 	case VAR_NUMBER:
 	case VAR_UNKNOWN:
 	case VAR_ANY:
@@ -2306,7 +2309,6 @@ push_default_value(
 	case VAR_VOID:
 	case VAR_INSTR:
 	case VAR_CLASS:
-	case VAR_OBJECT:
 	case VAR_SPECIAL:  // cannot happen
 	    // This is skipped for local variables, they are always
 	    // initialized to zero.  But in a "for" or "while" loop

src/vim9instr.c

@@ -662,11 +662,11 @@ generate_SETTYPE(
 /*
  * Generate an ISN_PUSHOBJ instruction.  Object is always NULL.
  */
-    static int
+    int
 generate_PUSHOBJ(cctx_T *cctx)
 {
     RETURN_OK_IF_SKIP(cctx);
-    if (generate_instr_type(cctx, ISN_PUSHOBJ, &t_any) == NULL)
+    if (generate_instr_type(cctx, ISN_PUSHOBJ, &t_object) == NULL)
 	return FAIL;
     return OK;
 }

src/vim9type.c

@@ -907,6 +907,8 @@ check_type_maybe(
 		return MAYBE;	// use runtime type check
 	    if (actual->tt_type != VAR_OBJECT)
 		return FAIL;	// don't use tt_class
+	    if (actual->tt_class == NULL)
+		return OK;	// A null object matches
 
 	    if (class_instance_of(actual->tt_class, expected->tt_class) == FALSE)
 		ret = FAIL;