patch 8.2.4642: Vim9: in :def function script var cannot be null

Problem:    Vim9: in :def function script var cannot be null.
Solution:   Only initialize a script variable when not set to a null value.
            (closes #10034)

src/evalvars.c

@@ -2823,7 +2823,7 @@ eval_variable(
 	    {
 		if (tv->v_type == VAR_DICT && tv->vval.v_dict == NULL
 			  && ((type != NULL && type != &t_dict_empty)
-							   || !in_vim9script()))
+							  || !in_vim9script()))
 		{
 		    tv->vval.v_dict = dict_alloc();
 		    if (tv->vval.v_dict != NULL)
@@ -2843,6 +2843,14 @@ eval_variable(
 			tv->vval.v_list->lv_type = alloc_type(type);
 		    }
 		}
+		else if (tv->v_type == VAR_BLOB && tv->vval.v_blob == NULL
+				    && ((type != NULL && type != &t_blob_null)
+							  || !in_vim9script()))
+		{
+		    tv->vval.v_blob = blob_alloc();
+		    if (tv->vval.v_blob != NULL)
+			++tv->vval.v_blob->bv_refcount;
+		}
 	    }
 	    copy_tv(tv, rettv);
 	}

src/globals.h

@@ -405,6 +405,7 @@ EXTERN type_T t_number_bool INIT6(VAR_NUMBER, 0, 0, TTFLAG_STATIC|TTFLAG_BOOL_OK
 EXTERN type_T t_float INIT6(VAR_FLOAT, 0, 0, TTFLAG_STATIC, NULL, NULL);
 EXTERN type_T t_string INIT6(VAR_STRING, 0, 0, TTFLAG_STATIC, NULL, NULL);
 EXTERN type_T t_blob INIT6(VAR_BLOB, 0, 0, TTFLAG_STATIC, NULL, NULL);
+EXTERN type_T t_blob_null INIT6(VAR_BLOB, 0, 0, TTFLAG_STATIC, &t_void, NULL);
 EXTERN type_T t_job INIT6(VAR_JOB, 0, 0, TTFLAG_STATIC, NULL, NULL);
 EXTERN type_T t_channel INIT6(VAR_CHANNEL, 0, 0, TTFLAG_STATIC, NULL, NULL);
 

src/testdir/test_vim9_expr.vim

@@ -890,6 +890,93 @@ def Test_expr4_compare_null()
   unlet g:null_dict
   unlet g:not_null_list
 
+  # variables declared at script level used in a :def function
+  lines =<< trim END
+      vim9script
+      
+      var l_decl: list<number>
+      var l_empty = []
+      var l_null = null_list
+
+      def TestList()
+        assert_false(l_decl == null)
+        assert_false(l_decl is null_list)
+        assert_false(l_empty == null)
+        assert_false(l_empty is null_list)
+        assert_true(l_null == null)
+        assert_true(l_null is null_list)
+        assert_true(l_null == null_list)
+
+        add(l_decl, 6)
+        assert_equal([6], l_decl)
+        add(l_empty, 7)
+        assert_equal([7], l_empty)
+        var caught = false
+        try
+          add(l_null, 9)
+        catch /E1130:/
+          caught = true
+        endtry
+        assert_true(caught)
+      enddef
+      TestList()
+      
+      var b_decl: blob
+      var b_empty = 0z
+      var b_null = null_blob
+
+      def TestBlob()
+        assert_false(b_decl == null)
+        assert_false(b_decl is null_blob)
+        assert_false(b_empty == null)
+        assert_false(b_empty is null_blob)
+        assert_true(b_null == null)
+        assert_true(b_null is null_blob)
+        assert_true(b_null == null_blob)
+
+        add(b_decl, 6)
+        assert_equal(0z06, b_decl)
+        add(b_empty, 7)
+        assert_equal(0z07, b_empty)
+        var caught = false
+        try
+          add(b_null, 9)
+        catch /E1131:/
+          caught = true
+        endtry
+        assert_true(caught)
+      enddef
+      TestBlob()
+      
+      var d_decl: dict<number>
+      var d_empty = {}
+      var d_null = null_dict
+
+      def TestDict()
+        assert_false(d_decl == null)
+        assert_false(d_decl is null_dict)
+        assert_false(d_empty == null)
+        assert_false(d_empty is null_dict)
+        assert_true(d_null == null)
+        assert_true(d_null is null_dict)
+        assert_true(d_null == null_dict)
+
+        d_decl['a'] = 6
+        assert_equal({a: 6}, d_decl)
+        d_empty['b'] = 7
+        assert_equal({b: 7}, d_empty)
+        var caught = false
+        try
+          d_null['c'] = 9
+        catch /E1103:/
+          caught = true
+        endtry
+        assert_true(caught)
+      enddef
+      TestDict()
+  END
+  v9.CheckScriptSuccess(lines)
+
   lines =<< trim END
       var d: dict<func> = {f: null_function}
       assert_equal(null_function, d.f)

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4642,
 /**/
     4641,
 /**/

src/vim.h

@@ -2229,6 +2229,7 @@ typedef enum {
 #define ASSIGN_UNPACK	0x10  // using [a, b] = list
 #define ASSIGN_NO_MEMBER_TYPE 0x20 // use "any" for list and dict member type
 #define ASSIGN_FOR_LOOP 0x40 // assigning to loop variable
+#define ASSIGN_INIT	0x80 // not assigning a value, just a declaration
 
 #include "ex_cmds.h"	    // Ex command defines
 #include "spell.h"	    // spell checking stuff

src/vim9execute.c

@@ -1336,20 +1336,22 @@ do_2string(typval_T *tv, int is_2string_any, int tolerant)
  * When the value of "sv" is a null list of dict, allocate it.
  */
     static void
-allocate_if_null(typval_T *tv)
+allocate_if_null(svar_T *sv)
 {
+    typval_T *tv = sv->sv_tv;
+
     switch (tv->v_type)
     {
 	case VAR_LIST:
-	    if (tv->vval.v_list == NULL)
+	    if (tv->vval.v_list == NULL && sv->sv_type != &t_list_empty)
 		(void)rettv_list_alloc(tv);
 	    break;
 	case VAR_DICT:
-	    if (tv->vval.v_dict == NULL)
+	    if (tv->vval.v_dict == NULL && sv->sv_type != &t_dict_empty)
 		(void)rettv_dict_alloc(tv);
 	    break;
 	case VAR_BLOB:
-	    if (tv->vval.v_blob == NULL)
+	    if (tv->vval.v_blob == NULL && sv->sv_type != &t_blob_null)
 		(void)rettv_blob_alloc(tv);
 	    break;
 	default:
@@ -2891,7 +2893,7 @@ exec_instructions(ectx_T *ectx)
 		    sv = get_script_svar(sref, ectx->ec_dfunc_idx);
 		    if (sv == NULL)
 			goto theend;
-		    allocate_if_null(sv->sv_tv);
+		    allocate_if_null(sv);
 		    if (GA_GROW_FAILS(&ectx->ec_stack, 1))
 			goto theend;
 		    copy_tv(sv->sv_tv, STACK_TV_BOT(0));

src/vim9script.c

@@ -822,7 +822,7 @@ vim9_declare_scriptvar(exarg_T *eap, char_u *arg)
 	init_tv.v_type = VAR_NUMBER;
     else
 	init_tv.v_type = type->tt_type;
-    set_var_const(name, 0, type, &init_tv, FALSE, 0, 0);
+    set_var_const(name, 0, type, &init_tv, FALSE, ASSIGN_INIT, 0);
 
     vim_free(name);
     return p;
@@ -925,6 +925,13 @@ update_vim9_script_var(
 	if (*type == NULL)
 	    *type = typval2type(tv, get_copyID(), &si->sn_type_list,
 					       do_member ? TVTT_DO_MEMBER : 0);
+	else if ((flags & ASSIGN_INIT) == 0
+		&& (*type)->tt_type == VAR_BLOB && tv->v_type == VAR_BLOB
+						    && tv->vval.v_blob == NULL)
+	{
+	    // "var b: blob = null_blob" has a different type.
+	    *type = &t_blob_null;
+	}
 	if (sv->sv_type_allocated)
 	    free_type(sv->sv_type);
 	if (*type != NULL && ((*type)->tt_type == VAR_FUNC

src/vim9type.c

@@ -337,7 +337,11 @@ typval2type_int(typval_T *tv, int copyID, garray_T *type_gap, int flags)
     if (tv->v_type == VAR_STRING)
 	return &t_string;
     if (tv->v_type == VAR_BLOB)
+    {
+	if (tv->vval.v_blob == NULL)
+	    return &t_blob_null;
 	return &t_blob;
+    }
 
     if (tv->v_type == VAR_LIST)
     {