patch 8.2.4576: Vim9: error for comparing with null can be annoying

Problem:    Vim9: error for comparing with null can be annoying.
Solution:   Allow comparing anything with null. (closes #9948)

src/testdir/test_vim9_expr.vim

@@ -716,6 +716,35 @@ def Test_expr4_compare_null()
   g:null_dict = test_null_dict()
   g:not_null_list = []
   var lines =<< trim END
+      assert_false(true == null)
+      assert_false(false == null)
+      assert_false(null == true)
+      assert_false(null == false)
+      assert_true(true != null)
+      assert_true(false != null)
+      assert_true(null != true)
+      assert_true(null != false)
+
+      assert_false(123 == null)
+      assert_false(0 == null)
+      assert_false(null == 123)
+      assert_false(null == 0)
+      assert_true(123 != null)
+      assert_true(0 != null)
+      assert_true(null != 123)
+      assert_true(null != 0)
+
+      if has('float')
+        assert_false(12.3 == null)
+        assert_false(0.0 == null)
+        assert_false(null == 12.3)
+        assert_false(null == 0.0)
+        assert_true(12.3 != null)
+        assert_true(0.0 != null)
+        assert_true(null != 12.3)
+        assert_true(null != 0.0)
+      endif
+
       assert_true(test_null_blob() == v:null)
       assert_true(null_blob == null)
       assert_true(v:null == test_null_blob())
@@ -818,16 +847,6 @@ def Test_expr4_compare_null()
       assert_equal(null_function, d.f)
   END
   v9.CheckDefAndScriptSuccess(lines)
-
-  v9.CheckDefAndScriptFailure(['echo 123 == v:null'], 'E1072: Cannot compare number with special')
-  v9.CheckDefAndScriptFailure(['echo v:null == 123'], 'E1072: Cannot compare special with number')
-  v9.CheckDefAndScriptFailure(['echo 123 != v:null'], 'E1072: Cannot compare number with special')
-  v9.CheckDefAndScriptFailure(['echo v:null != 123'], 'E1072: Cannot compare special with number')
-  v9.CheckDefAndScriptFailure(['echo true == v:null'], 'E1072: Cannot compare bool with special')
-  v9.CheckDefAndScriptFailure(['echo v:null == true'], 'E1072: Cannot compare special with bool')
-  v9.CheckDefAndScriptFailure(['echo true != v:null'], 'E1072: Cannot compare bool with special')
-  v9.CheckDefAndScriptFailure(['echo v:null != true'], 'E1072: Cannot compare special with bool')
-  v9.CheckDefAndScriptFailure(['echo false == v:null'], 'E1072: Cannot compare bool with special')
 enddef
 
 def Test_expr4_compare_none()

src/typval.c

@@ -1417,12 +1417,9 @@ typval_compare_null(typval_T *tv1, typval_T *tv2)
 	    default: break;
 	}
     }
-    if (!in_vim9script())
-	return FALSE;  // backwards compatible
-
-    semsg(_(e_cannot_compare_str_with_str),
-			 vartype_name(tv1->v_type), vartype_name(tv2->v_type));
-    return MAYBE;
+    // although comparing null with number, float or bool is not very usefule
+    // we won't give an error
+    return FALSE;
 }
 
 /*

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4576,
 /**/
     4575,
 /**/

src/vim9instr.c

@@ -397,20 +397,8 @@ get_compare_isn(
 			       vartype_name(vartype1), vartype_name(vartype2));
 	    return ISN_DROP;
 	}
-	switch (vartype1 == VAR_SPECIAL ? vartype2 : vartype1)
-	{
-	    case VAR_BLOB: break;
-	    case VAR_CHANNEL: break;
-	    case VAR_DICT: break;
-	    case VAR_FUNC: break;
-	    case VAR_JOB: break;
-	    case VAR_LIST: break;
-	    case VAR_PARTIAL: break;
-	    case VAR_STRING: break;
-	    default: semsg(_(e_cannot_compare_str_with_str),
-			       vartype_name(vartype1), vartype_name(vartype2));
-		     return ISN_DROP;
-	}
+	// although comparing null with number, float or bool is not useful, we
+	// allow it
 	isntype = ISN_COMPARENULL;
     }