missing tag - turn:lanes:forward without lanes - although lanes:forward and lanes:backward are correct

[Polarbear-j]

How to reproduce:

create New Layer
draw street, tag highway and name
tag lanes:forward=2 and lanes:backward=1

try upload: validator ok

add turn:lanes:forward=left|through

try upload: Validator: "missing tag - turn:lanes:forward without lanes (1)"
===
Thus the validator requires the sum of lanes:forward + lanes:backward to be added in lanes, which is redundant and error-prone.
In particular it requires the sum only in the presence of other *:lanes tags, which is inconsequent.

Might be a side effect of #19609 ?
===
Might be unrelated but in the startup messages I found:
2020-11-03 20:39:15.377 INFO: Could not load tool definition turnlanes-tagging
===
from Status:

URL:https://josm.openstreetmap.de/svn/trunk
Repository:UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
Last:Changed Date: 2020-11-03 00:09:37 +0100 (Tue, 03 Nov 2020)
Build-Date:2020-11-03 02:30:51
Revision:17292
Relative:URL: ^/trunk

Identification: JOSM/1.5 (17292 en_GB) Mac OS X 10.14.6
OS Build number: Mac OS X 10.14.6 (18G6032)
Memory Usage: 1559 MB / 3641 MB (1372 MB allocated, but free)
Java version: 1.8.0_251-b08, Oracle Corporation, Java HotSpot(TM) 64-Bit Server VM

Plugins:
[...]
+ turnlanes-tagging (288)
+ turnrestrictions (35583)
[...]
validator.bounds=1177,534,743,587
validator.geometry=x=759,y=-772,width=1161,height=772
validator.lastHeight=76
validator.layer=false
validator.org.openstreetmap.josm.data.validation.tests.MapCSSTagChecker.entries=[{title=Addresses, active=true, url=resource://data/validator/addresses.mapcss},

{title=Tag combinations, active=true, url=resource://data/validator/combinations.mapcss},
{title=Deprecated features, active=true, url=resource://data/validator/deprecated.mapcss},
{title=Geometry, active=true, url=resource://data/validator/geometry.mapcss},
{title=Highways, active=true, url=resource://data/validator/highway.mapcss},
{title=Multiple values, active=true, url=resource://data/validator/multiple.mapcss},
{title=Numeric values, active=true, url=resource://data/validator/numeric.mapcss},
{title=Religion, active=true, url=resource://data/validator/religion.mapcss},
{title=Relations, active=true, url=resource://data/validator/relation.mapcss},
{title=Unnecessary tags, active=true, url=resource://data/validator/unnecessary.mapcss},
{title=Wikipedia, active=true, url=resource://data/validator/wikipedia.mapcss},
{active=true, title=Territories, url=resource://data/validator/territories.mapcss}

]
validator.other=true
validator.skip=[]
validator.skipBeforeUpload=[]
validator.visible=true

[skyper]

-1
lanes=* should always be present. lanes:forward and lanes:backward are additional tags.

Adding both, lanes:forward and lanes:backward, used to be optional but there are situations where only one is not completely sufficient. E.g. it is common practice to add both.

[Polarbear-j]

I strongly disagree.
The wiki says on ​https://wiki.openstreetmap.org/wiki/Lanes :

In the common case of two driving directions either :forward or :backward is added to the end of the key; i.e., 
<key>:lanes:forward describes the properties of the lanes in the same direction as the osm-way, whereas 
<key>:lanes:backward described the properties of the lanes in the opposite direction of the osm-way.

lanes=N as the simple sum of lanes:forward=F + lanes:backward=B is an extra effort for the mapper, since N needs to be adjusted according to any changes of F and B.
On the other side it is a very simple calculation for the data consumer, as all values are available directly in the tagged object.

I'm not a friend of letting the data consumer do complex geographical is_in calculations, as with the idea of tagging the address on the building outline and leaving the POI without. But here the calculation is simple enough for the consumer and an extra burden for the mapper.