by= should do the same type promotion as rbindlist when combining across groups

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
r
Domain
data

Research direction

Start by reproducing the two examples comparing rbindlist() with grouped by= assembly, then inspect those entry points and the related type-consistency handling. Done means grouped results promote integer and numeric values consistently, matching rbindlist() without the reported error.

Written by the indexing model from the issue text.

Description

consistency feature request

Follow-up to #6924. Actually I'm surprised by this inconsistency:

rbindlist(list(data.table(a=1), data.table(a=1L)))
#        a
#    <num>
# 1:     1
# 2:     1

data.table(a=1:2)[, list(1, 1L)[[.GRP]], by=a]
# Error in `[.data.table`(data.table(a = 1:2), , list(1, 1L)[[.GRP]], by = a) : 
#   Column 1 of result for group 2 is type 'integer' but expecting type 'double'. Column types must be consistent for each group.

If rbindlist() is doing the "normal" type promotion integer->numeric, I don't see why by= assembly wouldn't do the same.

Dominant language
R
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