is.na returns FALSE for list element with scalar NA_integer64_

[tdhock]

hi @truecluster I noticed something inconsistent about the way NA_integer64_ is handled by is.na function of base R:

> is.na(list(NA_integer64_, NA_real_, NA_integer_))
[1] FALSE  TRUE  TRUE

I would have expected TRUE rather than FALSE above (same as below)

> sapply(list(NA_integer64_, NA_real_, NA_integer_), is.na)
[1] TRUE TRUE TRUE

The reason for this discrepancy is that base R is.na has a special method for lists which is hard coded in C: https://github.com/wch/r-source/blob/b560647e74459fa2f40262dcaf1abf171c197efc/src/main/coerce.c#L2247-L2271 (only for five types: logical/int, string, complex, real).
Do you think it would be possible to patch base R to fix this inconsistency? The fix would be to first check if there exists a method for is.na, and if so then use it (here the method is is.na.integer64) instead of the hard coded logic.
For reference I discovered this issue while fixing a bug in data.table::melt Rdatatable/data.table#5044 (comment)

[tdhock]

actually the is.na with base R is correct to not check the is.na.integer64 method, according to the documentation page ?is.na:

     For is.na, elementwise the result is false unless that element
     is a length-one atomic vector and the single element of that
     vector is regarded as NA or NaN (note that any is.na method
     for the class of the element is ignored).