diff --git a/NEWS.md b/NEWS.md
index 11a6e6999..14f676a86 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -321,6 +321,8 @@
 
 42. `DT[...,by={...}]` now handles expressions in `{`, [#3156](https://github.com/Rdatatable/data.table/issues/3156). Thanks to @tdhock for the report.
 
+43. `:=` could change a `data.table` creation statement in the body of the function calling it, or a variable in calling scope, [#3890](https://github.com/Rdatatable/data.table/issues/3890). Many thanks to @kirillmayantsev for the detailed reports.
+
 ## NOTES
 
 1. `rbindlist`'s `use.names="check"` now emits its message for automatic column names (`"V[0-9]+"`) too, [#3484](https://github.com/Rdatatable/data.table/pull/3484). See news item 5 of v1.12.2 below.
diff --git a/R/as.data.table.R b/R/as.data.table.R
index c077361be..71cbc310b 100644
--- a/R/as.data.table.R
+++ b/R/as.data.table.R
@@ -161,15 +161,16 @@ as.data.table.list = function(x,
       # TODO: port this as.data.table.list() to C and use MAYBE_REFERENCED(x) || ALTREP(x) to save some copies.
       #       That saving used to be done by CcopyNamedInList but the copies happened again as well, so removing CcopyNamedInList is
       #       not worse than before, and gets us in a better centralized place to port as.data.table.list to C and use MAYBE_REFERENCED
-      #       again in future.
+      #       again in future, for #617.
     }
     if (identical(x,list())) vector("list", nrow) else rep(x, length.out=nrow)   # new objects don't need copy
   }
   vnames = character(ncol)
   k = 1L
+  n_null = 0L
   for(i in seq_len(n)) {
     xi = x[[i]]
-    if (is.null(xi)) next
+    if (is.null(xi)) { n_null = n_null+1L; next }
     if (eachnrow[i]>1L && nrow%%eachnrow[i]!=0L)   # in future: eachnrow[i]!=nrow
       warning("Item ", i, " has ", eachnrow[i], " rows but longest item has ", nrow, "; recycled with remainder.")
     if (eachnrow[i]==0L && nrow>0L && is.atomic(xi))   # is.atomic to ignore list() since list() is a common way to initialize; let's not insist on list(NULL)
@@ -183,7 +184,7 @@ as.data.table.list = function(x,
       }
     } else {
       nm = names(x)[i]
-      vnames[k] = if (length(nm) && !is.na(nm) && nm!="") nm else paste0("V",i)  # i (not k) tested by 2058.14 to be the same as the past for now
+      vnames[k] = if (length(nm) && !is.na(nm) && nm!="") nm else paste0("V",i-n_null)  # i (not k) tested by 2058.14 to be the same as the past for now
       ans[[k]] = recycle(xi, nrow)
       k = k+1L
     }
diff --git a/R/data.table.R b/R/data.table.R
index 5ad029232..a3ae2cc0d 100644
--- a/R/data.table.R
+++ b/R/data.table.R
@@ -1281,24 +1281,11 @@ replace_dot_alias = function(e) {
           ))
         jval = lapply(jval, `[`, 0L)
       if (is.atomic(jval)) {
-        setattr(jval,"names",NULL)
-        jval = data.table(jval) # TO DO: should this be setDT(list(jval)) instead?
-      } else {
-        if (is.null(jvnames)) jvnames=names(jval)
-        lenjval = vapply(jval, length, 0L)
-        nulljval = vapply(jval, is.null, FALSE)
-        if (lenjval[1L]==0L || any(lenjval != lenjval[1L])) {
-          jval = as.data.table.list(jval)   # does the vector expansion to create equal length vectors, and drops any NULL items
-          jvnames = jvnames[!nulljval] # fix for #1477
-        } else {
-          # all columns same length and at least 1 row; avoid copy. TODO: remove when as.data.table.list is ported to C
-          setDT(jval)
-        }
+        setattr(jval,"names",NULL)  # discard names of named vectors otherwise each cell in the column would have a name
+        jval = list(jval)
       }
-      if (is.null(jvnames)) jvnames = character(length(jval)-length(bynames))
-      ww = which(jvnames=="")
-      if (any(ww)) jvnames[ww] = paste0("V",ww)
-      setnames(jval, jvnames)
+      if (!is.null(jvnames) && !all(jvnames=="")) setattr(jval, 'names', jvnames)  # e.g. jvnames=="N" for DT[,.N,]
+      jval = as.data.table.list(jval, .named=NULL)
     }
 
     if (is.data.table(jval)) {
diff --git a/inst/tests/tests.Rraw b/inst/tests/tests.Rraw
index c80a64b83..ce766eb14 100644
--- a/inst/tests/tests.Rraw
+++ b/inst/tests/tests.Rraw
@@ -15149,6 +15149,9 @@ L = list(1:3, 4:6, 7:9, 10:12)
 names(L) = c("","foo","","foo")
 test(2058.17, as.data.table(L),
               setnames(data.table(1:3, 4:6, 7:9, 10:12),c("","foo","","foo")))  # no
+L = list(1:3, NULL, 4:6)
+test(2058.18, length(L), 3L)
+test(2058.19, as.data.table(L), data.table(V1=1:3, V2=4:6))  # V2 not V3        # no
 
 # rbindlist improved error message, #3638
 DT = data.table(a=1)
@@ -15823,20 +15826,36 @@ test(2086.05, DT[ , sum(a), by = character()], data.table(V1=55L))
 test(2086.06, DT[ , sum(a), keyby = character()], data.table(V1=55L))
 
 # simple queries can create tables with columns sharing the same address, #3766
+# these tests were new in 1.12.4 dev. In late stage before release 2087.1 was changed to avoid the share for #3890
+# when #617 is done it will change back to being the same address
 x = data.table(a=1L, b=c(1L, 4L, 2L, 3L), c=4:1)
-test(2087.1, x[a == 1L, .(b, b2=b)][ , identical(address(b), address(b2))])
+test(2087.01, x[a == 1L, .(b, b2=b)][ , address(b)!=address(b2)])
 # setkey detects and copies shared address columns, #3496
 x = data.frame(a=paste0(2:1), stringsAsFactors=FALSE)
 x$b = x$a
 setDT(x)
-test(2087.2, setkey(x, a, verbose=TRUE), data.table(a=paste0(1:2), b=paste0(1:2), key="a"),
-             output='Found and copied 1 column with a shared memory address')
+test(2087.02, setkey(x, a, verbose=TRUE), data.table(a=paste0(1:2), b=paste0(1:2), key="a"),
+              output='Found and copied 1 column with a shared memory address')
 x = data.frame(a=paste0(2:1), stringsAsFactors=FALSE)
 x$b = x$a
 x$c = x$a
 setDT(x)
-test(2087.3, setkey(x, a, verbose=TRUE), data.table(a=paste0(1:2), b=paste0(1:2), c=paste0(1:2), key="a"),
-             output='Found and copied 2 columns with a shared memory address')
+test(2087.03, setkey(x, a, verbose=TRUE), data.table(a=paste0(1:2), b=paste0(1:2), c=paste0(1:2), key="a"),
+              output='Found and copied 2 columns with a shared memory address')
+# follow-up from #3890; function body and variable in calling scope
+f = function(flag=FALSE) {
+  dt1 = data.table(a = 1)
+  dt2 = data.table(a = 1)
+  dt3 = dt1[, .(a, b = 0)]  # (**)
+  if (flag) dt3[dt2, b := 999, on = "a"]
+  gsub(" ","",as.character(body(f)[[4L]]))  # (**) above; remove spaces just to isolate from potential future formatting changes in R
+}
+test(2087.10, f(TRUE), c("=","dt3","dt1[,.(a,b=0)]"))  # was "dt1[,.(a,b=999)]" in v1.12.2
+value = 0
+dt1 = data.table(a = 1)
+dt2 = dt1[, .(a, b = ..value)]
+dt2[1, b := 999]
+test(2087.11, value, 0)  # was 999 in v1.12.2
 
 # clear '.data.table.locked' even when is.null(irows), #2245
 x = data.table(a=c(0.85, -0.38, 1.19), b=c(0.56, 0.63, -1.30))