when as.data.frame dispatches to list method, force input to list

NEWS.md

@@ -57,6 +57,8 @@
 
 6. When `j` returns an object whose class `"X"` inherits from `data.table`; i.e. class `c("X", "data.table", "data.frame")`, the derived class `"X"` is no longer incorrectly dropped from the class of the `data.table` returned, [#4324](https://github.com/Rdatatable/data.table/issues/4324). Thanks to @HJAllen for reporting and @shrektan for the PR.
 
+7. `as.data.table()` failed with `.subset2(x, i, exact = exact): attempt to select less than one element in get1index` when passed an object inheriting from `data.table` with a different `[[` method, such as the class `dfidx` from the `dfidx` package, [#4526](https://github.com/Rdatatable/data.table/issues/4526). Thanks @RicoDiel for the report, and Michael Chirico for the PR.
+
 ## NOTES
 
 1. New feature 29 in v1.12.4 (Oct 2019) introduced zero-copy coercion. Our thinking is that requiring you to get the type right in the case of `0` (type double) vs `0L` (type integer) is too inconvenient for you the user. So such coercions happen in `data.table` automatically without warning. Thanks to zero-copy coercion there is no speed penalty, even when calling `set()` many times in a loop, so there's no speed penalty to warn you about either. However, we believe that assigning a character value such as `"2"` into an integer column is more likely to be a user mistake that you would like to be warned about. The type difference (character vs integer) may be the only clue that you have selected the wrong column, or typed the wrong variable to be assigned to that column. For this reason we view character to numeric-like coercion differently and will warn about it. If it is correct, then the warning is intended to nudge you to wrap the RHS with `as.<type>()` so that it is clear to readers of your code that a coercion from character to that type is intended. For example :

R/as.data.table.R

@@ -221,7 +221,8 @@ as.data.table.data.frame = function(x, keep.rownames=FALSE, key=NULL, ...) {
   }
   if (any(vapply_1i(x, function(xi) length(dim(xi))))) { # not is.atomic because is.atomic(matrix) is true
     # a data.frame with a column that is data.frame needs to be expanded; test 2013.4
-    return(as.data.table.list(x, keep.rownames=keep.rownames, ...))
+    # x may be a class with [[ method that behaves differently, so as.list first for default [[, #4526
+    return(as.data.table.list(as.list(x), keep.rownames=keep.rownames, ...))
   }
   ans = copy(x)  # TO DO: change this deep copy to be shallow.
   setattr(ans, "row.names", .set_row_names(nrow(x)))

inst/tests/tests.Rraw

@@ -17363,3 +17363,13 @@ A = data.table(COL = 'dt')
 class(A) = c('newclass', class(A))
 DT = data.table(LIST_COL = list(A, A))
 test(2172, class(DT[1, LIST_COL[[1]]]), class(A))
+
+# as.data.table.list edits list elements, so must be sure x does not use some other `[[` method, #4526
+x = data.frame(a = 1:5)
+x$b = matrix(6:15, ncol=2L)
+class(x) = c('foo', 'data.frame')
+`[[.foo` = function(x, i) {
+  if (any(sapply(x, inherits, 'data.table'))) stop('failure')
+  as.list(x)[[i]]
+}
+test(2173, as.data.table(x), data.table(a=1:5, b.V1=6:10, b.V2=11:15))