restore supporting data.table not inheriting from data.frame

R/data.table.R

@@ -2527,8 +2527,10 @@ copy = function(x) {
 }
 
 shallow = function(x, cols=NULL) {
-  if (!is.data.frame(x))
+  if (!is.data.frame(x) && !is.data.table(x)) {
+    #                      ^^ some revdeps do class(x)="data.table" without inheriting from data.frame, PR#5210
     stopf("x is not a data.table|frame. Shallow copy is a copy of the vector of column pointers (only), so is only meaningful for data.table|frame")
+  }
   ans = .shallow(x, cols=cols, retain.key=selfrefok(x))  # selfrefok for #5042
   ans
 }

inst/tests/tests.Rraw

@@ -640,7 +640,7 @@ test(211, ncol(TESTDT), 2L)
 DT = data.table(a=1:6,key="a")
 test(212, DT[J(3)]$a, 3L) # correct class c("data.table","data.frame")
 class(DT) = "data.table"  # incorrect class, but as from 1.8.1 it works. By accident when moving from colnames() to names(), it was dimnames() doing the check, but rather than add a check that identical(class(DT),c("data.frame","data.table")) at the top of [.data.table, we'll leave it flexible to user (user might not want to inherit from data.frame for some reason).
-test(213, DT[J(3)]$a, error="x is not a data.table|frame")  # from v1.14.2, data.table must inherit from data.frame (internals are too hard to reason if a data.table may not be data.frame too)
+test(213, DT[J(3)]$a, 3L)
 
 # setkey now auto coerces double and character for convenience, and
 # to solve bug #953
@@ -18266,3 +18266,8 @@ test(testnum+0.01, DT[, prod(l), g], error="GForce prod can only be applied to c
 # tables() error when called from inside a function(...), #5197
 test(2221, (function(...) tables())(), output = "No objects of class data.table exist")
 
+# some revdeps do class(x)="data.table" without inheriting from data.frame, PR#5210
+DT = data.table(A=1:3)
+class(DT) = "data.table"
+test(2222, print(DT), output="A.*3")
+