Don't allow casting to non-bare data frames

Author: lionel-

R/cast.R

@@ -209,11 +209,13 @@ vec_default_cast <- function(x,
       return(out)
     }
 
-    # Bare-class fallback for data frames
-    if (inherits(to, "tbl_df")) {
-      out <- df_as_tibble(out)
+    if (is_bare_df(to)) {
+      # Bare-class fallback for data frames
+      if (inherits(to, "tbl_df")) {
+        out <- df_as_tibble(out)
+      }
+      return(out)
     }
-    return(out)
   }
 
   if (is_same_type(x, to)) {

tests/testthat/_snaps/cast.md

@@ -62,6 +62,15 @@
       Error:
       ! Can't combine `..1$a$b` <factor<c1562>> and `..2$a$b` <double>.
 
+# vec_cast() only falls back when casting to base type
+
+    Code
+      (expect_error(vec_cast(mtcars, foobar(mtcars))))
+    Output
+      <error/vctrs_error_cast>
+      Error:
+      ! Can't convert `mtcars` <data.frame> to <vctrs_foobar>.
+
 # vec_cast() only attempts to fall back if `to` is a data frame (#1568)
 
     Code

tests/testthat/_snaps/type-misc.md

@@ -1,3 +1,12 @@
+# data.table and tibble do not have a common type
+
+    Code
+      (expect_error(vec_cast(tibble(y = 2), data.table(x = TRUE, y = 1L))))
+    Output
+      <error/vctrs_error_cast>
+      Error:
+      ! Can't convert `tibble(y = 2)` <tibble> to <data.table>.
+
 # data table has formatting methods
 
     Code

tests/testthat/test-cast.R

@@ -92,9 +92,11 @@ test_that("unspecified can be cast to shaped vectors", {
   expect_identical(out, exp)
 })
 
-test_that("vec_cast() falls back to base class even when casting to non-base type", {
+test_that("vec_cast() only falls back when casting to base type", {
   expect_equal(vec_cast(foobar(mtcars), mtcars), mtcars)
-  expect_equal(vec_cast(mtcars, foobar(mtcars)), mtcars)
+  expect_snapshot({
+    (expect_error(vec_cast(mtcars, foobar(mtcars))))
+  })
 })
 
 test_that("vec_cast() only attempts to fall back if `to` is a data frame (#1568)", {

tests/testthat/test-type-misc.R

@@ -77,10 +77,10 @@ test_that("data.table and tibble do not have a common type", {
     vec_cast(data.table(y = 2), tibble(x = TRUE, y = 1L)),
     tibble(x = lgl(NA), y = 2L)
   )
-  expect_identical(
-    vec_cast(tibble(y = 2), data.table(x = TRUE, y = 1L)),
-    data_frame(x = lgl(NA), y = 2L)
-  )
+
+  expect_snapshot({
+    (expect_error(vec_cast(tibble(y = 2), data.table(x = TRUE, y = 1L))))
+  })
 })
 
 test_that("data table has formatting methods", {