Adding difftime to date truncates the difftime to full dates

NEWS.md

@@ -5,3 +5,5 @@
 * Character indexing is now only supported for named objects, an error is raised for unnamed objects (#171).
 
 * `vec_c()` and `vec_rbind()` now handle data.frame columns properly (@yutannihilation, #182).
+
+* `vec_arith()` is consistent with base R when combining `difftime` and `date`, with a warning if casts are lossy (#192).

R/cast.R

@@ -354,3 +354,8 @@ report_lossy_cast <- function(x, y, lossy, details = NULL) {
   }
 }
 
+lossy_floor <- function(x, to) {
+  x_floor <- floor(x)
+  report_lossy_cast(x, to, x != x_floor)
+  x_floor
+}

R/type-date-time.R

@@ -427,12 +427,11 @@ vec_arith.difftime.POSIXct <- function(op, x, y) {
 #' @method vec_arith.Date difftime
 #' @export
 vec_arith.Date.difftime <- function(op, x, y) {
-  # Need to warn if non-integer number of days
   y <- vec_cast(y, new_duration(units = "days"))
 
   switch(op,
-    `+` = vec_restore(vec_arith_base(op, x, y), x),
-    `-` = vec_restore(vec_arith_base(op, x, y), x),
+    `+` = ,
+    `-` = vec_restore(vec_arith_base(op, x, lossy_floor(y, x)), x),
     stop_incompatible_op(op, x, y)
   )
 }
@@ -442,7 +441,7 @@ vec_arith.difftime.Date <- function(op, x, y) {
   x <- vec_cast(x, new_duration(units = "days"))
 
   switch(op,
-    `+` = vec_restore(vec_arith_base(op, x, y), y),
+    `+` = vec_restore(vec_arith_base(op, lossy_floor(x, y), y), y),
     stop_incompatible_op(op, x, y)
   )
 }

tests/testthat/test-type-date-time.R

@@ -207,14 +207,22 @@ test_that("date-time vs difftime", {
   d <- as.Date("2018-01-01")
   dt <- as.POSIXct(d)
   t <- as.difftime(1, units = "days")
+  th <- as.difftime(c(1, 24), units = "hours")
 
   expect_equal(vec_arith("+", dt, t), dt + t)
   expect_equal(vec_arith("+", d, t), d + t)
+  expect_equal(vec_arith("+", dt, th), dt + th)
+  expect_warning(expect_equal(vec_arith("+", d, th), d + th), class = "warning_lossy_cast")
   expect_equal(vec_arith("-", dt, t), dt - t)
   expect_equal(vec_arith("-", d, t), d - t)
+  expect_equal(vec_arith("-", dt, th), dt - th)
+  expect_warning(expect_equal(vec_arith("-", d, th), d - th), class = "warning_lossy_cast")
 
   expect_equal(vec_arith("+", t, dt), dt + t)
   expect_equal(vec_arith("+", t, d), d + t)
+  expect_equal(vec_arith("+", th, dt), dt + th)
+  expect_warning(expect_equal(vec_arith("+", th, d), d + th), class = "warning_lossy_cast")
+
   expect_error(vec_arith("-", t, dt), class = "vctrs_error_incompatible_op")
   expect_error(vec_arith("-", t, d), class = "vctrs_error_incompatible_op")
 })