Fixing crash when attempting to join on character(0)

NEWS.md

@@ -107,6 +107,8 @@ unit = "s")
 
 12. `rbindlist` no longer errors when coercing complex vectors to character vectors, [#4202](https:/Rdatatable/data.table/issues/4202). Thanks to @sritchie73 for reporting and the PR.
 
+13. `X[Y, on=character(0)]` and `merge(X, Y, by.x=character(0), by.y=character(0))` no longer crash, [#4272](https:/Rdatatable/data.table/pull/4272). Thanks to @tlapak for the PR.
+
 ## NOTES
 
 0. Retrospective license change permission was sought from and granted by 4 contributors who were missed in [PR#2456](https:/Rdatatable/data.table/pull/2456), [#4140](https:/Rdatatable/data.table/pull/4140). We had used [GitHub's contributor page](https:/Rdatatable/data.table/graphs/contributors) which omits 3 of these due to invalid email addresses, unlike GitLab's contributor page which includes the ids. The 4th omission was a PR to a script which should not have been excluded; a script is code too. We are sorry these contributors were not properly credited before. They have now been added to the contributors list as displayed on CRAN. All the contributors of code to data.table hold its copyright jointly; your contributions belong to you. You contributed to data.table when it had a particular license at that time, and you contributed on that basis. This is why in the last license change, all contributors of code were consulted and each had a veto.

R/data.table.R

@@ -3031,7 +3031,7 @@ isReallyReal = function(x) {
  onsub = as.call(c(quote(c), onsub))
  }
  on = eval(onsub, parent.frame(2L), parent.frame(2L))
- if (!is.character(on))
+ if (length(on) == 0L || !is.character(on))
  stop("'on' argument should be a named atomic vector of column names indicating which columns in 'i' should be joined with which columns in 'x'.")
  ## extract the operators and potential variable names from 'on'.
  ## split at backticks to take care about variable names like `col1<=`.

R/merge.R

@@ -21,8 +21,8 @@ merge.data.table = function(x, y, by = NULL, by.x = NULL, by.y = NULL, all = FAL
  if (!missing(by) && !missing(by.x))
  warning("Supplied both `by` and `by.x/by.y`. `by` argument will be ignored.")
  if (!is.null(by.x)) {
- if ( !is.character(by.x) || !is.character(by.y))
- stop("A non-empty vector of column names are required for `by.x` and `by.y`.")
+ if (length(by.x) == 0L || !is.character(by.x) || !is.character(by.y))
+ stop("A non-empty vector of column names is required for `by.x` and `by.y`.")
  if (!all(by.x %chin% names(x)))
  stop("Elements listed in `by.x` must be valid column names in x.")
  if (!all(by.y %chin% names(y)))

inst/tests/tests.Rraw

@@ -15869,7 +15869,7 @@ test(2074.31, dcast(DT, V1 ~ z, fun.aggregate=eval(quote(length)), value.var='z'
 test(2074.32, fwrite(DT, logical01=TRUE, logicalAsInt=TRUE), error="logicalAsInt has been renamed")
 
 # merge.data.table
-test(2074.33, merge(DT, DT, by.x = 1i, by.y=1i), error="A non-empty vector of column names are required")
+test(2074.33, merge(DT, DT, by.x = 1i, by.y=1i), error="A non-empty vector of column names is required")
 
 # shift naming
 test(2074.34, shift(list(a=1:5, b=6:10), give.names=TRUE), list(a_lag_1=c(NA, 1:4), b_lag_1=c(NA, 6:9)))
@@ -16846,3 +16846,10 @@ A = data.table(A=c(complex(real = 1:3, imaginary=c(0, -1, 1)), NaN))
 test(2138.3, rbind(A,B), data.table(A=c(as.character(A$A), B$A)))
 A = data.table(A=as.complex(rep(NA, 5)))
 test(2138.4, rbind(A,B), data.table(A=c(as.character(A$A), B$A)))
+
+# Attempting to join on character(0) shouldn't crash R
+A = data.table(A='a')
+B = data.table(B='b')
+test(2139.1, A[B, on=character(0)], error = "'on' argument should be a named atomic vector")
+test(2139.2, merge(A, B, by=character(0) ), error = "non-empty vector of column names for `by` is required.")
+test(2139.3, merge(A, B, by.x=character(0), by.y=character(0)), error = "non-empty vector of column names is required")