Recycle names in map2() (#783)

Closes #779

NEWS.md

@@ -11,6 +11,9 @@
 
 ## Features and fixes
 
+* `map2()` and `pmap()` now recycle names of their first input if
+  needed (#783).
+
 * `every()` now correctly propagates missing values using the same
   rules as `&&` (#751). Internally, it has become a wrapper around
   `&&`. This makes it consistent with `&&` and also with `some()`

src/map.c

@@ -7,14 +7,20 @@
 #include "utils.h"
 
 void copy_names(SEXP from, SEXP to) {
-  if (Rf_length(from) != Rf_length(to))
-    return;
-
   SEXP names = Rf_getAttrib(from, R_NamesSymbol);
-  if (Rf_isNull(names))
+  if (names == R_NilValue) {
     return;
+  }
+
+  R_len_t n = Rf_length(to);
+
+  if (Rf_length(names) != n) {
+    names = short_vec_recycle(names, n);
+  }
+  PROTECT(names);
 
   Rf_setAttrib(to, R_NamesSymbol, names);
+  UNPROTECT(1);
 }
 
 void check_vector(SEXP x, const char *name) {

tests/testthat/test-map2.R

@@ -57,3 +57,14 @@ test_that("map2() with empty input copies names", {
   expect_identical(map2_chr(named_list, list(), identity), named(chr()))
   expect_identical(map2_raw(named_list, list(), identity), named(raw()))
 })
+
+test_that("map2() and pmap() recycle names (#779)", {
+  expect_identical(
+    map2(c(a = 1), 1:2, ~ .x),
+    list(a = 1, a = 1)
+  )
+  expect_identical(
+    pmap(list(c(a = 1), 1:2), ~ .x),
+    list(a = 1, a = 1)
+  )
+})