setpos(): Set lowercase mark in other buffers (#5753)

Also make setpos("'A", [999, 1, 1, 0]) fail, i.e. return -1 (assuming there is no buffer 999).

Fixes #5713

Background:

`:help setpos()` mentions an argument `"bufnum"` that determines the buffer a mark should be put in.
This argument is respected for uppercase marks, but not for lowercase marks.
This is reasonable (though I personally would like `setpos()` to be able to set marks in other buffers), but the help doesn't mention this anywhere.

It's also strange that attempting to change buffers with `setpos('.', [bufnr('#'), 1, 1, 0])` alerts the user that having a different buffer is an error, while attempting to set a mark with `setpos("'d", [bufnr('#'), 1, 1, 0])` doesn't tell the user that the `"bufnum"` argument is an error.

1 files changed, 7 insertions, 1 deletions

diff --git a/src/nvim/mark.c b/src/nvim/mark.c
index bb5b8e8178..4e05845eb5 100644
--- a/src/nvim/mark.c
+++ b/src/nvim/mark.c
@@ -130,9 +130,15 @@ int setmark_pos(int c, pos_T *pos, int fnum)
     return OK;
   }
 
+  buf_T *buf = buflist_findnr(fnum);
+  // Can't set a mark in a non-existant buffer.
+  if (buf == NULL) {
+    return FAIL;
+  }
+
   if (ASCII_ISLOWER(c)) {
     i = c - 'a';
-    RESET_FMARK(curbuf->b_namedm + i, *pos, curbuf->b_fnum);
+    RESET_FMARK(buf->b_namedm + i, *pos, fnum);
     return OK;
   }
   if (ASCII_ISUPPER(c) || ascii_isdigit(c)) {