2 files changed, 8 insertions, 3 deletions
diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index 787d5aaf78..e090f3b37f 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -199,7 +199,7 @@ int call_internal_method(const char_u *const fname, const int argcount,
   FUNC_ATTR_NONNULL_ALL
 {
   const VimLFuncDef *const fdef = find_internal_func((const char *)fname);
-  if (fdef == NULL || fdef->base_arg == 0) {
+  if (fdef == NULL || fdef->base_arg == BASE_NONE) {
     return ERROR_UNKNOWN;
   } else if (argcount + 1 < fdef->min_argc) {
     return ERROR_TOOFEW;
@@ -208,7 +208,8 @@ int call_internal_method(const char_u *const fname, const int argcount,
   }
 
   typval_T argv[MAX_FUNC_ARGS + 1];
-  const ptrdiff_t base_index = fdef->base_arg - 1;
+  const ptrdiff_t base_index
+      = fdef->base_arg == BASE_LAST ? argcount : fdef->base_arg - 1;
   memcpy(argv, argvars, base_index * sizeof(typval_T));
   argv[base_index] = *basetv;
   memcpy(argv + base_index + 1, argvars + base_index,
diff --git a/src/nvim/eval/funcs.h b/src/nvim/eval/funcs.h
index 3ad0b8282a..c6a0cb959e 100644
--- a/src/nvim/eval/funcs.h
+++ b/src/nvim/eval/funcs.h
@@ -9,12 +9,16 @@ typedef void (*FunPtr)(void);
 /// Prototype of C function that implements VimL function
 typedef void (*VimLFunc)(typval_T *args, typval_T *rvar, FunPtr data);
 
+/// Special flags for base_arg @see VimLFuncDef
+#define BASE_NONE 0          ///< Not a method (no base argument).
+#define BASE_LAST UINT8_MAX  ///< Use the last argument as the method base.
+
 /// Structure holding VimL function definition
 typedef struct fst {
   char *name;        ///< Name of the function.
   uint8_t min_argc;  ///< Minimal number of arguments.
   uint8_t max_argc;  ///< Maximal number of arguments.
-  uint8_t base_arg;  ///< Method base arg # (1-indexed), or 0 if not a method.
+  uint8_t base_arg;  ///< Method base arg # (1-indexed), BASE_NONE or BASE_LAST.
   VimLFunc func;     ///< Function implementation.
   FunPtr data;       ///< Userdata for function implementation.
 } VimLFuncDef;