vim-patch:8.1.1863: confusing error when using a builtin function as method

Problem:    Confusing error when using a builtin function as method while it
            does not support that.
Solution:   Add a specific error message.
9174639a82

src/nvim/eval/funcs.c

@@ -199,8 +199,10 @@ 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 == BASE_NONE) {
+  if (fdef == NULL) {
     return ERROR_UNKNOWN;
+  } else if (fdef->base_arg == BASE_NONE) {
+    return ERROR_NOTMETHOD;
   } else if (argcount + 1 < fdef->min_argc) {
     return ERROR_TOOFEW;
   } else if (argcount + 1 > fdef->max_argc) {

src/nvim/eval/userfunc.c

@@ -1399,6 +1399,9 @@ static void user_func_error(int error, const char_u *name)
     case ERROR_UNKNOWN:
       emsg_funcname(N_("E117: Unknown function: %s"), name);
       break;
+    case ERROR_NOTMETHOD:
+      emsg_funcname(N_("E276: Cannot use function as a method: %s"), name);
+      break;
     case ERROR_DELETED:
       emsg_funcname(N_("E933: Function was deleted: %s"), name);
       break;

src/nvim/eval/userfunc.h

@@ -28,6 +28,7 @@ typedef enum {
   ERROR_OTHER,
   ERROR_BOTH,
   ERROR_DELETED,
+  ERROR_NOTMETHOD,
 } FnameTransError;
 
 /// Used in funcexe_T. Returns the new argcount.

src/nvim/testdir/test_method.vim

@@ -140,4 +140,8 @@ func Test_method_lambda()
   " call assert_fails('eval "text"->{x -> x .. " extended"}("more")', 'E99:')
 endfunc
 
+func Test_method_not_supported()
+  call assert_fails('eval 123->changenr()', 'E276:')
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab