Which example shows that the `dict` argument is not allowed when `{name}` is a partial, inside a `function()` call?

Question

From :h function(:

    {name} can also be a Funcref or a partial.  When it is a
    partial the dict stored in it will be used and the {dict}
    argument is not allowed. E.g.:
        let FuncWithArg = function(dict.Func, [arg])
        let Broken = function(dict.Func, [arg], dict)

The examples are incomplete, so I tried this:

let dict = { 'name': 'trees' }
fu! dict.Func(n)
    echo a:n.' '.self.name
endfu
let FuncWithArg = function(dict.Func, [3])
call FuncWithArg()

It works as expected and displays 3 trees.

Then, I tried this:

let Broken = function(dict.Func, [3], dict)
call Broken()

It works too, and displays 3 trees. But if this example is really equivalent to the one given in the help, it shouldn't work, since the help says that the {dict} argument is not allowed, when {name} is a partial.

I think the reason why it works, while it shouldn't, is because in my example, dict.Func isn't a partial.

---

So, I tried to rewrite these 2 examples, so that dict.Func is a partial:

fu! MyFunc(i,j) dict
    echo (a:i + a:j).' '. self.name
endfu

let dict = { 'name': 'trees', 'Func': function('MyFunc', [1]) }

let FuncWithArg = function(dict.Func, [2])
call FuncWithArg()

let Broken = function(dict.Func, [2], dict)
call Broken()

Both commands, call FuncWithArg() and call Broken(), work and display 3 trees.

---

But again, the 2nd one shouldn't work according to the {dict} argument is not allowed. So, I thought that maybe what the help meant was not:

let Broken = function(dict.Func, [arg], dict)

… but:

let Broken = function(dict.Func, [arg], other_dict)

So, I tried to find another example where dict is replaced with other_dict:

fu! MyFunc(i,j) dict
    echo (a:i + a:j).' '. self.name
endfu

let dict       = { 'name': 'trees', 'Func': function('MyFunc', [1]) }
let other_dict = { 'name': 'mountains', 'Func': function('MyFunc', [1]) }

let FuncWithArg = function(dict.Func, [2])
call FuncWithArg()

let Broken = function(dict.Func, [2], other_dict)
call Broken()

Both commands, call FuncWithArg() and call Broken(), work and display 3 trees and 3 mountains respectively.

I haven't been able to come up with an example where call Broken() would raise an error. Can someone provide one?

Answer 1

I don't think there is such an example. The help for function() is the same as the help for three errors: E700, E922 and E923. None of these refer to a situation like the one the text seems to describe:

• E700: Unknown function

  :call function('six',[1],[1])
  E700: Unknown function: six
  

• E922: expected a dict

  :call function('sin',[1],[1])
  E922: expected a dict
  

• E923: Second argument of function() must be a list or a dict

  :call function('sin',1)
  E923: Second argument of function() must be a list or a dict
  

Since no other errors are listed for function(), I think the help is probably describing something like C's undefined behaviour rather than an explicit error.