Steps to replicate the issue (include links if applicable):

• go to a function using a typed list as an input type, eg. https://wikifunctions.beta.wmflabs.org/view/en/Z11416
• click to Details
• click on Add test, https://wikifunctions.beta.wmflabs.org/w/index.php?title=Special:CreateObject&uselang=en&zid=Z20&Z20K1=Z11416
• click on Call Function under Call
• enter "concatenate list" and select the function
• click on the + / Plus button

What happens?:
It asks for a type

Since the input is a Typed List of String, this should just let us enter strings.

But ignoring that, I try to enter String in Select a type, and when I select it, it gives me a field for value, but leave an empty field in type.

If I then add a value and select a result validation function "string equality" and then enter a value there, and try to save it, I get a "Not wellformed" error on publishing

What should have happened instead?:

• ideally, it should not let me select the type but use the fixed type of string for the elements
• it should let me choose a type for the element

Software version (skip for WMF-hosted wikis like Wikipedia):

Other information (browser name/version, screenshots, etc.):

This also applies when creating an implementation, or using it when running a function, e.g. here: https://wikifunctions.beta.wmflabs.org/view/en/Z11416