Fix highlighting for function return types when highlight_function_arguments is enabled

[pocke]

Problem

vim-go does not highlight function return types with brackets correctly when g:go_highlight_function_arguments = 1 is setted.

expected highlight

actual highlight

vim-go does not highlight the returned types of b function, which are string and error.
It should be highlighted as goType syntax group (green color in this color scheme), but they actually have goSimpleArguments syntax group.

note: If let g:go_highlight_function_arguments = 0, it is highlighted below.

configuration to reproduce

test.vim. Need to replace /path/to/vim-go/ with the actual path.

let g:go_highlight_function_arguments = 1
set nocompatible
set rtp^=/path/to/vim-go/
syntax on

test.go

package main

type T struct{}

func a() error {
	return nil
}
func b(x int) (string, error) {
	return "", nil
}
func c(y bool, z T) (str string, err error) {
	return
}

$ vim --clean -u test.vim test.go

cause

goSImpleArtuments syntax matching definition has a problem.
It matches a function argument (e.g. (a int), (a int, b bool)).

it specifys nextgroup=goSimpleArguments, it means the group repeats itself. Return types with bracktes are concealed by this nextgroup definition.

func b(x int) (string, error) {
  //  ^^^^^^^                 goSimpleArguments
  //          ^^^^^^^^^^^^^^^ repeated goSimpleArguments

I'm not sure why the nextgroup is specified. I think it's unnecessary because I guess go syntax does not allow an argument list after an argument list.
If someone knows the reasons of the nextgroup, please tell me it 🙏

[bhcleek]

Thank you for contributing 🙇

I think it's unnecessary because I guess go syntax does not allow an argument list after an argument list

Without this, named return values may not be highlighted correctly. Perhaps there's an additional fix needed to highlight named return values correctly 🤔 ref: #1587

Before this PR:

This PR:

[bhcleek]

I've come up with an additional fix to address the named return value situation; you can leave this PR as-is, and I'll build on it.

Thanks again for contributing!