neovim/test/functional/plugin/lsp/handler_spec.lua

local helpers = require('test.functional.helpers')(after_each)

local eq = helpers.eq
local exec_lua = helpers.exec_lua
local pcall_err = helpers.pcall_err
local matches = helpers.matches

describe('lsp-handlers', function()
  describe('vim.lsp._with_extend', function()
    it('should return a table with the default keys', function()
      eq(
        { hello = 'world' },
        exec_lua [[
        return vim.lsp._with_extend('test', { hello = 'world' })
      ]]
      )
    end)

    it('should override with config keys', function()
      eq(
        { hello = 'universe', other = true },
        exec_lua [[
        return vim.lsp._with_extend('test', { other = true, hello = 'world' }, { hello = 'universe' })
      ]]
      )
    end)

    it('should not allow invalid keys', function()
      matches(
        '.*Invalid option for `test`.*',
        pcall_err(
          exec_lua,
          "return vim.lsp._with_extend('test', { hello = 'world' }, { invalid = true })"
        )
      )
    end)
  end)
end)