lsp: Accept text document edits with version zero

There were a couple of reports of "Buffer X newer than edits" problems.
We first assumed that it is incorrect for a server to send 0 as a
version - and stated that they should send a `null` instead, given that
in the specification the `textDocument` of a `TextDocumentEdit` is a
`OptionalVersionedTextDocumentIdentifier`.

But it turns out that this was a change in 3.16, and in 3.15 and earlier
versions of the specification it was a `VersionedTextDocumentIdentifier`
and language servers didn't have a better option than sending `0` if
they don't keep track of the version numbers.

So this changes the version check to always accept `0` values.

See

- https://github.com/neovim/neovim/issues/12970
- https://github.com/neovim/neovim/issues/14256
- https://github.com/haskell/haskell-language-server/pull/1727

runtime/lua/vim/lsp/util.lua

@@ -470,6 +470,8 @@ function M.apply_text_document_edit(text_document_edit, index)
   -- `VersionedTextDocumentIdentifier`s version may be null
   --  https://microsoft.github.io/language-server-protocol/specification#versionedTextDocumentIdentifier
   if should_check_version and (text_document.version
+      and text_document.version ~= vim.NIL
+      and text_document.version > 0
       and M.buf_versions[bufnr]
       and M.buf_versions[bufnr] > text_document.version) then
     print("Buffer ", text_document.uri, " newer than edits.")

test/functional/plugin/lsp_spec.lua

@@ -1124,6 +1124,19 @@ describe('LSP', function()
         '2nd line of 语text';
       }, buf_lines(target_bufnr))
     end)
+    it('always accepts edit with version = 0', function()
+      exec_lua([[
+        local args = {...}
+        local bufnr = select(1, ...)
+        local text_edit = select(2, ...)
+        vim.lsp.util.buf_versions[bufnr] = 10
+        vim.lsp.util.apply_text_document_edit(text_edit)
+      ]], target_bufnr, text_document_edit(0))
+      eq({
+        'First ↥ 🤦 🦄 line of text';
+        '2nd line of 语text';
+      }, buf_lines(target_bufnr))
+    end)
     it('correctly goes ahead with the edit if the version is vim.NIL', function()
       -- we get vim.NIL when we decode json null value.
       local json = exec_lua[[