feat(lsp): overwrite omnifunc/tagfunc set by ftplugin #22267

Problem:
Some built-in ftplugins set omnifunc/tagfunc/formatexpr which causes
lsp.lua:set_defaults() to skip setup of defaults for those filetypes.
For example the C++ ftplugin has:
    omnifunc=ccomplete#Complete
          Last set from /usr/share/nvim/runtime/ftplugin/c.vim line 30
so the changes done in #95c65a6b221fe6e1cf91e8322e7d7571dc511a71
will always be skipped for C++ files.

Solution:
Overwrite omnifunc/tagfunc/formatexpr options that were set by stock
ftplugin.

Fixes #21001

4 files changed, 153 insertions, 6 deletions

diff --git a/runtime/doc/lsp.txt b/runtime/doc/lsp.txt
index b73e2958ef..b21ef9d7d3 100644
--- a/runtime/doc/lsp.txt
+++ b/runtime/doc/lsp.txt
@@ -51,8 +51,11 @@ Starting a LSP client will automatically report diagnostics via
 |vim.diagnostic|. Read |vim.diagnostic.config()| to learn how to customize the
 display.
 
-It also sets some buffer options if the options are otherwise empty and if the
-language server supports the functionality.
+It also sets some buffer options if the language server supports the
+functionality and if the options are otherwise empty or have the default
+values set by Nvim runtime files (e.g. a ftplugin). In the latter case,
+the default values are not restored when the LSP client is detached from
+the buffer.
 
 - 'omnifunc' is set to |vim.lsp.omnifunc()|. This allows to trigger completion
   using |i_CTRL-X_CTRL-O|
diff --git a/runtime/lua/vim/lsp.lua b/runtime/lua/vim/lsp.lua
index 9a0b3f3100..cb77e9636f 100644
--- a/runtime/lua/vim/lsp.lua
+++ b/runtime/lua/vim/lsp.lua
@@ -1113,18 +1113,42 @@ function lsp.start_client(config)
   end
 
   ---@private
+  -- Determines whether the given option can be set by `set_defaults`.
+  local function is_empty_or_default(bufnr, option)
+    if vim.bo[bufnr][option] == '' then
+      return true
+    end
+
+    local old_bufnr = vim.fn.bufnr('')
+    local last_set_from = vim.fn.gettext('\n\tLast set from ')
+    local line = vim.fn.gettext(' line ')
+
+    vim.cmd.buffer(bufnr)
+    local scriptname = vim.fn
+      .execute('verbose set ' .. option .. '?')
+      :match(last_set_from .. '(.*)' .. line .. '%d+')
+    vim.cmd.buffer(old_bufnr)
+
+    if not scriptname then
+      return false
+    end
+    local vimruntime = vim.fn.getenv('VIMRUNTIME')
+    return vim.startswith(vim.fn.expand(scriptname), vim.fn.expand(vimruntime))
+  end
+
+  ---@private
   local function set_defaults(client, bufnr)
     local capabilities = client.server_capabilities
-    if capabilities.definitionProvider and vim.bo[bufnr].tagfunc == '' then
+    if capabilities.definitionProvider and is_empty_or_default(bufnr, 'tagfunc') then
       vim.bo[bufnr].tagfunc = 'v:lua.vim.lsp.tagfunc'
     end
-    if capabilities.completionProvider and vim.bo[bufnr].omnifunc == '' then
+    if capabilities.completionProvider and is_empty_or_default(bufnr, 'omnifunc') then
       vim.bo[bufnr].omnifunc = 'v:lua.vim.lsp.omnifunc'
     end
     if
       capabilities.documentRangeFormattingProvider
-      and vim.bo[bufnr].formatprg == ''
-      and vim.bo[bufnr].formatexpr == ''
+      and is_empty_or_default(bufnr, 'formatprg')
+      and is_empty_or_default(bufnr, 'formatexpr')
     then
       vim.bo[bufnr].formatexpr = 'v:lua.vim.lsp.formatexpr()'
     end
diff --git a/test/functional/fixtures/fake-lsp-server.lua b/test/functional/fixtures/fake-lsp-server.lua
index dbb66a42e8..9cc2f6fa27 100644
--- a/test/functional/fixtures/fake-lsp-server.lua
+++ b/test/functional/fixtures/fake-lsp-server.lua
@@ -932,6 +932,23 @@ function tests.basic_formatting()
   }
 end
 
+function tests.set_defaults_all_capabilities()
+  skeleton {
+    on_init = function(_)
+      return {
+        capabilities = {
+          definitionProvider = true,
+          completionProvider = true,
+          documentRangeFormattingProvider = true,
+        }
+      }
+    end;
+    body = function()
+      notify('test')
+    end;
+  }
+end
+
 -- Tests will be indexed by test_name
 local test_name = arg[1]
 local timeout = arg[2]
diff --git a/test/functional/plugin/lsp_spec.lua b/test/functional/plugin/lsp_spec.lua
index 5eb367b0b8..56c53a6fed 100644
--- a/test/functional/plugin/lsp_spec.lua
+++ b/test/functional/plugin/lsp_spec.lua
@@ -31,6 +31,11 @@ local fake_lsp_code = lsp_helpers.fake_lsp_code
 local fake_lsp_logfile = lsp_helpers.fake_lsp_logfile
 local test_rpc_server = lsp_helpers.test_rpc_server
 
+local function get_buf_option(name, bufnr)
+    bufnr = bufnr or "BUFFER"
+    return exec_lua(string.format("return vim.api.nvim_buf_get_option(%s, '%s')", bufnr, name))
+end
+
 -- TODO(justinmk): hangs on Windows https://github.com/neovim/neovim/pull/11837
 if skip(is_os('win')) then return end
 
@@ -313,6 +318,104 @@ describe('LSP', function()
       }
     end)
 
+    it('should set default options on attach', function()
+      local client
+      test_rpc_server {
+        test_name = "set_defaults_all_capabilities";
+        on_setup = function()
+          exec_lua [[
+            BUFFER = vim.api.nvim_create_buf(false, true)
+          ]]
+        end;
+        on_init = function(_client)
+          client = _client
+          exec_lua("lsp.buf_attach_client(BUFFER, TEST_RPC_CLIENT_ID)")
+        end;
+        on_handler = function(_, _, ctx)
+          if ctx.method == 'test' then
+            eq('v:lua.vim.lsp.tagfunc', get_buf_option("tagfunc"))
+            eq('v:lua.vim.lsp.omnifunc', get_buf_option("omnifunc"))
+            eq('v:lua.vim.lsp.formatexpr()', get_buf_option("formatexpr"))
+            client.stop()
+          end
+        end;
+        on_exit = function(_, _)
+          eq('', get_buf_option("tagfunc"))
+          eq('', get_buf_option("omnifunc"))
+          eq('', get_buf_option("formatexpr"))
+        end;
+      }
+    end)
+
+    it('should overwrite options set by ftplugins', function()
+      local client
+      test_rpc_server {
+        test_name = "set_defaults_all_capabilities";
+        on_setup = function()
+          exec_lua [[
+            vim.api.nvim_command('filetype plugin on')
+            BUFFER_1 = vim.api.nvim_create_buf(false, true)
+            BUFFER_2 = vim.api.nvim_create_buf(false, true)
+            vim.api.nvim_buf_set_option(BUFFER_1, 'filetype', 'man')
+            vim.api.nvim_buf_set_option(BUFFER_2, 'filetype', 'xml')
+          ]]
+          eq('v:lua.require\'man\'.goto_tag', get_buf_option("tagfunc", "BUFFER_1"))
+          eq('xmlcomplete#CompleteTags', get_buf_option("omnifunc", "BUFFER_2"))
+          eq('xmlformat#Format()', get_buf_option("formatexpr", "BUFFER_2"))
+        end;
+        on_init = function(_client)
+          client = _client
+          exec_lua("lsp.buf_attach_client(BUFFER_1, TEST_RPC_CLIENT_ID)")
+          exec_lua("lsp.buf_attach_client(BUFFER_2, TEST_RPC_CLIENT_ID)")
+        end;
+        on_handler = function(_, _, ctx)
+          if ctx.method == 'test' then
+            eq('v:lua.vim.lsp.tagfunc', get_buf_option("tagfunc", "BUFFER_1"))
+            eq('v:lua.vim.lsp.omnifunc', get_buf_option("omnifunc", "BUFFER_2"))
+            eq('v:lua.vim.lsp.formatexpr()', get_buf_option("formatexpr", "BUFFER_2"))
+            client.stop()
+          end
+        end;
+        on_exit = function(_, _)
+          eq('', get_buf_option("tagfunc", "BUFFER_1"))
+          eq('', get_buf_option("omnifunc", "BUFFER_2"))
+          eq('', get_buf_option("formatexpr", "BUFFER_2"))
+        end;
+      }
+    end)
+
+    it('should not overwrite user-defined options', function()
+      local client
+      test_rpc_server {
+        test_name = "set_defaults_all_capabilities";
+        on_setup = function()
+          exec_lua [[
+            BUFFER = vim.api.nvim_create_buf(false, true)
+            vim.api.nvim_buf_set_option(BUFFER, 'tagfunc', 'tfu')
+            vim.api.nvim_buf_set_option(BUFFER, 'omnifunc', 'ofu')
+            vim.api.nvim_buf_set_option(BUFFER, 'formatexpr', 'fex')
+          ]]
+        end;
+        on_init = function(_client)
+          client = _client
+          exec_lua("lsp.buf_attach_client(BUFFER, TEST_RPC_CLIENT_ID)")
+        end;
+        on_handler = function(_, _, ctx)
+          if ctx.method == 'test' then
+            eq('tfu', get_buf_option("tagfunc"))
+            eq('ofu', get_buf_option("omnifunc"))
+            eq('fex', get_buf_option("formatexpr"))
+            client.stop()
+          end
+        end;
+        on_exit = function(_, _)
+          eq('tfu', get_buf_option("tagfunc"))
+          eq('ofu', get_buf_option("omnifunc"))
+          eq('fex', get_buf_option("formatexpr"))
+        end;
+      }
+    end)
+
     it('should detach buffer on bufwipe', function()
       clear()
       exec_lua(create_server_definition)