fix(diagnostic): vim.diagnostic.setqflist() opens loclist on first call #31585

Problem:
Regression from de794f2d24: `vim.diagnostic.setqflist{open=true}` attempts to
open the location list instead of the diagnostics quickfix list if it didn't
exist before. This is because we are using `qf_id` to decide which to open, but
`qf_id=nil` when there is no existing diagnostics quickfix list with a given
title ("Diagnostics" by default).

Solution:
- Revert to using `loclist` to decide which to open.
- Add tests.

runtime/lua/vim/diagnostic.lua

@@ -871,7 +871,7 @@ local function set_list(loclist, opts)
   end
 
   if open then
-    if qf_id then
+    if not loclist then
       -- First navigate to the diagnostics quickfix list.
       local nr = vim.fn.getqflist({ id = qf_id, nr = 0 }).nr
       api.nvim_command(nr .. 'chistory')

test/functional/lua/diagnostic_spec.lua

@@ -5,6 +5,7 @@ local command = n.command
 local clear = n.clear
 local exec_lua = n.exec_lua
 local eq = t.eq
+local neq = t.neq
 local matches = t.matches
 local api = n.api
 local pcall_err = t.pcall_err
@@ -3212,6 +3213,74 @@ describe('vim.diagnostic', function()
     end)
   end)
 
+  describe('setqflist()', function()
+    it('updates existing diagnostics quickfix if one already exists', function()
+      local result = exec_lua(function()
+        vim.api.nvim_win_set_buf(0, _G.diagnostic_bufnr)
+
+        vim.fn.setqflist({}, ' ', { title = 'Diagnostics' })
+        local diagnostics_qf_id = vim.fn.getqflist({ id = 0 }).id
+
+        vim.diagnostic.setqflist({ title = 'Diagnostics' })
+        local qf_id = vim.fn.getqflist({ id = 0, nr = '$' }).id
+
+        return { diagnostics_qf_id, qf_id }
+      end)
+
+      eq(result[1], result[2])
+    end)
+
+    it('navigates to existing diagnostics quickfix if one already exists and open=true', function()
+      local result = exec_lua(function()
+        vim.api.nvim_win_set_buf(0, _G.diagnostic_bufnr)
+
+        vim.fn.setqflist({}, ' ', { title = 'Diagnostics' })
+        local diagnostics_qf_id = vim.fn.getqflist({ id = 0 }).id
+
+        vim.fn.setqflist({}, ' ', { title = 'Other' })
+
+        vim.diagnostic.setqflist({ title = 'Diagnostics', open = true })
+        local qf_id = vim.fn.getqflist({ id = 0 }).id
+
+        return { diagnostics_qf_id, qf_id }
+      end)
+
+      eq(result[1], result[2])
+    end)
+
+    it('sets new diagnostics quickfix as active when open=true', function()
+      local result = exec_lua(function()
+        vim.api.nvim_win_set_buf(0, _G.diagnostic_bufnr)
+
+        vim.fn.setqflist({}, ' ', { title = 'Other' })
+        local other_qf_id = vim.fn.getqflist({ id = 0 }).id
+
+        vim.diagnostic.setqflist({ title = 'Diagnostics', open = true })
+        local qf_id = vim.fn.getqflist({ id = 0 }).id
+
+        return { other_qf_id, qf_id }
+      end)
+
+      neq(result[1], result[2])
+    end)
+
+    it('opens quickfix window when open=true', function()
+      local qf_winid = exec_lua(function()
+        vim.api.nvim_win_set_buf(0, _G.diagnostic_bufnr)
+
+        vim.diagnostic.set(_G.diagnostic_ns, _G.diagnostic_bufnr, {
+          _G.make_error('Error', 1, 1, 1, 1),
+        })
+
+        vim.diagnostic.setqflist({ open = true })
+
+        return vim.fn.getqflist({ winid = 0 }).winid
+      end)
+
+      neq(0, qf_winid)
+    end)
+  end)
+
   describe('match()', function()
     it('matches a string', function()
       local msg = 'ERROR: george.txt:19:84:Two plus two equals five'