fix(diagnostic): change default severity_sort order

When severity_sort is true, higher severities should be displayed before
lower severities (e.g. ERROR is displayed over WARN).

Also improved the test case for this.

runtime/doc/diagnostic.txt

@@ -239,7 +239,9 @@ config({opts}, {namespace})                          *vim.diagnostic.config()*
                                  • severity_sort: (default false) Sort
                                    diagnostics by severity. This affects the
                                    order in which signs and virtual text are
-                                   displayed. Options:
+                                   displayed. When true, higher severities are
+                                   displayed before lower severities (e.g.
+                                   ERROR is displayed before WARN). Options:
                                    • reverse: (boolean) Reverse sort order
                     {namespace}  number|nil Update the options for the given
                                  namespace. When omitted, update the global

runtime/lua/vim/diagnostic.lua

@@ -452,7 +452,9 @@ end
 ---       - update_in_insert: (default false) Update diagnostics in Insert mode (if false,
 ---                           diagnostics are updated on InsertLeave)
 ---       - severity_sort: (default false) Sort diagnostics by severity. This affects the order in
----                         which signs and virtual text are displayed. Options:
+---                         which signs and virtual text are displayed. When true, higher severities
+---                         are displayed before lower severities (e.g. ERROR is displayed before WARN).
+---                         Options:
 ---                         * reverse: (boolean) Reverse sort order
 ---@param namespace number|nil Update the options for the given namespace. When omitted, update the
 ---                            global diagnostic options.
@@ -998,9 +1000,9 @@ function M.show(namespace, bufnr, diagnostics, opts)
 
   if vim.F.if_nil(opts.severity_sort, false) then
     if type(opts.severity_sort) == "table" and opts.severity_sort.reverse then
-      table.sort(diagnostics, function(a, b) return a.severity > b.severity end)
-    else
       table.sort(diagnostics, function(a, b) return a.severity < b.severity end)
+    else
+      table.sort(diagnostics, function(a, b) return a.severity > b.severity end)
     end
   end
 

test/functional/lua/diagnostic_spec.lua

@@ -540,11 +540,11 @@ describe('vim.diagnostic', function()
     end)
 
     it('allows sorting by severity', function()
-      local result = exec_lua([[
+      exec_lua [[
         vim.diagnostic.config({
-          underline = true,
+          underline = false,
-          virtual_text = false,
+          signs = true,
-          severity_sort = false,
+          virtual_text = true,
         })
 
         vim.diagnostic.set(diagnostic_ns, diagnostic_bufnr, {
@@ -553,31 +553,41 @@ describe('vim.diagnostic', function()
           make_info('Info', 4, 4, 4, 4),
         })
 
-        local extmarks = vim.api.nvim_buf_get_extmarks(diagnostic_bufnr, diagnostic_ns, 0, -1, {details = true})
+        function get_virt_text_and_signs(severity_sort)
+          vim.diagnostic.config({
+            severity_sort = severity_sort,
+          })
 
-        local warn_highlight = extmarks[1][4].hl_group
+          local virt_text = vim.api.nvim_buf_get_extmarks(diagnostic_bufnr, diagnostic_ns, 0, -1, {details = true})[1][4].virt_text
 
-        vim.diagnostic.config({
+          local virt_texts = {}
-          severity_sort = true,
+          for i = 2, #virt_text do
-        })
+            table.insert(virt_texts, (string.gsub(virt_text[i][2], "DiagnosticVirtualText", "")))
+          end
 
-        extmarks = vim.api.nvim_buf_get_extmarks(diagnostic_bufnr, diagnostic_ns, 0, -1, {details = true})
+          local signs = {}
+          for _, v in ipairs(vim.fn.sign_getplaced(diagnostic_bufnr, {group = "*"})[1].signs) do
+            table.insert(signs, (string.gsub(v.name, "DiagnosticSign", "")))
+          end
 
-        local err_highlight = extmarks[1][4].hl_group
+          return {virt_texts, signs}
+        end
+      ]]
 
-        vim.diagnostic.config({
+      local result = exec_lua [[return get_virt_text_and_signs(false)]]
-          severity_sort = { reverse = true },
-        })
 
-        extmarks = vim.api.nvim_buf_get_extmarks(diagnostic_bufnr, diagnostic_ns, 0, -1, {details = true})
+      -- Virt texts are defined lowest priority to highest, signs from
+      -- highest to lowest
+      eq({'Warn', 'Error', 'Info'}, result[1])
+      eq({'Info', 'Error', 'Warn'}, result[2])
 
-        local info_highlight = extmarks[1][4].hl_group
+      result = exec_lua [[return get_virt_text_and_signs(true)]]
+      eq({'Info', 'Warn', 'Error'}, result[1])
+      eq({'Error', 'Warn', 'Info'}, result[2])
 
-        return { warn_highlight, err_highlight, info_highlight }
+      result = exec_lua [[return get_virt_text_and_signs({ reverse = true })]]
-      ]])
+      eq({'Error', 'Warn', 'Info'}, result[1])
-      eq('DiagnosticUnderlineWarn', result[1])
+      eq({'Info', 'Warn', 'Error'}, result[2])
-      eq('DiagnosticUnderlineError', result[2])
-      eq('DiagnosticUnderlineInfo', result[3])
     end)
   end)