vim-patch:9.2.0489: filetype: some Objective-C files are not recognized

Problem:  filetype: some Objective-C files are not recognized
Solution: Add g:filetype_mm override variable, improve the objective c
          pattern detection (Keith Smiley).

closes: vim/vim#20221

bc7f736a39

Co-authored-by: Keith Smiley <keithbsmiley@gmail.com>

runtime/doc/filetype.txt

@@ -158,6 +158,7 @@ variables can be used to overrule the filetype used for certain extensions:
 	`*.int`		g:filetype_int
 	`*.lsl`		g:filetype_lsl
 	`*.m`		g:filetype_m		|ft-mathematica-syntax|
+	`*.mm`		g:filetype_mm
 	`*.mac`		g:filetype_mac
 	`*[mM]makefile,*.mk,*.mak,[mM]akefile*`
 			g:make_flavor		|ft-make-syntax|

runtime/lua/vim/filetype/detect.lua

@@ -1258,8 +1258,12 @@ end
 
 --- @type vim.filetype.mapfn
 function M.mm(_, bufnr)
+  if vim.g.filetype_mm then
+    return vim.g.filetype_mm
+  end
+
   for _, line in ipairs(getlines(bufnr, 1, 20)) do
-    if matchregex(line, [[\c^\s*\(#\s*\(include\|import\)\>\|@import\>\|/\*\)]]) then
+    if matchregex(line, [[\c^\s*\(//\|#\s*\(include\|import\)\>\|@import\>\|/\*\)]]) then
       return 'objcpp'
     end
   end

test/old/testdir/test_filetype.vim

@@ -2054,6 +2054,37 @@ func Test_m4_file()
   endtry
 endfunc
 
+func Test_mm_file()
+  filetype on
+
+  call writefile(['#import "test.h"'], 'Xfile.mm', 'D')
+  split Xfile.mm
+  call assert_equal('objcpp', &filetype)
+  bwipe!
+
+  call writefile(['// Objective-C++ line comment'], 'Xfile.mm', 'D')
+  split Xfile.mm
+  call assert_equal('objcpp', &filetype)
+  bwipe!
+
+  call writefile(['.TH VIM 1 "YYYY Mth DD"'], 'Xfile.mm', 'D')
+  split Xfile.mm
+  call assert_equal('nroff', &filetype)
+  bwipe!
+
+  try
+    let g:filetype_mm = 'objcpp'
+    call writefile(['.TH VIM 1 "YYYY Mth DD"'], 'Xfile_override.mm', 'D')
+    split Xfile_override.mm
+    call assert_equal('objcpp', &filetype)
+    bwipe!
+  finally
+    unlet! g:filetype_mm
+  endtry
+
+  filetype off
+endfunc
+
 func Test_mod_file()
   filetype on