vim-patch:9.1.1327: filetype: nroff detection can be improved

Problem:  filetype: nroff detection can be improved
Solution: improve nroff detection (Eisuke Kawashima)

- explicitly check roff comments and macros typically found in manpages
- do not try to detect alphabetically-sectioned files, except for n, as
  nroff
    - l: > 'l' happens to be a section for historical reasons
         <https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=391977>
    - n: e.g. /usr/share/man/mann/Tcl.n.gz
    - o: unsure (perhaps fedora-specific)
    - p: unsure (perhaps fedora-specific)

closes: vim/vim#17160

2cb42efc18

Co-authored-by: Eisuke Kawashima <e-kwsm@users.noreply.github.com>

runtime/lua/vim/filetype.lua

@@ -696,7 +696,7 @@ local extension = {
   lex = 'lex',
   lxx = 'lex',
   ['l++'] = 'lex',
-  l = detect_seq('lex', detect.nroff),
+  l = 'lex',
   lhs = 'lhaskell',
   lidr = 'lidris2',
   ly = 'lilypond',
@@ -862,7 +862,6 @@ local extension = {
   ['3posix'] = detect.nroff,
   ['3type'] = detect.nroff,
   n = detect.nroff,
-  o = detect.nroff,
   roff = 'nroff',
   tmac = 'nroff',
   man = 'nroff',
@@ -1409,7 +1408,7 @@ local extension = {
   pp = detect.pp,
   i = detect.i,
   w = detect.progress_cweb,
-  p = detect_seq(detect.nroff, detect.progress_pascal),
+  p = detect.progress_pascal,
   pro = detect_seq(detect.proto, 'idlang'),
   patch = detect.patch,
   r = detect.r,

runtime/lua/vim/filetype/detect.lua

@@ -1170,12 +1170,17 @@ function M.news(_, bufnr)
   end
 end
 
---- This function checks if one of the first five lines start with a dot. In
+--- This function checks if one of the first five lines start with a typical
---- that case it is probably an nroff file.
+--- nroff pattern in man files.  In that case it is probably an nroff file.
 --- @type vim.filetype.mapfn
 function M.nroff(_, bufnr)
   for _, line in ipairs(getlines(bufnr, 1, 5)) do
-    if line:find('^%.%S%S?') then
+    if
+      matchregex(
+        line,
+        [[^\%([.']\s*\%(TH\|D[dt]\|S[Hh]\|d[es]1\?\|so\)\s\+\S\|[.'']\s*ig\>\|\%([.'']\s*\)\?\\"\)]]
+      )
+    then
       return 'nroff'
     end
   end

test/old/testdir/test_filetype.vim

@@ -2921,11 +2921,21 @@ endfunc
 func Test_nroff_file()
   filetype on
 
-  call writefile(['.TH vim 1 "YYYY Mth DD"'], 'Xfile.1', 'D')
+  call writefile(['.TH VIM 1 "YYYY Mth DD"'], 'Xfile.1', 'D')
   split Xfile.1
   call assert_equal('nroff', &filetype)
   bwipe!
 
+  call writefile(['.Dd $Mdocdate$', '.Dt "DETECTION TEST" "7"', '.Os'], 'Xfile.7', 'D')
+  split Xfile.7
+  call assert_equal('nroff', &filetype)
+  bwipe!
+
+  call writefile(['''\" t'], 'Xfile.3p', 'D')
+  split Xfile.3p
+  call assert_equal('nroff', &filetype)
+  bwipe!
+
   call writefile(['. /etc/profile'], 'Xfile.1', 'D')
   split Xfile.1
   call assert_notequal('nroff', &filetype)