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: #17160

Signed-off-by: Eisuke Kawashima <e-kwsm@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/autoload/dist/ft.vim

@@ -3,7 +3,7 @@ vim9script
 # Vim functions for file type detection
 #
 # Maintainer:		The Vim Project <https://github.com/vim/vim>
-# Last Change:		2025 Apr 19
+# Last Change:		2025 Apr 21
 # Former Maintainer:	Bram Moolenaar <Bram@vim.org>
 
 # These functions are moved here from runtime/filetype.vim to make startup
@@ -639,13 +639,14 @@ export def FTmms()
   setf mmix
 enddef
 
-# 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: 'filetype' is set and 1 is returned.
+# nroff pattern in man files.  In that case it is probably an nroff file:
+# 'filetype' is set and 1 is returned.
 export def FTnroff(): number
   var n = 1
-  while n < 5
+  while n <= 5
     var line = getline(n)
-    if line =~ '^\.\S\S\?'
+    if line =~ '^\%([.'']\s*\%(TH\|D[dt]\|S[Hh]\|d[es]1\?\|so\)\s\+\S\|[.'']\s*ig\>\|\%([.'']\s*\)\?\\"\)'
       setf nroff
       return 1
     endif

runtime/filetype.vim

@@ -1,7 +1,7 @@
 " Vim support file to detect file types
 "
 " Maintainer:	The Vim Project <https://github.com/vim/vim>
-" Last Change:	2025 Apr 19
+" Last Change:	2025 Apr 21
 " Former Maintainer:	Bram Moolenaar <Bram@vim.org>
 
 " Listen very carefully, I will say this only once
@@ -1728,7 +1728,7 @@ au BufNewFile,BufRead *.me
 	\   setf nroff |
 	\ endif
 au BufNewFile,BufRead *.tr,*.nr,*.roff,*.tmac,*.mom	setf nroff
-au BufNewFile,BufRead *.[0-9],*.[013]p,*.[1-8]x,*.3{am,perl,pm,posix,type},*.[nlop]	call dist#ft#FTnroff()
+au BufNewFile,BufRead *.[0-9],*.[013]p,*.[1-8]x,*.3{am,perl,pm,posix,type},*.n	call dist#ft#FTnroff()
 
 " Nroff or Objective C++
 au BufNewFile,BufRead *.mm			call dist#ft#FTmm()

src/testdir/test_filetype.vim

@@ -2955,11 +2955,21 @@ 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)

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1327,
 /**/
     1326,
 /**/