vim-patch:9.1.1713: filetype: fvwm2m4 files are no longer detected

Problem:  filetype: fvwm2m4 files are no longer recognized
          (after 9.1.1687).
Solution: Add a special case in m4 filetype detection (zeertzjq).

closes: vim/vim#18146

5355e81868

Co-authored-by: Damien Lejay <damien@lejay.be>

runtime/lua/vim/filetype/detect.lua

@@ -688,10 +688,7 @@ function M.fvwm_v1(_, _)
 end
 
 --- @type vim.filetype.mapfn
-function M.fvwm_v2(path, _)
-  if fn.fnamemodify(path, ':e') == 'm4' then
-    return 'fvwm2m4'
-  end
+function M.fvwm_v2(_, _)
   return 'fvwm', function(bufnr)
     vim.b[bufnr].fvwm_version = 2
   end
@@ -1026,22 +1023,30 @@ function M.m(_, bufnr)
   end
 end
 
+--- For files ending in *.m4, distinguish:
+---  – *.html.m4 files
+---  - *fvwm2rc*.m4 files
+---  – files in the Autoconf M4 dialect
+---  – files in POSIX M4
 --- @type vim.filetype.mapfn
 function M.m4(path, bufnr)
   local fname = fn.fnamemodify(path, ':t')
   path = fn.fnamemodify(path, ':p:h')
 
-  -- Case 0: canonical Autoconf file
-  if fname == 'aclocal.m4' then
-    return 'config'
-  end
-
-  -- Case 1: html.m4
   if fname:find('html%.m4$') then
     return 'htmlm4'
   end
 
-  -- Case 2: repo heuristic (nearby configure.ac)
+  if fname:find('fvwm2rc') then
+    return 'fvwm2m4'
+  end
+
+  -- Canonical Autoconf file
+  if fname == 'aclocal.m4' then
+    return 'config'
+  end
+
+  -- Repo heuristic for Autoconf M4 (nearby configure.ac)
   if
     fn.filereadable(path .. '/../configure.ac') ~= 0
     or fn.filereadable(path .. '/configure.ac') ~= 0
@@ -1049,7 +1054,7 @@ function M.m4(path, bufnr)
     return 'config'
   end
 
-  -- Case 3: content heuristic (scan first ~200 lines)
+  -- Content heuristic for Autoconf M4 (scan first ~200 lines)
   -- Signals:
   --   - Autoconf macro prefixes: AC_/AM_/AS_/AU_/AT_
   for _, line in ipairs(getlines(bufnr, 1, 200)) do
@@ -1058,7 +1063,7 @@ function M.m4(path, bufnr)
     end
   end
 
-  -- Case 4: default to POSIX M4
+  -- Default to POSIX M4
   return 'm4'
 end
 

test/old/testdir/test_filetype.vim

@@ -308,7 +308,8 @@ func s:GetFilenameChecks() abort
     \ 'fstab': ['fstab', 'mtab'],
     \ 'func': ['file.fc'],
     \ 'fusion': ['file.fusion'],
-    \ 'fvwm': ['/.fvwm/file', 'any/.fvwm/file'],
+    \ 'fvwm': ['/.fvwm/file', 'any/.fvwm/file', '.fvwmrc', 'foo.fvwmrc', 'fvwmrc.foo', '.fvwm2rc', 'foo.fvwm2rc', 'fvwm2rc.foo', 'foo.fvwm95.hook', 'fvwm95.foo.hook'],
+    \ 'fvwm2m4': ['.fvwm2rc.m4', 'foo.fvwm2rc.m4', 'fvwm2rc.foo.m4'],
     \ 'gdb': ['.gdbinit', 'gdbinit', '.cuda-gdbinit', 'cuda-gdbinit', 'file.gdb', '.config/gdbearlyinit', '.gdbearlyinit'],
     \ 'gdmo': ['file.mo', 'file.gdmo'],
     \ 'gdresource': ['file.tscn', 'file.tres'],