vim-patch:51a06ec: runtime(sh): consider sh as POSIX shell by default (#33078)

Also, do not set g:is_kornshell when g:is_posix is set. BSD shells are
POSIX but many are derived from the ash shell.

closes: vim/vim#16939

51a06ecee0

Co-authored-by: Mohamed Akram <mohd.akram@outlook.com>

runtime/doc/syntax.txt

@@ -3032,24 +3032,24 @@ cases pertain, then the first line of the file is examined (ex. looking for
 /bin/sh  /bin/ksh  /bin/bash).  If the first line specifies a shelltype, then
 that shelltype is used.  However some files (ex. .profile) are known to be
 shell files but the type is not apparent.  Furthermore, on many systems sh is
-symbolically linked to "bash" (Linux, Windows+cygwin) or "ksh" (Posix).
+symbolically linked to "bash" (Linux, Windows+cygwin) or "ksh" (POSIX).
 
 One may specify a global default by instantiating one of the following
 variables in your vimrc:
 
    ksh: >
 	let g:is_kornshell = 1
-<   posix:  (using this is nearly the same as setting g:is_kornshell to 1) >
+<   posix: (default) >
 	let g:is_posix     = 1
 <   bash: >
 	let g:is_bash	   = 1
-<   sh: (default) Bourne shell >
+<   dash: >
+	let g:is_dash	   = 1
+<   sh: Bourne shell >
 	let g:is_sh	   = 1
 
-<   (dash users should use posix)
-
 If there's no "#! ..." line, and the user hasn't availed themself of a default
-sh.vim syntax setting as just shown, then syntax/sh.vim will assume the Bourne
+sh.vim syntax setting as just shown, then syntax/sh.vim will assume the POSIX
 shell syntax.  No need to quote RFCs or market penetration statistics in error
 reports, please -- just select the default version of the sh your system uses
 and install the associated "let..." in your <.vimrc>.

runtime/syntax/sh.vim

@@ -7,6 +7,7 @@
 "		2024 Nov 03 by Aliaksei Budavei <0x000c70 AT gmail DOT com> (improved bracket expressions, #15941)
 "		2025 Jan 06 add $PS0 to bashSpecialVariables (#16394)
 "		2025 Jan 18 add bash coproc, remove duplicate syn keywords (#16467)
+"		2025 Mar 21 update shell capability detection (#16939)
 " Version:		208
 " Former URL:		http://www.drchip.org/astronaut/vim/index.html#SYNTAX_SH
 " For options and settings, please use:      :help ft-sh-syntax
@@ -17,6 +18,9 @@ if exists("b:current_syntax")
   finish
 endif
 
+" Ensure this is set unless we find another shell
+let b:is_sh = 1
+
 " If the shell script itself specifies which shell to use, use it
 if getline(1) =~ '\<ksh\>'
  let b:is_kornshell = 1
@@ -24,7 +28,25 @@ elseif getline(1) =~ '\<bash\>'
  let b:is_bash      = 1
 elseif getline(1) =~ '\<dash\>'
  let b:is_dash      = 1
-elseif !exists("g:is_kornshell") && !exists("g:is_bash") && !exists("g:is_posix") && !exists("g:is_sh") && !exists("g:is_dash")
+" handling /bin/sh with is_kornshell/is_sh {{{1
+" b:is_sh will be set when "#! /bin/sh" is found;
+" However, it often is just a masquerade by bash (typically Linux)
+" or kornshell (typically workstations with Posix "sh").
+" So, when the user sets "g:is_kornshell", "g:is_bash",
+" "g:is_posix" or "g:is_dash", a b:is_sh is converted into
+" b:is_kornshell/b:is_bash/b:is_posix/b:is_dash, respectively.
+elseif !exists("b:is_kornshell") && !exists("b:is_bash") && !exists("b:is_posix") && !exists("b:is_dash")
+ if exists("g:is_kornshell")
+  let b:is_kornshell= 1
+ elseif exists("g:is_bash")
+  let b:is_bash= 1
+ elseif exists("g:is_dash")
+  let b:is_dash= 1
+ elseif exists("g:is_posix")
+  let b:is_posix= 1
+ elseif exists("g:is_sh")
+  let b:is_sh= 1
+ else
   " user did not specify which shell to use, and
   " the script itself does not specify which shell to use. FYI: /bin/sh is ambiguous.
   " Assuming /bin/sh is executable, and if its a link, find out what it links to.
@@ -40,38 +62,10 @@ elseif !exists("g:is_kornshell") && !exists("g:is_bash") && !exists("g:is_posix"
    let b:is_bash = 1
   elseif s:shell =~ '\<dash\>'
    let b:is_dash = 1
+  else
+   let b:is_posix = 1
   endif
   unlet s:shell
-endif
-
-" handling /bin/sh with is_kornshell/is_sh {{{1
-" b:is_sh will be set when "#! /bin/sh" is found;
-" However, it often is just a masquerade by bash (typically Linux)
-" or kornshell (typically workstations with Posix "sh").
-" So, when the user sets "g:is_bash", "g:is_kornshell",
-" or "g:is_posix", a b:is_sh is converted into b:is_bash/b:is_kornshell,
-" respectively.
-if !exists("b:is_kornshell") && !exists("b:is_bash") && !exists("b:is_dash")
-  if exists("g:is_posix") && !exists("g:is_kornshell")
-   let g:is_kornshell= g:is_posix
-  endif
-  if exists("g:is_kornshell")
-    let b:is_kornshell= 1
-    if exists("b:is_sh")
-      unlet b:is_sh
-    endif
-  elseif exists("g:is_bash")
-    let b:is_bash= 1
-    if exists("b:is_sh")
-      unlet b:is_sh
-    endif
-  elseif exists("g:is_dash")
-    let b:is_dash= 1
-    if exists("b:is_sh")
-      unlet b:is_sh
-    endif
-  else
-    let b:is_sh= 1
  endif
 endif
 
@@ -80,6 +74,12 @@ if exists("b:is_dash")
  let b:is_posix= 1
 endif
 
+if exists("b:is_kornshell") || exists("b:is_bash")
+ if exists("b:is_sh")
+  unlet b:is_sh
+ endif
+endif
+
 " set up default g:sh_fold_enabled {{{1
 " ================================
 if !exists("g:sh_fold_enabled")