patch 8.2.3314: behavior of exists() in a :def function is unpredictable

Problem:    Behavior of exists() in a :def function is unpredictable.
Solution:   Add exists_compiled().

runtime/doc/eval.txt

@@ -2473,14 +2473,14 @@ browse({save}, {title}, {initdir}, {default})
 				String	put up a file requester
 browsedir({title}, {initdir})	String	put up a directory requester
 bufadd({name})			Number	add a buffer to the buffer list
-bufexists({expr})		Number	|TRUE| if buffer {expr} exists
+bufexists({buf})		Number	|TRUE| if buffer {buf} exists
-buflisted({expr})		Number	|TRUE| if buffer {expr} is listed
+buflisted({buf})		Number	|TRUE| if buffer {buf} is listed
-bufload({expr})			Number	load buffer {expr} if not loaded yet
+bufload({buf})			Number	load buffer {buf} if not loaded yet
-bufloaded({expr})		Number	|TRUE| if buffer {expr} is loaded
+bufloaded({buf})		Number	|TRUE| if buffer {buf} is loaded
-bufname([{expr}])		String	Name of the buffer {expr}
+bufname([{buf}])		String	Name of the buffer {buf}
-bufnr([{expr} [, {create}]])	Number	Number of the buffer {expr}
+bufnr([{buf} [, {create}]])	Number	Number of the buffer {buf}
-bufwinid({expr})		Number	window ID of buffer {expr}
+bufwinid({buf})			Number	window ID of buffer {buf}
-bufwinnr({expr})		Number	window number of buffer {expr}
+bufwinnr({buf})			Number	window number of buffer {buf}
 byte2line({byte})		Number	line number at byte count {byte}
 byteidx({expr}, {nr})		Number	byte index of {nr}'th char in {expr}
 byteidxcomp({expr}, {nr})	Number	byte index of {nr}'th char in {expr}
@@ -2562,6 +2562,7 @@ executable({expr})		Number	1 if executable {expr} exists
 execute({command})		String	execute {command} and get the output
 exepath({expr})			String	full path of the command {expr}
 exists({expr})			Number	|TRUE| if {expr} exists
+exists_compiled({expr})		Number	|TRUE| if {expr} exists at compile time
 exp({expr})			Float	exponential of {expr}
 expand({expr} [, {nosuf} [, {list}]])
 				any	expand special keywords in {expr}
@@ -4442,8 +4443,10 @@ exepath({expr})						*exepath()*
 							*exists()*
 exists({expr})	The result is a Number, which is |TRUE| if {expr} is defined,
 		zero otherwise.
-		Note: In a compiled |:def| function local variables and
+
-		arguments are not visible to `exists()`.
+		Note: In a compiled |:def| function the evaluation is done at
+		runtime.  Use `exists_compiled()` to evaluate the expression
+		at compile time.
 
 		For checking for a supported feature use |has()|.
 		For checking if a file exists use |filereadable()|.
@@ -4534,8 +4537,23 @@ exists({expr})	The result is a Number, which is |TRUE| if {expr} is defined,
 
 		Can also be used as a |method|: >
 			Varname()->exists()
+<
 
-exp({expr})						*exp()*
+exists_compiled({expr})						*exists()*
+		Like `exists()` but evaluated at compile time.  This is useful
+		to skip a block where a function is used that would otherwise
+		give an error: >
+			if exists_compiled('*ThatFunction')
+			   ThatFunction('works')
+			endif
+<		If `exists()` were used then a compilation error would be
+		given if ThatFunction() is not defined.
+
+		{expr} must be a literal string. *E1232*
+		Can only be used in a |:def| function. *E1233*
+
+
+exp({expr})							*exp()*
 		Return the exponential of {expr} as a |Float| in the range
 		[0, inf].
 		{expr} must evaluate to a |Float| or a |Number|.
@@ -6434,7 +6452,7 @@ has({feature} [, {check}])
 		features that have been abandoned will not be known by the
 		current Vim version.
 
-		Also see |exists()|.
+		Also see |exists()| and |exists_compiled()|.
 
 		Note that to skip code that has a syntax error when the
 		feature is not available, Vim may skip the rest of the line

runtime/doc/usr_41.txt

@@ -1182,6 +1182,7 @@ Various:					*various-functions*
 	state()			get current busy state
 	visualmode()		last visual mode used
 	exists()		check if a variable, function, etc. exists
+	exists_compiled()	like exists() but check at compile time
 	has()			check if a feature is supported in Vim
 	changenr()		return number of most recent change
 	cscope_connection()	check if a cscope connection exists

src/errors.h

@@ -646,3 +646,7 @@ EXTERN char e_encryption_sodium_mlock_failed[]
 	INIT(= N_("E1230: Encryption: sodium_mlock() failed"));
 EXTERN char e_cannot_use_bar_to_separate_commands_here_str[]
 	INIT(= N_("E1231: Cannot use a bar to separate commands here: %s"));
+EXTERN char e_argument_of_exists_compiled_must_be_literal_string[]
+	INIT(= N_("E1232: Argument of exists_compiled() must be a literal string"));
+EXTERN char e_exists_compiled_can_only_be_used_in_def_function[]
+	INIT(= N_("E1233: exists_compiled() can only be used in a :def function"));

src/evalfunc.c

@@ -49,6 +49,7 @@ static void f_escape(typval_T *argvars, typval_T *rettv);
 static void f_eval(typval_T *argvars, typval_T *rettv);
 static void f_eventhandler(typval_T *argvars, typval_T *rettv);
 static void f_execute(typval_T *argvars, typval_T *rettv);
+static void f_exists_compiled(typval_T *argvars, typval_T *rettv);
 static void f_expand(typval_T *argvars, typval_T *rettv);
 static void f_expandcmd(typval_T *argvars, typval_T *rettv);
 static void f_feedkeys(typval_T *argvars, typval_T *rettv);
@@ -1329,6 +1330,8 @@ static funcentry_T global_functions[] =
 			ret_string,	    f_exepath},
     {"exists",		1, 1, FEARG_1,	    arg1_string,
 			ret_number_bool,    f_exists},
+    {"exists_compiled",	1, 1, FEARG_1,	    arg1_string,
+			ret_number_bool,    f_exists_compiled},
     {"exp",		1, 1, FEARG_1,	    arg1_float_or_nr,
 			ret_float,	    FLOAT_FUNC(f_exp)},
     {"expand",		1, 3, FEARG_1,	    arg3_string_bool_bool,
@@ -3626,6 +3629,12 @@ f_exists(typval_T *argvars, typval_T *rettv)
     rettv->vval.v_number = n;
 }
 
+    static void
+f_exists_compiled(typval_T *argvars UNUSED, typval_T *rettv UNUSED)
+{
+    emsg(_(e_exists_compiled_can_only_be_used_in_def_function));
+}
+
 /*
  * "expand()" function
  */

src/testdir/test_vim9_builtin.vim

@@ -793,42 +793,57 @@ def Test_exists()
   CheckDefAndScriptFailure2(['exists(10)'], 'E1013: Argument 1: type mismatch, expected string but got number', 'E1174: String required for argument 1')
   call assert_equal(1, exists('&tabstop'))
 
-  if exists('+newoption')
+  var lines =<< trim END
+    if exists('+newoption')
+      if &newoption == 'ok'
+      endif
+    endif
+  END
+  CheckDefFailure(lines, 'E113:')
+  CheckScriptSuccess(lines)
+enddef
+
+def Test_exists_compiled()
+  call assert_equal(1, exists_compiled('&tabstop'))
+  CheckDefAndScriptFailure2(['exists_compiled(10)'], 'E1232:', 'E1233:')
+  CheckDefAndScriptFailure2(['exists_compiled(v:progname)'], 'E1232:', 'E1233:')
+
+  if exists_compiled('+newoption')
     if &newoption == 'ok'
     endif
   endif
-  if exists('&newoption')
+  if exists_compiled('&newoption')
     if &newoption == 'ok'
     endif
   endif
-  if exists('+tabstop')
+  if exists_compiled('+tabstop')
     assert_equal(8, &tabstop)
   else
     assert_report('tabstop option not existing?')
   endif
-  if exists('&tabstop')
+  if exists_compiled('&tabstop')
     assert_equal(8, &tabstop)
   else
     assert_report('tabstop option not existing?')
   endif
 
-  if exists(':DoSomeCommand') >= 2
+  if exists_compiled(':DoSomeCommand') >= 2
     DoSomeCommand
   endif
   assert_equal(4, g:didSomeCommand)
-  if exists(':NoSuchCommand') >= 2
+  if exists_compiled(':NoSuchCommand') >= 2
     NoSuchCommand
   endif
 
   var found = false
-  if exists('*CheckScriptSuccess')
+  if exists_compiled('*CheckScriptSuccess')
     found = true
   endif
   assert_true(found)
-  if exists('*NoSuchFunction')
+  if exists_compiled('*NoSuchFunction')
     NoSuchFunction()
   endif
-  if exists('*no_such_function')
+  if exists_compiled('*no_such_function')
     no_such_function()
   endif
 enddef

src/version.c

@@ -755,6 +755,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3314,
 /**/
     3313,
 /**/

src/vim9compile.c

@@ -3415,9 +3415,9 @@ compile_call(
     int		is_searchpair;
 
     // We can evaluate "has('name')" at compile time.
-    // We can evaluate some "exists()" values at compile time.
+    // We always evaluate "exists_compiled()" at compile time.
     if ((varlen == 3 && STRNCMP(*arg, "has", 3) == 0)
-	    || (varlen == 6 && STRNCMP(*arg, "exists", 6) == 0))
+	    || (varlen == 15 && STRNCMP(*arg, "exists_compiled", 6) == 0))
     {
 	char_u	    *s = skipwhite(*arg + varlen + 1);
 	typval_T    argvars[2];
@@ -3431,8 +3431,7 @@ compile_call(
 	s = skipwhite(s);
 	if (*s == ')' && argvars[0].v_type == VAR_STRING
 	       && ((is_has && !dynamic_feature(argvars[0].vval.v_string))
-		    || (!is_has && vim_strchr((char_u *)"+&:*",
+		    || !is_has))
-						  *argvars[0].vval.v_string))))
 	{
 	    typval_T	*tv = &ppconst->pp_tv[ppconst->pp_used];
 
@@ -3449,6 +3448,11 @@ compile_call(
 	    return OK;
 	}
 	clear_tv(&argvars[0]);
+	if (!is_has)
+	{
+	    emsg(_(e_argument_of_exists_compiled_must_be_literal_string));
+	    return FAIL;
+	}
     }
 
     if (generate_ppconst(cctx, ppconst) == FAIL)