patch 8.2.3137: Vim9: no error when a line only has a variable name

Problem:    Vim9: no error when a line only has a variable name.
Solution:   Give an error when an expression is evaluated without an effect.
            (closes #8538)

src/errors.h

@@ -494,3 +494,5 @@ EXTERN char e_no_white_space_allowed_between_option_and[]
 	INIT(= N_("E1205: No white space allowed between option and"));
 EXTERN char e_dict_required_for_argument_nr[]
 	INIT(= N_("E1206: Dictionary required for argument %d"));
+EXTERN char e_expression_without_effect_str[]
+	INIT(= N_("E1207: Expression without an effect: %s"));

src/ex_eval.c

@@ -208,7 +208,7 @@ cause_errthrow(
      * not skipped.  Errors in those commands may affect what of the subsequent
      * commands are regarded part of catch and finally clauses.  Catching the
      * exception would then cause execution of commands not intended by the
-     * user, who wouldn't even get aware of the problem.  Therefor, discard the
+     * user, who wouldn't even get aware of the problem.  Therefore, discard the
      * exception currently being thrown to prevent it from being caught.  Just
      * execute finally clauses and terminate.
      */
@@ -896,11 +896,28 @@ ex_eval(exarg_T *eap)
 {
     typval_T	tv;
     evalarg_T	evalarg;
+    int		name_only = FALSE;
+    char_u	*p;
+    long	lnum = SOURCING_LNUM;
+
+    if (in_vim9script())
+    {
+	char_u	*alias;
+
+	p = eap->arg;
+	get_name_len(&p, &alias, FALSE, FALSE);
+	name_only = ends_excmd2(eap->arg, skipwhite(p));
+	vim_free(alias);
+    }
 
     fill_evalarg_from_eap(&evalarg, eap, eap->skip);
 
     if (eval0(eap->arg, &tv, eap, &evalarg) == OK)
+    {
 	clear_tv(&tv);
+	if (in_vim9script() && name_only && lnum == SOURCING_LNUM)
+	    semsg(_(e_expression_without_effect_str), eap->arg);
+    }
 
     clear_evalarg(&evalarg, eap);
 }
@@ -1287,7 +1304,7 @@ ex_continue(exarg_T *eap)
     {
 	// Try to find the matching ":while".  This might stop at a try
 	// conditional not in its finally clause (which is then to be executed
-	// next).  Therefor, inactivate all conditionals except the ":while"
+	// next).  Therefore, inactivate all conditionals except the ":while"
 	// itself (if reached).
 	idx = cleanup_conditionals(cstack, CSF_WHILE | CSF_FOR, FALSE);
 	if (idx >= 0 && (cstack->cs_flags[idx] & (CSF_WHILE | CSF_FOR)))

src/testdir/test_vim9_expr.vim

@@ -647,7 +647,7 @@ def Test_expr4_equal()
   CheckDefFailure(["var x = 'a' == "], 'E1097:', 3)
   CheckScriptFailure(['vim9script', "var x = 'a' == "], 'E15:', 2)
 
-  CheckDefExecAndScriptFailure2(['var items: any', 'eval 1', 'eval 2', 'if items == []', 'endif'], 'E691:', 'E1072:', 4)
+  CheckDefExecAndScriptFailure2(['var items: any', 'eval 1 + 1', 'eval 2 + 2', 'if items == []', 'endif'], 'E691:', 'E1072:', 4)
 
   CheckDefExecAndScriptFailure(['var x: any = "a"', 'echo x == true'], 'E1072: Cannot compare string with bool', 2)
   CheckDefExecAndScriptFailure(["var x: any = true", 'echo x == ""'], 'E1072: Cannot compare bool with string', 2)

src/testdir/test_vim9_func.vim

@@ -2538,7 +2538,7 @@ def Test_restore_modifiers()
       set eventignore=
       autocmd QuickFixCmdPost * copen
       def AutocmdsDisabled()
-        eval 0
+        eval 1 + 2
       enddef
       func Func()
         noautocmd call s:AutocmdsDisabled()
@@ -2551,8 +2551,8 @@ def Test_restore_modifiers()
 enddef
 
 def StackTop()
-  eval 1
-  eval 2
+  eval 1 + 2
+  eval 2 + 3
   # call not on fourth line
   StackBot()
 enddef

src/testdir/test_vim9_script.vim

@@ -691,7 +691,7 @@ enddef
 def Test_cnext_works_in_catch()
   var lines =<< trim END
       vim9script
-      au BufEnter * eval 0
+      au BufEnter * eval 1 + 2
       writefile(['text'], 'Xfile1')
       writefile(['text'], 'Xfile2')
       var items = [
@@ -1754,6 +1754,21 @@ def Test_script_var_shadows_function()
   CheckScriptFailure(lines, 'E1041:', 5)
 enddef
 
+def Test_script_var_shadows_command()
+  var lines =<< trim END
+      var undo = 1
+      undo = 2
+      assert_equal(2, undo)
+  END
+  CheckDefAndScriptSuccess(lines)
+
+  lines =<< trim END
+      var undo = 1
+      undo
+  END
+  CheckDefAndScriptFailure(lines, 'E1207:', 2)
+enddef
+
 def s:RetSome(): string
   return 'some'
 enddef
@@ -2270,7 +2285,7 @@ def Test_if_const_expr()
   assert_equal(false, res)
 
   # with constant "false" expression may be invalid so long as the syntax is OK
-  if false | eval 0 | endif
+  if false | eval 1 + 2 | endif
   if false | eval burp + 234 | endif
   if false | echo burp 234 'asd' | endif
   if false

src/version.c

@@ -755,6 +755,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3137,
 /**/
     3136,
 /**/

src/vim9compile.c

@@ -8563,6 +8563,37 @@ compile_throw(char_u *arg, cctx_T *cctx UNUSED)
     return p;
 }
 
+    static char_u *
+compile_eval(char_u *arg, cctx_T *cctx)
+{
+    char_u	*p = arg;
+    int		name_only;
+    char_u	*alias;
+    long	lnum = SOURCING_LNUM;
+
+    // find_ex_command() will consider a variable name an expression, assuming
+    // that something follows on the next line.  Check that something actually
+    // follows, otherwise it's probably a misplaced command.
+    get_name_len(&p, &alias, FALSE, FALSE);
+    name_only = ends_excmd2(arg, skipwhite(p));
+    vim_free(alias);
+
+    p = arg;
+    if (compile_expr0(&p, cctx) == FAIL)
+	return NULL;
+
+    if (name_only && lnum == SOURCING_LNUM)
+    {
+	semsg(_(e_expression_without_effect_str), arg);
+	return NULL;
+    }
+
+    // drop the result
+    generate_instr_drop(cctx, ISN_DROP, 1);
+
+    return skipwhite(p);
+}
+
 /*
  * compile "echo expr"
  * compile "echomsg expr"
@@ -9630,13 +9661,7 @@ compile_def_function(
 		    break;
 
 	    case CMD_eval:
-		    if (compile_expr0(&p, &cctx) == FAIL)
-			goto erret;
-
-		    // drop the result
-		    generate_instr_drop(&cctx, ISN_DROP, 1);
-
-		    line = skipwhite(p);
+		    line = compile_eval(p, &cctx);
 		    break;
 
 	    case CMD_echo: