patch 8.2.3054: Vim9: unpack assignment using "_" after semicolon fails

Problem:    Vim9: unpack assignment using "_" after semicolon fails.
Solution:   Drop the expression result. (closes #8453)

src/errors.h

@@ -318,7 +318,7 @@ EXTERN char e_using_bool_as_number[]
 EXTERN char e_missing_matching_bracket_after_dict_key[]
 	INIT(= N_("E1139: Missing matching bracket after dict key"));
 EXTERN char e_for_argument_must_be_sequence_of_lists[]
-	INIT(= N_("E1140: For argument must be a sequence of lists"));
+	INIT(= N_("E1140: :for argument must be a sequence of lists"));
 EXTERN char e_indexable_type_required[]
 	INIT(= N_("E1141: Indexable type required"));
 EXTERN char e_non_empty_string_required[]

src/testdir/test_vim9_assign.vim

@@ -289,6 +289,16 @@ def Test_assign_unpack()
     assert_equal(1, v1)
     assert_equal(2, v2)
 
+    var reslist = []
+    for text in ['aaa {bbb} ccc', 'ddd {eee} fff']
+      var before: string
+      var middle: string
+      var after: string
+      [_, before, middle, after; _] = text->matchlist('\(.\{-\}\){\(.\{-\}\)}\(.*\)')
+      reslist->add(before)->add(middle)->add(after)
+    endfor
+    assert_equal(['aaa ', 'bbb', ' ccc', 'ddd ', 'eee', ' fff'], reslist)
+
     var a = 1
     var b = 3
     [a, b] += [2, 4]

src/version.c

@@ -755,6 +755,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3054,
 /**/
     3053,
 /**/

src/vim9compile.c

@@ -6598,6 +6598,7 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
     int		var_count = 0;
     int		var_idx;
     int		semicolon = 0;
+    int		did_generate_slice = FALSE;
     garray_T	*instr = &cctx->ctx_instr;
     garray_T    *stack = &cctx->ctx_type_stack;
     char_u	*op;
@@ -6801,6 +6802,7 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
 		else if (semicolon && var_idx == var_count - 1)
 		{
 		    // For "[var; var] = expr" get the rest of the list
+		    did_generate_slice = TRUE;
 		    if (generate_SLICE(cctx, var_count - 1) == FAIL)
 			goto theend;
 		}
@@ -7010,8 +7012,9 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
 	    var_start = skipwhite(lhs.lhs_dest_end + 1);
     }
 
-    // for "[var, var] = expr" drop the "expr" value
-    if (var_count > 0 && !semicolon)
+    // For "[var, var] = expr" drop the "expr" value.
+    // Also for "[var, var; _] = expr".
+    if (var_count > 0 && (!semicolon || !did_generate_slice))
     {
 	if (generate_instr_drop(cctx, ISN_DROP, 1) == NULL)
 	    goto theend;