patch 8.2.3106: Vim9: confusing line number reported for error

Problem:    Vim9: confusing line number reported for error.
Solution:   Use the start line number for the store instruction.
            (closes #8488)

Author: brammool

src/testdir/test_vim9_assign.vim

@@ -388,6 +388,16 @@ def Test_assign_linebreak()
           ->copy()
   END
   CheckDefFailure(lines, 'E1012:', 2)
+
+  lines =<< trim END
+      var x: any
+      x.key = 1
+          + 2
+          + 3
+          + 4
+          + 5
+  END
+  CheckDefExecAndScriptFailure2(lines, 'E1148:', 'E1203:', 2)
 enddef
 
 def Test_assign_index()

src/version.c

@@ -755,6 +755,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3106,
 /**/
     3105,
 /**/

src/vim9compile.c

@@ -6726,7 +6726,8 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
 	var_start = arg;
     for (var_idx = 0; var_idx == 0 || var_idx < var_count; var_idx++)
     {
-	int		instr_count = -1;
+	int	instr_count = -1;
+	int	save_lnum;
 
 	if (var_start[0] == '_' && !eval_isnamec(var_start[1]))
 	{
@@ -6979,13 +6980,20 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
 		goto theend;
 	}
 
+	// Use the line number of the assignment for store instruction.
+	save_lnum = cctx->ctx_lnum;
+	cctx->ctx_lnum = start_lnum - 1;
+
 	if (lhs.lhs_has_index)
 	{
 	    // Use the info in "lhs" to store the value at the index in the
 	    // list or dict.
 	    if (compile_assign_unlet(var_start, &lhs, TRUE, rhs_type, cctx)
 								       == FAIL)
+	    {
+		cctx->ctx_lnum = save_lnum;
 		goto theend;
+	    }
 	}
 	else
 	{
@@ -7006,8 +7014,12 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
 		generate_SETTYPE(cctx, lhs.lhs_type);
 
 	    if (generate_store_lhs(cctx, &lhs, instr_count) == FAIL)
+	    {
+		cctx->ctx_lnum = save_lnum;
 		goto theend;
+	    }
 	}
+	cctx->ctx_lnum = save_lnum;
 
 	if (var_idx + 1 < var_count)
 	    var_start = skipwhite(lhs.lhs_dest_end + 1);