patch 8.2.2705: Vim9: misleading reported line number for wrong type

Problem:    Vim9: misleading reported line number for wrong type.
Solution:   Remember and use the line number at the start. (closes #8059)

src/testdir/test_vim9_assign.vim

@@ -315,6 +315,14 @@ def Test_assign_linebreak()
   assert_equal(34, n2)
 
   CheckDefFailure(["var x = #"], 'E1097:', 3)
+
+  var lines =<< trim END
+      var x: list<string> = ['a']
+      var y: list<number> = x
+          ->copy()
+          ->copy()
+  END
+  CheckDefFailure(lines, 'E1012:', 2)
 enddef
 
 def Test_assign_index()

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2705,
 /**/
     2704,
 /**/

src/vim9compile.c

@@ -6166,6 +6166,7 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
     char_u	*sp;
     int		is_decl = is_decl_command(cmdidx);
     lhs_T	lhs;
+    long	start_lnum = SOURCING_LNUM;
 
     // Skip over the "var" or "[var, var]" to get to any "=".
     p = skip_var_list(arg, TRUE, &var_count, &semicolon, TRUE);
@@ -6393,7 +6394,9 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
 		    {
 			type_T *use_type = lhs.lhs_lvar->lv_type;
 
-			// without operator check type here, otherwise below
+			// Without operator check type here, otherwise below.
+			// Use the line number of the assignment.
+			SOURCING_LNUM = start_lnum;
 			if (lhs.lhs_has_index)
 			    use_type = lhs.lhs_member_type;
 			if (need_type(rhs_type, use_type, -1, 0, cctx,