patch 8.2.3239: Vim9: no error using heredoc for a number variable

Problem:    Vim9: no error using heredoc for a number variable.
Solution:   Add a type check. (closes #8627)

src/evalvars.c

@@ -817,6 +817,7 @@ ex_let(exarg_T *eap)
     else if (expr[0] == '=' && expr[1] == '<' && expr[2] == '<')
     {
 	list_T	*l;
+	long	cur_lnum = SOURCING_LNUM;
 
 	// HERE document
 	l = heredoc_get(eap, expr + 3, FALSE);
@@ -825,6 +826,8 @@ ex_let(exarg_T *eap)
 	    rettv_list_set(&rettv, l);
 	    if (!eap->skip)
 	    {
+		// errors are for the assignment, not the end marker
+		SOURCING_LNUM = cur_lnum;
 		op[0] = '=';
 		op[1] = NUL;
 		(void)ex_let_vars(eap->arg, &rettv, FALSE, semicolon, var_count,

src/testdir/test_vim9_assign.vim

@@ -1392,6 +1392,14 @@ def Test_heredoc()
   [END]
   CheckScriptFailure(lines, 'E1145: Missing heredoc end marker: END')
   delfunc! g:Func
+
+  lines =<< trim END
+      var lines: number =<< trim STOP
+        aaa
+        bbb
+      STOP
+  END
+  CheckDefAndScriptFailure(lines, 'E1012: Type mismatch; expected number but got list<string>', 1)
 enddef
 
 def Test_var_func_call()

src/version.c

@@ -755,6 +755,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3239,
 /**/
     3238,
 /**/

src/vim9compile.c

@@ -6875,7 +6875,15 @@ compile_assignment(char_u *arg, exarg_T *eap, cmdidx_T cmdidx, cctx_T *cctx)
 	if (compile_assign_lhs(var_start, &lhs, cmdidx,
 					is_decl, heredoc, oplen, cctx) == FAIL)
 	    goto theend;
-	if (!heredoc)
+	if (heredoc)
+	{
+	    SOURCING_LNUM = start_lnum;
+	    if (lhs.lhs_has_type
+		    && need_type(&t_list_string, lhs.lhs_type,
+					    -1, 0, cctx, FALSE, FALSE) == FAIL)
+		goto theend;
+	}
+	else
 	{
 	    if (cctx->ctx_skip == SKIP_YES)
 	    {