patch 8.2.3367: Vim9: :@r executing a register is inconsistent

Problem:    Vim9: :@r executing a register is inconsistent.
Solution:   Use "@r" as the start of an expression. (issue #8779)

Author: brammool

src/ex_docmd.c

@@ -3432,12 +3432,23 @@ find_ex_command(
 	    int	    heredoc;
 	    char_u  *swp;
 
-	    if (*eap->cmd == '&')
+	    if (*eap->cmd == '&' || (eap->cmd[0] == '@'
+					&& (valid_yank_reg(eap->cmd[1], FALSE)
+						       || eap->cmd[1] == '@')))
 	    {
-		p = to_name_end(eap->cmd + 1, FALSE);
+		if (*eap->cmd == '&')
+		{
+		    p = eap->cmd + 1;
+		    if (STRNCMP("l:", p, 2) == 0 || STRNCMP("g:", p, 2) == 0)
+			p += 2;
+		    p = to_name_end(p, FALSE);
+		}
+		else
+		    p = eap->cmd + 2;
 		if (ends_excmd(*skipwhite(p)))
 		{
-		    // "&option <NL>" is the start of an expression.
+		    // "&option <NL>" and "@r <NL>" is the start of an
+		    // expression.
 		    eap->cmdidx = CMD_eval;
 		    return eap->cmd;
 		}
@@ -3548,10 +3559,6 @@ find_ex_command(
 	    // "&opt = expr"
 	    // "var = expr"  where "var" is a variable name or we are skipping
 	    // (variable declaration might have been skipped).
-	    if (*eap->cmd == '@')
-		p = eap->cmd + 2;
-	    else if (*eap->cmd == '&')
-		p = skiptowhite_esc(eap->cmd + 1);
 	    oplen = assignment_len(skipwhite(p), &heredoc);
 	    if (oplen > 0)
 	    {

src/testdir/test_vim9_cmd.vim

@@ -537,6 +537,22 @@ def Test_option_use_linebreak()
   CheckDefAndScriptSuccess(lines)
 enddef
 
+def Test_register_use_linebreak()
+  var lines =<< trim END
+      new
+      @a = 'one'
+      @a->setline(1)
+      @b = 'two'
+      @b   ->setline(2)
+      @c = 'three'
+      @c  
+          ->setline(3)
+      assert_equal(['one', 'two', 'three'], getline(1, '$'))
+      bwipe!
+  END
+  CheckDefAndScriptSuccess(lines)
+enddef
+
 def Test_skipped_expr_linebreak()
   if 0
     var x = []

src/version.c

@@ -755,6 +755,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3367,
 /**/
     3366,
 /**/