patch 9.1.0301: Vim9: heredoc start may be recognized in string

Problem:  Vim9: heredoc start may be recognized in string.
Solution: Don't skip to closing bracket for invalid list assignment.
          (zeertzjq)

closes: #14472

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_let.vim b/src/testdir/test_let.vim
index 1d61635..5ee2e9b 100644
--- a/src/testdir/test_let.vim
+++ b/src/testdir/test_let.vim
@@ -409,10 +409,16 @@
   endtry
 
   try
+    let [] =<< trim TEXT
+    TEXT
+    call assert_report('No exception thrown')
+  catch /E475:/
+  catch
+    call assert_report('Caught exception: ' .. v:exception)
+  endtry
+
+  try
     let [a b c] =<< trim TEXT
-      change
-      insert
-      append
     TEXT
     call assert_report('No exception thrown')
   catch /E475:/
@@ -422,9 +428,6 @@
 
   try
     let [a; b; c] =<< trim TEXT
-      change
-      insert
-      append
     TEXT
     call assert_report('No exception thrown')
   catch /E452:/
diff --git a/src/testdir/test_vim9_assign.vim b/src/testdir/test_vim9_assign.vim
index 0d6d783..0beaa44 100644
--- a/src/testdir/test_vim9_assign.vim
+++ b/src/testdir/test_vim9_assign.vim
@@ -2029,6 +2029,20 @@
   END
   v9.CheckScriptSuccess(lines)
 
+  # heredoc start should not be recognized in string
+  lines =<< trim END
+      vim9script
+      def Func()
+        new
+        @" = 'bar'
+        ['foo', @"]->setline("]=<<"->count('='))
+        assert_equal(['foo', 'bar'], getline(1, '$'))
+        bwipe!
+      enddef
+      Func()
+  END
+  v9.CheckScriptSuccess(lines)
+
   v9.CheckDefFailure(['var lines =<< trim END X', 'END'], 'E488:')
   v9.CheckDefFailure(['var lines =<< trim END " comment', 'END'], 'E488:')