builtin _ is now a reserved keyword

Similar to `test`, `_` is so likely to at least slow down if not
break all things catastrophically that it ought not be allowed as a
function name. Fixes #8342

CHANGELOG.rst

@@ -45,7 +45,8 @@ Scripting improvements
 - ``return`` can now be used outside of functions. In scripts it does the same thing as :ref:`exit <cmd-exit>`, in the commandline it sets ``$status`` without exiting (:issue:`8148`).
 - An oversight prevented all syntax checks from running on commands given to ``fish -c`` (:issue:`8171`). This includes checks like e.g. ``exec`` not being allowed in a pipeline and ``$$`` not being a valid variable. Most of these would have triggered an assert or other error before.
 - ``fish_indent`` now correctly reformats tokens that end with a backslash followed by a newline (:issue:`8197`).
-- ``set`` learned a new option ``--function`` to set a variable in the function's top scope. This should be a more familiar way of scoping variables and avoids issues with ``--local``, which is actually block-scoped. (:issue:`565`, :issue:`8145`)::
+- ``set`` learned a new option ``--function`` to set a variable in the function's top scope. This should be a more familiar way of scoping variables and avoids issues with ``--local``, which is actually block-scoped (:issue:`565`, :issue:`8145`).
+- ``_`` is now a reserved keyword (:isues:`8342`).::
 
     function demonstration
         if true

src/parser_keywords.cpp

@@ -24,7 +24,7 @@ static const string_set_t block_keywords = {L"for",      L"while",  L"if",
 
 static const wcstring reserved_keywords[] = {
     L"end",  L"case",   L"else", L"return", L"continue", L"break", L"argparse",
-    L"read", L"string", L"set",  L"status", L"test",     L"["};
+    L"read", L"string", L"set",  L"status", L"test",     L"[",     L"_"};
 
 // The lists above are purposely implemented separately from the logic below, so that future
 // maintainers may assume the contents of the list based off their names, and not off what the