compile_lhs(
	char_u	*var_start,
	lhs_T	*lhs,
	int	cmdidx,
	int	heredoc,
	int	oplen,
	cctx_T	*cctx)
{
    char_u	*var_end;
    int		is_decl = is_decl_command(cmdidx);

    CLEAR_POINTER(lhs);
    lhs->lhs_dest = dest_local;
    lhs->lhs_vimvaridx = -1;
    lhs->lhs_scriptvar_idx = -1;

    // "dest_end" is the end of the destination, including "[expr]" or
    // ".name".
    // "var_end" is the end of the variable/option/etc. name.
    lhs->lhs_dest_end = skip_var_one(var_start, FALSE);
    if (*var_start == '@')
	var_end = var_start + 2;
    else
    {
	// skip over the leading "&", "&l:", "&g:" and "$"
	var_end = skip_option_env_lead(var_start);
	var_end = to_name_end(var_end, TRUE);
    }

    // "a: type" is declaring variable "a" with a type, not dict "a:".
    if (is_decl && lhs->lhs_dest_end == var_start + 2
					       && lhs->lhs_dest_end[-1] == ':')
	--lhs->lhs_dest_end;
    if (is_decl && var_end == var_start + 2 && var_end[-1] == ':')
	--var_end;
    lhs->lhs_end = lhs->lhs_dest_end;

    // compute the length of the destination without "[expr]" or ".name"
    lhs->lhs_varlen = var_end - var_start;
    lhs->lhs_varlen_total = lhs->lhs_varlen;
    lhs->lhs_name = vim_strnsave(var_start, lhs->lhs_varlen);
    if (lhs->lhs_name == NULL)
	return FAIL;

    if (lhs->lhs_dest_end > var_start + lhs->lhs_varlen)
	// Something follows after the variable: "var[idx]" or "var.key".
	lhs->lhs_has_index = TRUE;

    if (heredoc)
	lhs->lhs_type = &t_list_string;
    else
	lhs->lhs_type = &t_any;

    if (cctx->ctx_skip != SKIP_YES)
    {
	int	    declare_error = FALSE;

	if (get_var_dest(lhs->lhs_name, &lhs->lhs_dest, cmdidx,
				      &lhs->lhs_opt_flags, &lhs->lhs_vimvaridx,
						 &lhs->lhs_type, cctx) == FAIL)
	    return FAIL;
	if (lhs->lhs_dest != dest_local
				 && cmdidx != CMD_const && cmdidx != CMD_final)
	{
	    // Specific kind of variable recognized.
	    declare_error = is_decl;
	}
	else
	{
	    // No specific kind of variable recognized, just a name.
	    if (check_reserved_name(lhs->lhs_name) == FAIL)
		return FAIL;

	    if (lookup_local(var_start, lhs->lhs_varlen,
					     &lhs->lhs_local_lvar, cctx) == OK)
		lhs->lhs_lvar = &lhs->lhs_local_lvar;
	    else
	    {
		CLEAR_FIELD(lhs->lhs_arg_lvar);
		if (arg_exists(var_start, lhs->lhs_varlen,
			 &lhs->lhs_arg_lvar.lv_idx, &lhs->lhs_arg_lvar.lv_type,
			    &lhs->lhs_arg_lvar.lv_from_outer, cctx) == OK)
		{
		    if (is_decl)
		    {
			semsg(_(e_str_is_used_as_argument), lhs->lhs_name);
			return FAIL;
		    }
		    lhs->lhs_lvar = &lhs->lhs_arg_lvar;
		}
	    }
	    if (lhs->lhs_lvar != NULL)
	    {
		if (is_decl)
		{
		    semsg(_(e_variable_already_declared), lhs->lhs_name);
		    return FAIL;
		}
	    }
	    else
	    {
		int script_namespace = lhs->lhs_varlen > 1
				       && STRNCMP(var_start, "s:", 2) == 0;
		int script_var = (script_namespace
			? script_var_exists(var_start + 2, lhs->lhs_varlen - 2,
									  cctx)
			  : script_var_exists(var_start, lhs->lhs_varlen,
								  cctx)) == OK;
		imported_T  *import =
			       find_imported(var_start, lhs->lhs_varlen, cctx);

		if (script_namespace || script_var || import != NULL)
		{
		    char_u	*rawname = lhs->lhs_name
					   + (lhs->lhs_name[1] == ':' ? 2 : 0);

		    if (is_decl)
		    {
			if (script_namespace)
			    semsg(_(e_cannot_declare_script_variable_in_function),
								lhs->lhs_name);
			else
			    semsg(_(e_variable_already_declared_in_script_str),
								lhs->lhs_name);
			return FAIL;
		    }
		    else if (cctx->ctx_ufunc->uf_script_ctx_version
							 == SCRIPT_VERSION_VIM9
				    && script_namespace
				    && !script_var && import == NULL)
		    {
			semsg(_(e_unknown_variable_str), lhs->lhs_name);
			return FAIL;
		    }

		    lhs->lhs_dest = dest_script;

		    // existing script-local variables should have a type
		    lhs->lhs_scriptvar_sid = current_sctx.sc_sid;
		    if (import != NULL)
			lhs->lhs_scriptvar_sid = import->imp_sid;
		    if (SCRIPT_ID_VALID(lhs->lhs_scriptvar_sid))
		    {
			// Check writable only when no index follows.
			lhs->lhs_scriptvar_idx = get_script_item_idx(
					       lhs->lhs_scriptvar_sid, rawname,
			      lhs->lhs_has_index ? ASSIGN_FINAL : ASSIGN_CONST,
									 cctx);
			if (lhs->lhs_scriptvar_idx >= 0)
			{
			    scriptitem_T *si = SCRIPT_ITEM(
						       lhs->lhs_scriptvar_sid);
			    svar_T	 *sv =
					    ((svar_T *)si->sn_var_vals.ga_data)
						      + lhs->lhs_scriptvar_idx;
			    lhs->lhs_type = sv->sv_type;
			}
		    }
		}
		else if (check_defined(var_start, lhs->lhs_varlen, cctx, FALSE)
								       == FAIL)
		    return FAIL;
	    }
	}

	if (declare_error)
	{
	    vim9_declare_error(lhs->lhs_name);
	    return FAIL;
	}
    }

    // handle "a:name" as a name, not index "name" in "a"
    if (lhs->lhs_varlen > 1 || var_start[lhs->lhs_varlen] != ':')
	var_end = lhs->lhs_dest_end;

    if (lhs->lhs_dest != dest_option && lhs->lhs_dest != dest_func_option)
    {
	if (is_decl && *var_end == ':')
	{
	    char_u *p;

	    // parse optional type: "let var: type = expr"
	    if (!VIM_ISWHITE(var_end[1]))
	    {
		semsg(_(e_white_space_required_after_str_str), ":", var_end);
		return FAIL;
	    }
	    p = skipwhite(var_end + 1);
	    lhs->lhs_type = parse_type(&p, cctx->ctx_type_list, TRUE);
	    if (lhs->lhs_type == NULL)
		return FAIL;
	    lhs->lhs_has_type = TRUE;
	    lhs->lhs_end = p;
	}
	else if (lhs->lhs_lvar != NULL)
	    lhs->lhs_type = lhs->lhs_lvar->lv_type;
    }

    if (oplen == 3 && !heredoc
		   && lhs->lhs_dest != dest_global
		   && !lhs->lhs_has_index
		   && lhs->lhs_type->tt_type != VAR_STRING
		   && lhs->lhs_type->tt_type != VAR_ANY)
    {
	emsg(_(e_can_only_concatenate_to_string));
	return FAIL;
    }

    if (lhs->lhs_lvar == NULL && lhs->lhs_dest == dest_local
						 && cctx->ctx_skip != SKIP_YES)
    {
	if (oplen > 1 && !heredoc)
	{
	    // +=, /=, etc. require an existing variable
	    semsg(_(e_cannot_use_operator_on_new_variable), lhs->lhs_name);
	    return FAIL;
	}
	if (!is_decl)
	{
	    semsg(_(e_unknown_variable_str), lhs->lhs_name);
	    return FAIL;
	}

	// Check the name is valid for a funcref.
	if ((lhs->lhs_type->tt_type == VAR_FUNC
				      || lhs->lhs_type->tt_type == VAR_PARTIAL)
		&& var_wrong_func_name(lhs->lhs_name, TRUE))
	    return FAIL;

	// New local variable.
	lhs->lhs_lvar = reserve_local(cctx, var_start, lhs->lhs_varlen,
		    cmdidx == CMD_final || cmdidx == CMD_const, lhs->lhs_type);
	if (lhs->lhs_lvar == NULL)
	    return FAIL;
	lhs->lhs_new_local = TRUE;
    }

    lhs->lhs_member_type = lhs->lhs_type;
    if (lhs->lhs_has_index)
    {
	char_u	*after = var_start + lhs->lhs_varlen;
	char_u	*p;

	// Something follows after the variable: "var[idx]" or "var.key".
	if (is_decl)
	{
	    emsg(_(e_cannot_use_index_when_declaring_variable));
	    return FAIL;
	}

	// Now: var_start[lhs->lhs_varlen] is '[' or '.'
	// Only the last index is used below, if there are others
	// before it generate code for the expression.  Thus for
	// "ll[1][2]" the expression is "ll[1]" and "[2]" is the index.
	for (;;)
	{
	    p = skip_index(after);
	    if (*p != '[' && *p != '.')
	    {
		lhs->lhs_varlen_total = p - var_start;
		break;
	    }
	    after = p;
	}
	if (after > var_start + lhs->lhs_varlen)
	{
	    lhs->lhs_varlen = after - var_start;
	    lhs->lhs_dest = dest_expr;
	    // We don't know the type before evaluating the expression,
	    // use "any" until then.
	    lhs->lhs_type = &t_any;
	}

	if (lhs->lhs_type->tt_member == NULL)
	    lhs->lhs_member_type = &t_any;
	else
	    lhs->lhs_member_type = lhs->lhs_type->tt_member;
    }
    return OK;
}