`PageableHandlerMethodArgumentResolver` should resolve `Optional<Pageable>` arguments

[vhbcm]

I have OpenAPI generated controllers that use Optional for non-required parameters. This results in API like this where, e.g., Pageable is wrapped in the Optional:

    @RequestMapping(
            method = RequestMethod.GET,
            value = "/discovery/agent",
            produces = {"application/json"})
    default ResponseEntity<DiscoveryAgentPageRestDto> _discoveryAgentGet(
            @Parameter(name = "pageable", description = "Defines page number, page size, sort property and order", schema = @Schema(description = "")) @Valid Optional<Pageable> pageable,
            @Parameter(name = "q", description = "Search query", schema = @Schema(description = "")) @Valid @RequestParam(value = "q", required = false) Optional<String> q)
            throws Exception {
        return discoveryAgentGet(pageable, q);
    }

However, the stock Spring Framework PageableHandlerMethodArgumentResolver does not handle this. PageableHandlerMethodArgumentResolver should resolve Optional<Pageable> arguments

Cross reference: spring-projects/spring-framework#17589 (comment)

[quaff]

It's too risky to change method signature of PageableHandlerMethodArgumentResolver, I created dedicated resolver for Optional<Pageable>, see #3156.

[mp911de]

We do not have a case of an absent Pageable, even an unpaged Pageable can contain some sort. Spring Data represents an empty pageable through Pageable.unpaged() and we do not want to introduce another alternative as that makes its usage much more difficult and introduces ambiguity over how the API should be used.

Instead, the OpenAPI part should be fixed.