Handling composite template variables in links and UriTemplate expansion

I'm having some issues with building a link to a method where I think the template variable should be a composite parameter (i.e. {paramName*}).

I have a controller with a "search-like" method, which can take arbitrary key=[value] parameters, with the declaration:

    @PostMapping("/{entityId}/populateFromSearch")
    @Transactional
    public ResponseEntity populateFromSearch(@PathVariable UUID entitytId, @RequestParam MultiValueMap<String, String> searchParameters)

I'm adding a link to this on my EntityModels, using the normal form: linkTo(methodOn(MyController.class).populateFromSearch(null, null).withRel("populateFromSearch")

This produces a URI template ending /populateFromSearch?searchParameters={searchParameters}. But this can't be expanded as a composite parameter by the UriTemplate class. Even if the parameterised part were correct ({searchParameters*}) the expansion leaves the original query parameter in place, i.e. ?searchParameters=&foo=qux&baz=buz.

My other approach is to manually set up the Link from a UriTemplate and manually add a TemplateVariable:

        resource.add(Link.of(
                UriTemplate.of(linkTo(DataBasketRepositoryExtension.class).slash(entityId).slash("populateFromSearch")
                                .toUriComponentsBuilder().build().toUriString(),
                        new TemplateVariables(TemplateVariable.compositeParameter("searchParameters"))),
                "populateFromSearch"
        ));

This produces a 'working' URI (ending with /inputSearch{*searchParameters}) but this brings me to the second issue: there's an incompatibility between TemplateVariable.compositeParameter("foo") and uriTemplate.expand.

The TemplateVariable is rendered {*paramName} (ref), while the expansion expects the composite marker to be at the end of the parameter (i.e. {paramName*} (ref).

This is a bit frustrating, but can be worked around using the manual link approach - I can just append the {searchParameters*} string manually when building the UriTemplate.

Start with TemplateVariable.java around compositeParameter and UriTemplate.java around the expansion logic referenced in the issue. Reproduce the link-generation and expansion examples for searchParameters, then inspect how composite query variables are rendered and parsed. Done means the two paths agree on composite-variable syntax and the resulting URI expands without retaining an incorrect placeholder.