A declaration request is similar to a definition request, except that it is expected to return (potentially) many results across the workspace for a given reference. For example, an inline function or macro may have many declarations in the workspace, but only one "good" or canonical definition. For now, this is only implemented by forwarding the request on to clangd since I'm unfamiliar with a SourceKit query for this.
For languages like Swift that lack such a sharp declaration/definition split, we could potentially use this request to provide navigable metadata on linked definitions. For example, the declaration for a type reference would include all extensions of that type in the workspace.
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