This works (kind of):

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let inline get (arr:^a) (ind:^b) = 
  (^a : (member get_Item : ^b -> ^c)(arr,ind)) 

Unfortunately, this will only work with types that truly expose a "get_Item" method, but not with types that F# augments with implicit "get_Item" methods (such as arrays). So this will work:

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let hello = get (Map.ofList [1,"hello"]) 1

But this will fail at runtime:

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let test = get [|1;2;3|] 1