<Copy and Update Record Expressions> with SRTP

[AvalonWot]

There are some records with same filed name and type: a1: string, examples:

type t1 = {a1: string; a2: int}
type t2 = {a1: string; a3: float}
...

I just want to update a1 with one simple function:

let inline update_a1<'a when 'a:(member a1: string)> (t:'a) =
    let v = match t.a1 with
            | "A" -> "@"
            | _ -> "#"
    {t with a1 = v}

but complier complains:

this expression was expected to have type
    ''a'    
but here has type
    't2'

So could anybody give some suggestions to implement this point?