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open System
   
// In this case we receive error "The address of the variable 'r' may not be used at this point"
let inline ParseDefB (str:string) (def:^a) : ^a =
    let mutable r = LanguagePrimitives.GenericZero<(^a)>
    let success = (^a : (static member TryParse: string * ^a byref -> bool ) (str, (* X *) &r))
    if success then r else def
    
// In this case we don't receive result of TryParse to r ceil        
let inline ParseDef (str:string) (def:^a) : ^a =
    let r = ref LanguagePrimitives.GenericZero<(^a)>
    let success = (^a : (static member TryParse: string * ^a ref -> bool ) (str,r))
    if success then !r else def

printfn "%d" ( ParseDef "" 1 ) //-> print 1
printfn "%d" ( ParseDef "12" 1 ) //-> print 0 instead 12