Andreas Klebinger authored Feb 02, 2021 and Marge Bot committed Feb 06, 2021

When a user writes code like:

    unsafePerformIO $ do
        let x = f x
        writeIORef ref x
        return x

We might expect that the write happens before we evaluate `f x`.
Sadly this wasn't to case for reasons detailed in #19181.

We fix this by avoiding the strict demand by turning:

    unsafeDupablePerformIO (IO m) = case runRW# m of (# _, a #) -> a

into

    unsafeDupablePerformIO (IO m) = case runRW# m of (# _, a #) -> lazy a

This makes the above code lazy in x. And ensures the side effect of the
write happens before the evaluation of `f x`. If a user *wants* the code
to be strict on the returned value he can simply use `return $! x`.

This fixes #19181