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> When in doubt, stick a (double) in front of it

If casting from an integer type, this loses precision. Why would this make tests pass? Are there any secondary effects? The author apparently made no effort to find out.

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This may lose precision. Depending on architecture & compiler & flags, your mantissa may be big enough to hold your whole int. An 80 bit extended double has 64 bits of significand.

https://en.wikipedia.org/wiki/Extended_precision#x86_extende...

This will lose precision, as any floating point operation is only accurate to 1 unit in the last place (ULP). Even simple addition.

This conversion will not lose precision!

With a 64-bit mantissa, ULP(x) is ≤ 1 for x in [-2e64, 2e64]. If you have the time you can count up to 2^64 by repeatedly incrementing a float80(0.0) by 1.

Addition, subtraction and multiplication on float80s are perfectly stable provided the results stays within [-2e64, 2e64].

There is no guarantee on stability of those ops. Intel and IEEE guarantees 1 ULP. Even if you use the 80 bit type. The error is not guaranteed to not be additive over multiple operations either. And for multiplication and division? Let's not even go there.

This will mess up all kinds of sharp comparisons on the result of casted operations.

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