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[DOCS] Semantics of negation. #5087

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[DOCS] Semantics of negation. #5087

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15 changes: 14 additions & 1 deletion docs/types.rst
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Expand Up @@ -82,10 +82,23 @@ Addition, Subtraction and Multiplication
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Addition, subtraction and multiplication have the usual semantics.
They wrap in two's complement notation, meaning that
They wrap in two's complement representation, meaning that
for example ``uint256(0) - uint256(1) == 2**256 - 1``. You have to take these overflows
into account when designing safe smart contracts.

The expression ``-x`` is equivalent to ``(T(0) - x)`` where
``T`` is the type of ``x``. This means that ``-x`` will not be negative
if the type of ``x`` is an unsigned integer type. Also, ``-x`` can be
positive if ``x`` is negative. There is another caveat also resulting
from two's complement representation::

int x = -2**255;
assert(-x == x);
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Hm, just realised, do we have a test case for this in the EndToEnd suite?

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We could add it, but that's just EVM's semantics.

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Yeah but we're perhaps going to have runtime overflow checks, etc., so better to add a test case when we see it.


This means that even if a number is negative, you cannot assume that
its negation will be positive.


Division
^^^^^^^^

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