> This is puzzling, isn't it?

I don't see why.  There's nothing in -Wall -Wextra -Wsigned-overflow
that asks for warnings for code that might overflow. Indeed, I don't
see how any compiler could reasonably provide such warnings without
flagging (almost) every occurrence of arithmetic on signed integers
as suspect.[*] 

The -ftrapv option is useful for catching genuine signed-integer
overflows at runtime, but it can still only catch those cases
that actually get exercised (e.g., by the Python test suite).


[*] Even some operations on unsigned integers would have to be
flagged: the C expression "(unsigned short)x * (unsigned short)y"
also has the potential to invoke undefined behaviour, thanks to
C's integer promotion rules.