From a mutation-testing pass: about sixty small, plausible changes to the
compiler and runtime, each applied, run and restored. Nineteen of them left the
whole suite green. The compiler was right in every case - what was missing was
anything that looked.
The two programs here close the severe cluster. cleanup.flan covers six claims:
an early return runs the defers registered above it, and runs them innermost
first; a defer that calls something, which is what puts a guard inside a defer
on the transfer path; a transfer out of a handler-bind pops its frames; a
two-clause handler-bind pops both; and a signal stops once a handler has
answered it by transferring. The numbers differ per failure, so a wrong answer
names its own cause rather than just being wrong.
signedness.flan covers the ashr/lshr and slt/ult choices. Either could have been
hardcoded to one arm and nothing would have noticed, because no program in the
corpus shifted a negative integer right or compared an unsigned value above
2^31 - where a signed compare answers the other way on every operator.
Each was verified able to fail, with the numbers the report predicted: hardcode
lshr and -4 becomes 9223372036854775804; drop the defers from the return path
and 21 becomes 0; reverse them and it becomes 12; let the signal walk continue
past a handler that transferred and the outer handler runs too.
The ones left open are recorded for the next pass: Reach's walk of index
expressions, addr places and restart clause bodies; the dev registry's
size-change guard; a local shadowing an imported name; and the 4K result cap,
which has no coverage at all rather than a missing assertion.
Joseph Ferano
a5980734dc