…raints
The IntegerSet constructor of FlatLinearValueConstraints asserts when
flattening fails, which happens for semi-affine integer sets. Since the
failure is signalled from inside a constructor, callers have no way to
detect the unsupported case ahead of time and no way to work around it;
they simply crash.
Add an optional `bool *error` parameter to the constructor. When it is
non-null, hitting the unimplemented case sets `*error` to true and
returns instead of asserting, letting callers bail out gracefully.
`*error` is set to false on success. When the pointer is null the
previous assert behavior is retained, so existing callers are
unaffected.
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