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Fix UnionWeight constructed or populated with non-member elements. - #343
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`UnionWeight<W, O>` represents the empty set (`Zero()`) with `!first_.Member()` and `rest_.empty()`, and represents `NoWeight()` (`UnionWeight(W::Zero(), W::NoWeight())`) with `first_ = W::Zero()` and `rest_ = {W::NoWeight()}`. However, constructing `UnionWeight(W weight)` from a non-member `weight` or calling `PushBack(weight, srt)` with a non-member `weight` on an empty `UnionWeight` left `!first_.Member()` while pushing `NoWeight()` to `rest_`. Because `Size()` returns `0` and `UnionWeightIterator::Done()` is immediately `true` when `!first_.Member()`, such weights reported `Member() == true`, compared equal to `Zero()`, printed as `"EmptySet"` instead of `"BadSet"`, and failed to propagate `NoWeight()` in `Plus`, `Times`, `Divide`, `Quantize`, `Reverse`, and `Write`.
- In `UnionWeight(W weight)` and `UnionWeight::PushBack`, set `first_ = W::Zero()` when `!weight.Member()` and `!first_.Member()`, matching `UnionWeight::NoWeight()`.
- In `UnionWeight::Member()`, check `if (!first_.Member()) return rest_.empty();` and verify `w.Member()` across `rest_`.
PiperOrigin-RevId: 995132105
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Fix
UnionWeightconstructed or populated with non-member elements.UnionWeight<W, O>represents the empty set (Zero()) with!first_.Member()andrest_.empty(), and representsNoWeight()(UnionWeight(W::Zero(), W::NoWeight())) withfirst_ = W::Zero()andrest_ = {W::NoWeight()}. However, constructingUnionWeight(W weight)from a non-memberweightor callingPushBack(weight, srt)with a non-memberweighton an emptyUnionWeightleft!first_.Member()while pushingNoWeight()torest_. BecauseSize()returns0andUnionWeightIterator::Done()is immediatelytruewhen!first_.Member(), such weights reportedMember() == true, compared equal toZero(), printed as"EmptySet"instead of"BadSet", and failed to propagateNoWeight()inPlus,Times,Divide,Quantize,Reverse, andWrite.UnionWeight(W weight)andUnionWeight::PushBack, setfirst_ = W::Zero()when!weight.Member()and!first_.Member(), matchingUnionWeight::NoWeight().UnionWeight::Member(), checkif (!first_.Member()) return rest_.empty();and verifyw.Member()acrossrest_.