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Sema: Fix accidental mutation of DenseMap in ConstraintSystem::selectDisjunction() [6.3] #86379

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slavapestov merged 1 commit into from
Jan 9, 2026
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Sema: Fix accidental mutation of DenseMap in ConstraintSystem::selectDisjunction() [6.3] #86379

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21 changes: 11 additions & 10 deletions lib/Sema/CSOptimizer.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1945,10 +1945,11 @@ ConstraintSystem::selectDisjunction() {
if (auto preference = isPreferable(*this, first, second))
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@jamieQ jamieQ Jan 8, 2026

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@slavapestov out of curiosity: i thought there was some frontend/driver setting to try and catch nondeterminism that would like compile twice and compare the outputs – would that have caught this? (assuming i'm not hallucinating...)

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@slavapestov slavapestov Jan 8, 2026

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No, the non-determinism here can only be caught by invoking the predicate more than once on the same pair of inputs and observing that it produces a different result.

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@slavapestov slavapestov Jan 8, 2026

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So it’s more accurate to say the predicate had an unexpected side effect which changed subsequent calls, instead of being non-deterministic. The overall compiler invocation is still deterministic in its output.

return preference.value();

auto &[firstScore, firstFavoredChoices, isFirstSpeculative] =
favorings[first];
auto &[secondScore, secondFavoredChoices, isSecondSpeculative] =
favorings[second];
const auto &firstFavorings = favorings[first];
const auto &secondFavorings = favorings[second];

auto firstScore = firstFavorings.Score;
auto secondScore = secondFavorings.Score;

bool isFirstOperator = isOperatorDisjunction(first);
bool isSecondOperator = isOperatorDisjunction(second);
Expand All @@ -1968,10 +1969,10 @@ ConstraintSystem::selectDisjunction() {
// when nothing concrete is known about them, let's reset the score
// and compare purely based on number of choices.
if (isFirstOperator != isSecondOperator) {
if (isFirstOperator && isFirstSpeculative)
if (isFirstOperator && firstFavorings.IsSpeculative)
firstScore = 0;

if (isSecondOperator && isSecondSpeculative)
if (isSecondOperator && secondFavorings.IsSpeculative)
secondScore = 0;
}

Expand All @@ -1997,17 +1998,17 @@ ConstraintSystem::selectDisjunction() {
// because it would fail and prune the other overload if parameter
// type (aka contextual type) is `Int`.
if (isFirstOperator && isSecondOperator &&
isFirstSpeculative != isSecondSpeculative)
return isSecondSpeculative;
firstFavorings.IsSpeculative != secondFavorings.IsSpeculative)
return secondFavorings.IsSpeculative;
}

// Use favored choices only if disjunction score is higher
// than zero. This means that we can maintain favoring
// choices without impacting selection decisions.
unsigned numFirstFavored =
firstScore.value_or(0) ? firstFavoredChoices.size() : 0;
firstScore.value_or(0) ? firstFavorings.FavoredChoices.size() : 0;
unsigned numSecondFavored =
secondScore.value_or(0) ? secondFavoredChoices.size() : 0;
secondScore.value_or(0) ? secondFavorings.FavoredChoices.size() : 0;

if (numFirstFavored == numSecondFavored) {
if (firstActive != secondActive)
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