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Port TypeScript PR #62604: Propagate variance reliability #1916

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Port TypeScript PR #62604: Propagate variance reliability #1916

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Initial plan
Copilot Oct 20, 2025
da27ef1
Port PR 62604: Propagate variance reliability flags
Copilot Oct 20, 2025
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44 changes: 25 additions & 19 deletions internal/checker/relater.go
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Original file line number Diff line number Diff line change
Expand Up @@ -3878,26 +3878,32 @@ func (r *Relater) typeArgumentsRelatedTo(sources []*Type, targets []*Type, varia
} else {
related = r.c.compareTypesIdentical(s, t)
}
} else if variance == VarianceFlagsCovariant {
related = r.isRelatedToEx(s, t, RecursionFlagsBoth, reportErrors, nil /*headMessage*/, intersectionState)
} else if variance == VarianceFlagsContravariant {
related = r.isRelatedToEx(t, s, RecursionFlagsBoth, reportErrors, nil /*headMessage*/, intersectionState)
} else if variance == VarianceFlagsBivariant {
// In the bivariant case we first compare contravariantly without reporting
// errors. Then, if that doesn't succeed, we compare covariantly with error
// reporting. Thus, error elaboration will be based on the covariant check,
// which is generally easier to reason about.
related = r.isRelatedTo(t, s, RecursionFlagsBoth, false /*reportErrors*/)
if related == TernaryFalse {
related = r.isRelatedToEx(s, t, RecursionFlagsBoth, reportErrors, nil /*headMessage*/, intersectionState)
}
} else {
// In the invariant case we first compare covariantly, and only when that
// succeeds do we proceed to compare contravariantly. Thus, error elaboration
// will typically be based on the covariant check.
related = r.isRelatedToEx(s, t, RecursionFlagsBoth, reportErrors, nil /*headMessage*/, intersectionState)
if related != TernaryFalse {
related &= r.isRelatedToEx(t, s, RecursionFlagsBoth, reportErrors, nil /*headMessage*/, intersectionState)
// Propagate unreliable variance flag
if r.c.inVarianceComputation && varianceFlags&VarianceFlagsUnreliable != 0 {
r.c.instantiateType(s, r.c.reportUnreliableMapper)
}
if variance == VarianceFlagsCovariant {
related = r.isRelatedToEx(s, t, RecursionFlagsBoth, reportErrors, nil /*headMessage*/, intersectionState)
} else if variance == VarianceFlagsContravariant {
related = r.isRelatedToEx(t, s, RecursionFlagsBoth, reportErrors, nil /*headMessage*/, intersectionState)
} else if variance == VarianceFlagsBivariant {
// In the bivariant case we first compare contravariantly without reporting
// errors. Then, if that doesn't succeed, we compare covariantly with error
// reporting. Thus, error elaboration will be based on the covariant check,
// which is generally easier to reason about.
related = r.isRelatedTo(t, s, RecursionFlagsBoth, false /*reportErrors*/)
if related == TernaryFalse {
related = r.isRelatedToEx(s, t, RecursionFlagsBoth, reportErrors, nil /*headMessage*/, intersectionState)
}
} else {
// In the invariant case we first compare covariantly, and only when that
// succeeds do we proceed to compare contravariantly. Thus, error elaboration
// will typically be based on the covariant check.
related = r.isRelatedToEx(s, t, RecursionFlagsBoth, reportErrors, nil /*headMessage*/, intersectionState)
if related != TernaryFalse {
related &= r.isRelatedToEx(t, s, RecursionFlagsBoth, reportErrors, nil /*headMessage*/, intersectionState)
}
}
}
if related == TernaryFalse {
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