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Perform structural fallbacks for reliable variance results less liberally #54754

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Perform structural fallbacks for reliable variance results less liberally #54754

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40 changes: 17 additions & 23 deletions src/compiler/checker.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -21114,9 +21114,9 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
return result;
}

function typeArgumentsRelatedTo(sources: readonly Type[] = emptyArray, targets: readonly Type[] = emptyArray, variances: readonly VarianceFlags[] = emptyArray, reportErrors: boolean, intersectionState: IntersectionState): Ternary {
function typeArgumentsRelatedTo(sources: readonly Type[] = emptyArray, targets: readonly Type[] = emptyArray, variances: readonly VarianceFlags[] = emptyArray, reportErrors: boolean, intersectionState: IntersectionState): { result: Ternary, allowStructuralFallback: boolean } {
if (sources.length !== targets.length && relation === identityRelation) {
return Ternary.False;
return { result: Ternary.False, allowStructuralFallback: false };
}
const length = sources.length <= targets.length ? sources.length : targets.length;
let result = Ternary.True;
Expand Down Expand Up @@ -21163,12 +21163,12 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
}
}
if (!related) {
return Ternary.False;
return { result: Ternary.False, allowStructuralFallback: !!(varianceFlags & VarianceFlags.AllowStructuralFallback) };
}
result &= related;
}
}
return result;
return { result, allowStructuralFallback: false };
}

// Determine if possibly recursive types are related. First, check if the result is already available in the global cache.
Expand Down Expand Up @@ -21915,10 +21915,11 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
}

function relateVariances(sourceTypeArguments: readonly Type[] | undefined, targetTypeArguments: readonly Type[] | undefined, variances: VarianceFlags[], intersectionState: IntersectionState) {
if (result = typeArgumentsRelatedTo(sourceTypeArguments, targetTypeArguments, variances, reportErrors, intersectionState)) {
const { result, allowStructuralFallback } = typeArgumentsRelatedTo(sourceTypeArguments, targetTypeArguments, variances, reportErrors, intersectionState);
if (result) {
return result;
}
if (some(variances, v => !!(v & VarianceFlags.AllowsStructuralFallback))) {
if (allowStructuralFallback) {
// If some type parameter was `Unmeasurable` or `Unreliable`, and we couldn't pass by assuming it was identical, then we
// have to allow a structural fallback check
// We elide the variance-based error elaborations, since those might not be too helpful, since we'll potentially
Expand All @@ -21927,16 +21928,16 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
resetErrorInfo(saveErrorInfo);
return undefined;
}
const allowStructuralFallback = targetTypeArguments && hasCovariantVoidArgument(targetTypeArguments, variances);
varianceCheckFailed = !allowStructuralFallback;
const allowStructuralFallbackForCovariantVoid = targetTypeArguments && hasCovariantVoidArgument(targetTypeArguments, variances);
varianceCheckFailed = !allowStructuralFallbackForCovariantVoid;
// The type arguments did not relate appropriately, but it may be because we have no variance
// information (in which case typeArgumentsRelatedTo defaulted to covariance for all type
// arguments). It might also be the case that the target type has a 'void' type argument for
// a covariant type parameter that is only used in return positions within the generic type
// (in which case any type argument is permitted on the source side). In those cases we proceed
// with a structural comparison. Otherwise, we know for certain the instantiations aren't
// related and we can return here.
if (variances !== emptyArray && !allowStructuralFallback) {
if (variances !== emptyArray && !allowStructuralFallbackForCovariantVoid) {
// In some cases generic types that are covariant in regular type checking mode become
// invariant in --strictFunctionTypes mode because one or more type parameters are used in
// both co- and contravariant positions. In order to make it easier to diagnose *why* such
Expand Down Expand Up @@ -22809,33 +22810,26 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
modifiers & ModifierFlags.In ? VarianceFlags.Invariant : VarianceFlags.Covariant :
modifiers & ModifierFlags.In ? VarianceFlags.Contravariant : undefined;
if (variance === undefined) {
let unmeasurable = false;
let unreliable = false;
const oldHandler = outofbandVarianceMarkerHandler;
outofbandVarianceMarkerHandler = (onlyUnreliable) => onlyUnreliable ? unreliable = true : unmeasurable = true;
// We first compare instantiations where the type parameter is replaced with
// marker types that have a known subtype relationship. From this we can infer
// invariance, covariance, contravariance or bivariance.
const typeWithSuper = createMarkerType(symbol, tp, markerSuperType);
const typeWithSub = createMarkerType(symbol, tp, markerSubType);
variance = (isTypeAssignableTo(typeWithSub, typeWithSuper) ? VarianceFlags.Covariant : 0) |
(isTypeAssignableTo(typeWithSuper, typeWithSub) ? VarianceFlags.Contravariant : 0);
variance = VarianceFlags.Invariant;
outofbandVarianceMarkerHandler = (onlyUnreliable) => onlyUnreliable ? (variance! |= VarianceFlags.UnreliableCovariance) : (variance! |= VarianceFlags.Unmeasurable);
const covariant = (isTypeAssignableTo(typeWithSub, typeWithSuper) ? VarianceFlags.Covariant : 0);
outofbandVarianceMarkerHandler = (onlyUnreliable) => onlyUnreliable ? (variance! |= VarianceFlags.UnreliableContravariance) : (variance! |= VarianceFlags.Unmeasurable);
const contravariant = (isTypeAssignableTo(typeWithSuper, typeWithSub) ? VarianceFlags.Contravariant : 0);
variance |= covariant | contravariant;
// If the instantiations appear to be related bivariantly it may be because the
// type parameter is independent (i.e. it isn't witnessed anywhere in the generic
// type). To determine this we compare instantiations where the type parameter is
// replaced with marker types that are known to be unrelated.
if (variance === VarianceFlags.Bivariant && isTypeAssignableTo(createMarkerType(symbol, tp, markerOtherType), typeWithSuper)) {
if ((variance & VarianceFlags.Bivariant) === VarianceFlags.Bivariant && isTypeAssignableTo(createMarkerType(symbol, tp, markerOtherType), typeWithSuper)) {
variance = VarianceFlags.Independent;
}
outofbandVarianceMarkerHandler = oldHandler;
if (unmeasurable || unreliable) {
if (unmeasurable) {
variance |= VarianceFlags.Unmeasurable;
}
if (unreliable) {
variance |= VarianceFlags.Unreliable;
}
}
}
variances.push(variance);
}
Expand Down
6 changes: 4 additions & 2 deletions src/compiler/types.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -6401,8 +6401,10 @@ export const enum VarianceFlags {
Independent = 1 << 2, // Unwitnessed type parameter
VarianceMask = Invariant | Covariant | Contravariant | Independent, // Mask containing all measured variances without the unmeasurable flag
Unmeasurable = 1 << 3, // Variance result is unusable - relationship relies on structural comparisons which are not reflected in generic relationships
Unreliable = 1 << 4, // Variance result is unreliable - checking may produce false negatives, but not false positives
AllowsStructuralFallback = Unmeasurable | Unreliable,
UnreliableCovariance = 1 << 4, // Covariant variance result is unreliable - checking may produce false negatives, but not false positives
UnreliableContravariance = 1 << 5, // Contravariant variance result is unreliable - checking may produce false negatives, but not false positives
Unreliable = UnreliableCovariance | UnreliableContravariance,
AllowStructuralFallback = Unreliable | Unmeasurable,
}

// Generic class and interface types
Expand Down